Vibrational Spectroscopy of Ionic Liquids - Chemical Reviews (ACS

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Vibrational Spectroscopy of Ionic Liquids Vitor H. Paschoal, Luiz F. O. Faria, and Mauro C. C. Ribeiro* Laboratório de Espectroscopia Molecular, Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, São Paulo 05508-000, Brazil S Supporting Information *

ABSTRACT: Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990’s. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on different anions and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

CONTENTS 1. 2. 3. 4.

Introduction Experimental Details Computational Details Vibrational Spectroscopy of Pure Ionic Liquids in the Mid-Frequency Range 4.1. Vibrational Frequencies of Anions 4.1.1. Small Symmetric Anions 4.1.2. More Complex Fluorinated Anions 4.1.3. Alkylsulfates and Hydrogen Sulfate 4.1.4. Carboxilates 4.2. Vibrational Frequencies of Cations 4.2.1. Imidazolium 4.2.2. Pyridinium 4.2.3. Pyrrolidinium 4.2.4. Piperidinium 4.2.5. Ammonium 4.2.6. Other Cations 4.3. Applications 5. Vibrational Spectroscopy of Pure Ionic Liquids in the Low-Frequency Range 6. Vibrational Spectroscopy for Studying Ionic Liquid Phase Transitions 7. Vibrational Spectroscopy of Ionic Liquid Solutions 8. Concluding Remarks Associated Content Supporting Information Author Information © XXXX American Chemical Society

Corresponding Author ORCID Notes Biographies Acknowledgments References

A C E H H H K N O Q Q U U V W Y Y

AS AS AS AS AS AS

1. INTRODUCTION Vibrational and NMR spectroscopies are fundamental tools to characterize ionic liquids. The vibrational spectroscopy techniques of infrared (IR) and Raman permeate the literature on essentially all of the actual or potential applications envisaged for ionic liquids. IR and Raman spectroscopies have provided deep insights on the nature of ionic interactions, the role played by anion−cation hydrogen bonds, molecular conformations, and their modifications as pressure and temperature is varied in the normal liquid phase, during phase transition to crystalline or amorphous (glassy) solid phases, after vaporization, etc. The user of a vibrational spectroscopy technique needs to have in hand reliable interpretation of vibrational spectra, in particular, assignment of experimental frequencies to vibrational motions of the common ions of ionic liquids. This information is usually

AC AH AL AR AS AS AS

Special Issue: Ionic Liquids Received: July 15, 2016

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DOI: 10.1021/acs.chemrev.6b00461 Chem. Rev. XXXX, XXX, XXX−XXX

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available from studies which are more dedicated to theoretical and computational issues related to vibrational spectroscopy. Quantum chemistry methods are now routinely used to calculate vibrational frequencies to be compared to experimental data. However, strong ionic interactions may imply that comparison between experimental and calculated frequencies by a relatively fast ab initio calculation is far from straightforward. In fact, scrutinizing the large body of literature on vibrational spectroscopy of ionic liquids, eventually there is no full agreement between different authors for the same system. Other schemes have been considered for assigning vibrational spectra of ionic liquids (e.g., taking advantage of molecular dynamics simulations of liquids). On the other hand, even though vibrational spectroscopy has a long history in studying high temperature molten salts and some ions are common species for both high and room temperature molten salts (e.g., relatively simple polyatomic anions), sometimes previous knowledge on vibrational spectroscopy of molten salts is not fully appreciated within the context of ionic liquids. Moreover, vibrational spectroscopy is well appropriate for studying intermolecular interactions not only in pure ionic liquids but also in ionic liquids mixtures, solutions of molecular or ionic solutes in ionic liquids, gas absorption, etc. This review addresses the above-mentioned issues and others, aiming to be useful for users who employ IR and Raman spectroscopies combined with other techniques while investigating ionic liquids and also for those who are involved in assignments of vibrational spectra of ionic liquids. Papatheodorou et al.1,2 published important reviews on Raman spectroscopy of high temperature molten salts, but previous reviews on vibrational spectroscopy of ionic liquids are scarce. In 2007, Berg3 published a review on Raman spectroscopy and ab initio calculations of ionic liquids. Berg’s review focused on the spectroscopic signatures of molecular conformations achieved by the ions, mainly 1-alkyl-3methylimidazolium and N,N-dialkylpyrrolidinium cations, and the bis(trifluoromethanesulfonyl)imide anion, [NTf2]−. These Raman spectroscopic studies have been reviewed more recently by Saha et al.4 The more specific issue of vibrational spectroscopy of ionic liquid surfaces using linear and nonlinear techniques has also been reviewed recently.5 In this review, we will address both IR and Raman studies encompassing a wide group of cations and anions commonly used in forming ionic liquids. Figure 1 shows molecular structures of several cations and anions whose vibrational spectra will be discussed in this review, together with notation used throughout this work. This review addresses many issues in which vibrational spectroscopy has given fundamental contributions for our current understanding about interactions and structure of ionic liquids along the last two decades. The review focusesd on linear IR and Raman spectroscopies, being beyond the scope of techniques such as time-resolved IR spectroscopy and coherent anti-Stokes Raman scattering (CARS) which have recently been applied for investigating ionic liquids.6−11 On the other hand, we will address far-infrared (FIR) and low-frequency Raman spectroscopy studies of ionic liquids. Thus, results obtained from optical Kerr effect (OKE) spectroscopy, being the counterpart of low-frequency Raman spectroscopy, will be mentioned, even though OKE is a time-resolved spectroscopy technique, because it has been the most common technique probing the low-frequency vibrations of ionic liquids. The review is organized as follows. We provide experimental details in section 2 since most of spectra shown below have

Figure 1. Molecular structures of some cations and anions whose vibrational spectra are discussed in this review. The Ri, Rj, Rk, and Rl indicate a hydrogen atom or an alkyl chain group. We adopt the usual notation of cations throughout this work (e.g., 1-alkyl-3-methylimidazolium, [CnC1im]+, where n is the number of carbon atoms in the alkyl chain, and 1-alkyl-2,3-dimethylimidazolium, [CnC1C1im]+). Derivatives of ammonium cations are also indicated by the length of the alkyl chains (e.g., [C4C1C1C1N]+ means butyl-trimethylammonium and [C3NH3]+ is the propylammonium cation which forms protic ionic liquids). Similar notation follows for derivatives of pyridinium, pyrrolidinium, and piperidinium and usual notation for the anions.

been especially obtained for the purpose of this review. Raman spectroscopy of high temperature molten salts demand nontrivial experimental skills for building furnaces and sample containers for handling corrosive and air-sensitive melts.2 In contrast, IR and Raman measurements of ionic liquids are more easily carried out since the liquid sample may be simply accommodated in quartz tubes. Nevertheless, some warnings on the experimental side are timely. All of IR and Raman spectra recorded in this work are available for the reader in TXT files. The usefulness of vibrational spectroscopy is heavily linked to the calculation of vibrational frequencies, so that section 3 reviews the methods which have been applied for calculating vibrational spectra of ionic liquids. These methods include classical normal coordinate analysis, ab initio calculation for an isolated ion or for a cluster of ions, and classical or ab initio molecular dynamics simulations of liquids. The scope of section 3 is not technical details of all of these methods; instead the section focuses on the conceptual basis of them, helping the reader to distinguish the assumptions and the need for different methods of calculation. Section 4 reviews vibrational spectroscopy studies of pure ionic liquids, being the longest section of this work. This section discusses the assignment of vibrational frequencies of the most common anions (section 4.1) and cations (section 4.2) which form ionic liquids. These two sections are already plenty of applications of vibrational spectroscopy in studying pure ionic liquids, but further applications are given in section 4.3. It is our hope that the vibrational assignments discussed thoroughly in this section helps the reader to interpret vibrational spectra when working with a given ionic liquid based on a different combination of anions and cations. We separated in section 5 the discussion of the low-frequency range, as this range directly manifests the liquid structure and intermolecular dynamics of ionic liquids. Studies concerning the low-frequency range of vibrational spectra of ionic liquids have been developed along two different research lines, one by workers using far-infrared (FIR) and other by workers using low-frequency Raman spectroscopy, in B


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transmission and ATR infrared spectroscopy has been used by Burba et al.20−22 as a methodology to infer about charge organization in the series [CnC1im][CF3SO3], n = 2−8. They considered the intense IR band of the anion symmetric stretching mode, νs(SO3), with maximum at 1031 cm−1. The approach is based on a first estimate of the dipole moment derivative from a transmission measurement according to the dipolar coupling theory, which assumes a quasilattice organization of the ions. A second estimate of the dipole moment derivative is obtained from optical constants obtained from an ATR measurement, which is not based on the quasillatice model. The ratio between these two values of dipole moment derivatives indicates the degree of quasilattice structure in the ionic liquid. Burba et al.21,22 showed that such charge ordering decreases with increasing length of the alkyl chain in [CnC1im][CF3SO3]. It should be noted, however, that the method also relies on assigning the asymmetric shape of the νs(SO3) IR band as the result of longitudinal optic− transverse optic (LO-TO) splitting23,24 to be considered within the dipolar coupling theory.20−22 All of the IR spectra recorded in this work will be reported in transmittance, except Figures 2 and 5 where IR spectra are shown in absorbance. Raman spectra were obtained with a Horiba-Jobin-Yvon T64000 triple monochromator spectrometer equipped with CCD. The need of triple or double monochromator, or a single monochromator spectrometer with notch and bandpass filters suitable to reduce the Rayleigh scattering line, is particularly important for the low-frequency range, 5 < ω < 100 cm−1, to be discussed in section 5. Raman spectra were obtained in 180° scattering geometry. There was no selection of polarization of scattered light for most of the Raman spectra shown in this review, otherwise some polarized and depolarized Raman spectra will be shown as mentioned in text. The excitation line used was the 647.1 nm line of a mixed argon−krypton Coherent laser, typically with 200 mW of output power. Fluorescence background is a known issue in Raman spectroscopy of (high temperature) molten salts.2 Even though most ionic liquids are colorless, we use a red laser line to excite Raman spectra in order to reduce eventual fluorescence background. For instance, Figure 3 shows Raman spectra obtained with excitations at 514.5 and 647.1 nm for the same sample of a colorless ionic liquid, [C4C1im][PF6]. The same laser power was used to obtain the spectra of Figure 3, and no baseline correction was done, so that it is clear that fluorescence background is strongly reduced when using the 647.1 nm laser line. Whenever fluorescence precluded obtaining suitable

particular OKE spectroscopy. Section 5 is an attempt at putting into a proper perspective the results from FIR and lowfrequency Raman spectroscopy that have been obtained along the past decade. The accumulated knowledge on the nature of vibrational motions and characteristic frequencies in the normal liquid phase discussed in sections 4 and 5 is then applied in section 6 for studying phase transitions, in particular crystallization and glass transition experienced by ionic liquids under low temperature or high pressure. In section 7, we move to ionic liquids solutions. Since ionic liquids as solvents encompass a very large range of applications, from gas molecules to cellulose, and in each of these areas vibrational spectroscopy has given its contribution, section 7 gives some representative examples of the capability of vibrational spectroscopy in shedding light on solute−ionic liquid interactions. Section 8 closes the review with some concluding remarks.

2. EXPERIMENTAL DETAILS IR and Raman spectra of several ionic liquids were recorded for specific purposes of this review. The spectra are available as TXT files. The ionic liquids used in this work were purchased from different suppliers (e.g., Iolitec, Solvionic, Aldrich, and Merck). Most of the purchased ionic liquids have purity superior to 98%, and they were used without further purification except for the drying process as discussed below. Fourier transform IR spectra were recorded with a Bruker Alpha equipment with a DTGS detector and KBr optics. IR spectra measured by transmission were obtained from thin films of liquid samples between KRS-5 windows. In the case of solid samples, we used the attenuated total reflection (ATR) Platinum accessory (Bruker) with diamond crystal and a single reflection. Spectral resolution was 2 cm−1. Optical effects can result in differences between transmission and reflection measurements.12−15 ATR is a technique appropriate for quantitative IR spectroscopy since it allows for better control of sample size and thickness.16,17 Moreover, ATR Fourier transform IR spectrum can be used to obtain optical constants12−14,16,18,19 (i.e., frequency-dependent refractive index and extinction coefficient), as shown by Buffeteau et al.15 for different ionic liquids. Figure 2 illustrates differences in relative intensities and vibrational frequencies that can be found between transmission and ATR measurements of IR spectra of a given ionic liquid, [C4C1im][CF3SO3]. A combination of

Figure 2. IR spectra of [C4C1im][CF3SO3] obtained in transmittance (black) and ATR (red). The IR spectra have their intensities normalized by the most intense band for comparison purpose.

Figure 3. Raman spectra of [C4C1im][PF6] obtained using the 514.5 nm (black) and the 647.1 nm (red) laser line. C


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All the samples used in this work were submitted to a drying process under high vacuum (10−5 mbar) at ∼60 °C for 48 h prior the analysis. The samples were then manipulated inside a drybox with argon atmosphere. It has been shown that small amount of water can have significant effect on ionic liquids properties.25−28 Karl Fischer coulometric titration is the usual method to quantify water, but IR spectroscopy has been applied to measure water content in ionic liquids. Andanson et al.27 used the O−H stretching mode of water in the range 3400− 3800 cm−1, and Fadeeva et al.28 used a combination band of water at ∼5250 cm−1 to measure the water content in ionic liquids. Cammarata et al.29 showed by using IR spectroscopy that interactions between water and the ions are dominated by anion−water interactions in the case of nonprotic ionic liquids based on imidazolium cations. In the case of a mixture of liquids, it is worth noting that accurate analysis of band intensity should take into account the fact that the effective path length eventually changes in ATR-IR measurements because the refractive index of the sample depends on the concentration of the solution.30 Figure 5 (panels A−C) illustrate the evolution of the drying process as monitored by IR spectroscopy for three ionic liquids with the same [C4C1im]+ cation but anions with distinct coordination strength or basicity, [NTf2]−, [CF3SO3]−, and [CH3COO]−. The Karl Fischer analyses of [C4C1im][NTf2], [C4C1im][CF3SO3], and [C4C1im][CH3COO] indicated water concentration of 284, 835, and 22353 ppm, respectively, for the samples as taken straight from their flask prior to any drying attempt. Water content dropped to 214, 298, and 18795 ppm after 48 h under high-vacuum at room temperature, and when the sample was simultaneously warmed to ∼60 °C, the water content achieved 45, 118, and 8580 ppm, respectively. This trend is manifested in the IR spectra of Figure 5 (panels A−C) by the comparison between relative intensities of water bands (3400−3800 cm−1) and ionic liquid bands (2800−3200 cm−1). The drying protocol using high vacuum at room temperature seems enough for a less viscous ionic liquid and less

Raman spectrum with the 647.1 nm laser line, we used a FTRaman Bruker RFS-100 spectrometer with a 1064 nm exciting radiation (Nd:YAG laser Coherent Compass 1064−500N) and Germanium detector. A relatively stringent spectral resolution is eventually required since the complex molecular structures of the ions result in overlapping of bands. Figure 4 illustrates the effect of

Figure 4. Raman spectra of [C4C1im][NTf2] in the range of SO2 wagging mode recorded with different spectrometer resolutions. Raman spectrum obtained with 0.5 cm−1 of spectral resolution is shown with the intensity multiplied by a factor of 20.

spectral resolution in the Raman spectrum of [C4C1im][NTf2] within the spectral range of the anion SO2 wagging mode. This figure exhibits a single broad band at ∼400 cm−1 when using poor spectral resolution, but the band is resolved into two peaks at 397 and 404 cm−1 when using better spectral resolution. In this work, Raman spectra were obtained with spectral resolution of 2 cm−1, taking into consideration compromise on good signal-to-noise ratio.

Figure 5. IR spectra of (A) [C4C1im][NTf2], (B) [C4C1im][CF3SO3], and (C) [C4C1im][CH3COO] before and after the drying processes showing the spectral range of water bands (3400−3800 cm−1). The asterisk in each panel marks the ionic liquid band used to normalize IR intensities within the spectral range shown in the figure. (C) also compares transmission (red) and ATR (green) measurements of IR spectra of the same sample of [C4C1im][CH3COO] after the drying process. (D) shows the molar absorption coefficient Em (black, scale at right), the real part of the refractive index n (green, scale at left), and the imaginary part of the dielectric constant ε″ (red, scale at left) of liquid H2O as provided by Bertie and Lan.31 D


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with the 647.1 nm laser line focused into the sample by a 20× Leica objective. We used a DAC from Almax EasyLab, model Diacell LeverDAC-Maxi, having a diamond culet size of 500 μm. The Boehler microDriller (Almax EasyLab) was used to drill a 250 μm hole in a stainless steel gasket (10 mm diameter, 250 μm thick) preindented to ∼150 μm. Pressure calibration has been done by the usual method of measuring the shift of the fluorescence line of ruby.38,39 Figure 6 illustrates the

coordinating anion such as [C4C1im][NTf2]. Otherwise heating is needed to reduce the viscosity and facilitate the drying process for more viscous ionic liquids and more strongly coordinating anions, [C4C1im][CF3SO3] and [C4C1im][CH3COO].29 It is worth mentioning that increase of temperature implies a certain degree of decomposition as discussed by Gurau et al.32 for [C4C1im][CH3COO]. Even in a situation where the minor amount of impurity is generated, this heating effect may have a direct consequence in Raman spectroscopy because of eventual increase in fluorescence background. The IR spectra reported by Andanson et al.27 for [C4C1im][NTf2] and [C4C1im][CF3SO3] and by Thomas et al.33 and Marekha et al.34 for [C4C1im][CH3COO] exhibit less intense water bands in comparison with the spectra shown in Figure 5. However, it should be noted that these authors reported IR spectra obtained by ATR while we report transmission IR spectra. In fact, Figure 5C compares transmission and ATR spectra of [C4C1im][CH3COO], the latter exhibiting lower intensity of water bands in comparison with the former, being the spectra normalized by an ionic liquid band. The strong variation of the real part of the refractive index, n(ω), within the range of wavenumbers ω of an IR absorption (i.e., the so-called anomalous dispersion as illustrated in Figure 5D for liquid water (green line),31 might imply significant differences between ATR and transmission measurements of IR spectra.12−14 The molar absorption coefficient Em, given by Em = A10/Cd, where d is the path length, C is the molar concentration, and A10 is the decadic absorbance following the Beer’s Law, is related to the imaginary 4πωk(ω) part of the refractive index, C = 2.303Em(ω).18,19,35 On the other hand, Stuchebryukov and Rudoy12 showed that an ATR spectrum can be considered as the spectrum of the imaginary part of the dielectric constant, ε″(ω). Since the complex dielectric constant, ε̂ = ε′ + iε″, and refractive index, n̂ = n + ik, are related by the fundamental equation ε̂ = n̂2, then ε′(ω) = n2(ω) − k2(ω) and ε″(ω) = 2n(ω)k(ω). As a consequence of the anomalous dispersion of n(ω) around an IR band, the frequency of the ε″(ω) spectrum is shifted toward the low-frequency side of the band. This is clearly seen in the comparison between ATR and transmission IR spectra of [C4C1im][CF3SO3] shown in Figure 2. Relative intensities of IR bands in ATR and transmission measurements are also affected by the different dependence on the optical constants. This is evident in Figure 5D, which shows Em(ω) and ε″(ω) spectra provided by Bertie and Lan31 from an investigation of the optical constants of liquid water. It is clear from Figure 5D the difference of intensities in the O−H stretching region for each spectrum. Therefore, the word of caution is that transmission IR spectrum might indicate that an apparent “dry” sample of ionic liquid still contains a significant amount of water, depending on the ionic liquid. Raman spectra as a function of temperature were obtained in this work for some ionic liquids. We used an OptistatDN cryostat (Oxford Instruments) filled with liquid nitrogen, thus allowing for the achievement of temperatures as low as 77 K at which ionic liquids are in crystalline or glassy phases. Raman spectra were also obtained for some ionic liquids under pressure within the GPa range by using a diamond anvil cell (DAC).36,37 Spectra as a function of pressure at room temperature were obtained with the already mentioned Horiba-Jobin-Yvon T64000 spectrometer having a coupled Olympus BX41 microscope. The Raman spectra were excited

Figure 6. Pressure dependence of the fluorescence emission spectra of ruby in the ionic liquid [Pyr14][NTf2]. Spectra are normalized by the most intense peak. The inset shows a photograph of a DAC chamber containing the ionic liquid sample and ruby spheres used for pressure calibration.

pressure-dependent fluorescence emission spectra of ruby in [Pyr14][NTf2]. The inset is a photograph of the gasket hole in between the diamonds of the DAC containing the liquid sample and the ruby spheres used for pressure calibration. In fact, Faria et al.40 showed that the characteristic [NTf2]− Raman band at 741 cm−1 can be used for pressure calibration. The pressure induced frequency shift of this vibrational mode of [NTf2]− exhibits a linear variation, ca. 4.2 cm−1/GPa, depending on the ionic liquid for pressures up to ∼2.5 GPa, so that within this pressure range the ionic liquid can be used as pressure-transmitting medium and pressure marker.

3. COMPUTATIONAL DETAILS Polyatomic ions usually involved in forming ionic liquids range from highly symmetric small ions to complex flexible organic ions, so that different theoretical methods have been used for the calculation of vibrational frequencies and normal mode assignment. These include classical normal coordinates analysis relying on an assumed intramolecular force field, ab initio quantum chemistry calculations at different levels of theory for an isolated ion, ion pair or cluster of ions, and molecular dynamics simulation (classical or ab initio) to calculate an appropriate time correlation function and then its Fourier transform to result in the vibrational spectrum. Different methods are chosen not merely on the basis of molecular structure complexity but also on the level of analysis one wishes and approximations allowed for. All of these theoretical methods have been used for assigning vibrational spectra of the most common ions to be discussed in section 4, so that it is useful to provide here a brief account of these methods for future reference along the review. Classical normal coordinate analysis is based on a ball-andspring model for the nuclei vibrations with no consideration of electronic degrees of freedom.41,42 Taking the [SCN]− anion as E


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an example, a quadratic force field proposed by Baddiel and Janz43 involves internal coordinates for stretching of C−N and C−S bonds, ΔrCN and ΔrCS, and bending of the SCN angle, Δθ,

There are some accounts in the literature of ionic liquids concerning the performance of quantum chemistry methods for calculating different properties (e.g., anion−cation binding energy44,45 or gas−ion interactions for studying gas capture by ionic liquids).46 On the other hand, there are no benchmark studies for ab initio calculations of vibrational spectra of ionic liquids, although some works provide specific comparisons of vibrational frequencies calculated with different levels of theory.47−49 Density functional theory (DFT) is one of the most widely used level of theory for the calculation of vibrational frequencies of ionic liquids since it provides a satisfactory compromise between accuracy and computational time. Overall, the DFT/B3LYP with 6-311++G(d,p) basis set gives good results without being computationally expensive. Second-order Møller−Plesset perturbation theory (MP2) has been also extensively used in the context of vibrational spectroscopy of ionic liquids. We used these methods to calculate vibrational frequencies for specific purposes of this review. The ultimate aim of calculating vibrational frequencies is the assignment of vibrational motions to observed frequencies. Most of the works dealing with calculation of vibrational spectra of ionic liquids have interpreted the normal mode composition by direct visualization of atomic displacements on the computer screen. This is a helpful and easy way of assigning the vibrations, although not a quantitative one. Furthermore, it might be misleading because large displacement of hydrogen atoms eventually is only a matter of the small mass of hydrogen but not implying large contribution to the normal mode energy. Potential energy distribution (PED) in the classical normal coordinate analysis is accomplished by evaluating the percent weight of each symmetry coordinate Sj contributing to the potential energy of the normal mode Qα. The PED can also be written in terms of internal coordinates, an approach which is chemically appealing as one understands the vibrations in terms of changes in bond lengths and angles of valence, out-of-plane, and torsional. Jamróz50 made available the computer code VEDA (vibrational energy distribution analysis) which takes the output of the commonly used Gaussian51 package of quantum chemistry. The matrixes containing atomic displacements and force constants in terms of Cartesian coordinates are available along the calculation of vibrational frequencies with the Gaussian program. The VEDA program then uses the molecular structure, automatically sets internal coordinates, and evaluates how much each one contributes to the energy of a given normal mode. We show in Figure 23 atomic displacements for two normal modes of [C 4 C 1 C 1 im] + calculated by the Gaussian program that illustrate the large amplitude of motions of hydrogen atoms, although PED calculated by the VEDA program indicate they contribute little to the normal modes energies. Harmonic vibrational frequencies are obtained in these ab initio quantum chemistry calculations. No negative eigenvalue (i.e., no imaginary frequency) is obtained as long as the minimum energy molecular structure has been correctly identified. However, theoretical harmonic frequencies are usually higher than experimentally observed, so that scaling factors multiplying the frequencies are needed to bring calculations into agreement with the experiment. Taking 1alkyl-3-methylimidazolim cations as examples, Talaty et al.52 and Heimer et al.53 found for [CnC1im][PF6] and [CnC1im][BF4], n = 2, 3, and 4, it was necessary to multiply harmonic frequencies calculated by the DFT/B3LYP level of theory by

2 2 2V = k CNΔrCN + k CSΔrCS + k CN,CSΔrCNΔrCS + kθ Δθ 2

(1)

with corresponding force constants kCN, kCS, and kθ, and interaction constant kCN,CS. Force constants are second derivatives of the potential energy function at the equilibrium configuration, and they are optimized in order to reproduce the experimental vibrational frequencies. A normal mode Qα is a coordinate in which all atoms execute harmonic vibrations with the same frequency leading to a Hamiltonian function, H = T + V, where T is the kinetic energy, as a collection of independent oscillators without cross-terms between different coordinates: 3N

H=

∑ α=1

1 ̇2 (Q + λαQ α2) 2 α

(2)

where the eigenvalue λα of mode α is related to the square of its vibrational frequency, λα = (2πνα)2, and the coordinates for convenience are weighted by the square root of atomic masses. The number of nuclei is N, and the number of actual vibrational motions, excluding rotational and translational motions of zero frequency, is 3N − 6 in general or 3N − 5 for linear molecules. If this classical Hamiltonian function is used to build the Hamiltonian operator in the quantum chemistry treatment, the set of independent harmonic oscillators implies that the total vibrational energy is the sum of the well-known textbook result for each normal mode, Eα = hνα (vα + 1/2), where h is the Planck constant, vα is the vibrational quantum number, vα = 0, 1, 2···, and να is the same as the classical vibrational frequency. Each Qα is a linear combination of the original coordinates of displacements for all of the atoms of the molecule. The linear combination can be done in terms of the 3N atomic Cartesian coordinates, in terms of internal coordinates of bond-length and angle variations, or in terms of symmetry coordinates, Sj. In the so-called method of GF matrixes of Wilson, Decius, and Cross,41 the symmetry coordinate is a new coordinate system mixing the internal coordinates, so that each Sj belongs to a given symmetry species of the point group of the molecule. The advantage of proposing symmetry coordinates relies on the fact that only Sj of the same symmetry species will mix together in the composition of a given normal mode Qα. Furthermore, defining symmetry coordinates makes easier the resolution of the secular equation for obtaining the eigenvalues because it factors into diagonal blocks for each symmetry species. In the context of ionic liquids, the classical normal coordinate analysis is more appropriate for simple ions, typically highly symmetric anions. Most of vibrational frequency calculations of ionic liquids referred to in section 4 are based instead on quantum chemistry methods. The electronic structure problem of the molecule is solved within a given approximation for the electronic wave function, and the minimum energy configuration of nuclei is found. For this configuration, one obtains the 3N × 3N Hessian matrix (i.e., the matrix containing second derivatives of potential energy in terms of the nuclei Cartesian coordinates). Diagonalization of the Hessian matrix gives the eigenvalues λα along the diagonal, and the eigenvector corresponding to each frequency gives the composition of Qα in terms of atomic displacements. F


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scaling factors within the range 0.963−0.967. Katsyuba et al.54 proposed sets of different scaling factors within the range of 0.889−1.0144 for each kind of normal mode (stretch, bend, torsion, and out of plane) of the [C2C1im]+ cation. In order to go beyond the calculation of harmonic frequencies and to avoid empirical scaling factors, anharmonicity should be included. The concept of independent normal coordinates is no longer rigorously valid when anharmonicity is considered because there appears additional potential terms in the Hamiltonian function of eq 2 mixing together different coordinates, ∑α,β,γgαβγQαQβQγ + ∑α,β,γ,δhαβγδQαQβQγQδ + ..., where gαβγ and hαβγδ are anharmonic constants corresponding, respectively, to third and fourth derivatives of the potential energy.42 Nevertheless, one still thinks in terms of normal modes because anharmonic effects are taken into account as perturbations on the states and energies of the harmonic model according to perturbation theory of quantum chemistry. The harmonic frequency of a given normal mode continues being related to the force constant as in the harmonic case, but the correction on energy levels mixes gαβγ and hαβγδ in complicated expressions depending on the order of the expansion of the potential energy function and the order of perturbation theory.42,55,56 Barone et al.57 made available a code including cubic and quartic anharmonicity constants at second-order perturbation theory (VPT2) to correct harmonic vibrational frequencies calculated by the Gaussian program. Anharmonic frequencies have been indeed calculated by the method of Barone for ionic liquids based on 1-alkyl-3-methylimidazolium cations.58 The vibrational frequency shift when anharmonicity is included in the calculation of an isolated 1-alkyl-3-methylimidazolium cation might be comparable in magnitude to the effect of considering a cation−anion pair, in particular for those vibrations involving C−H stretching motions. Including anharmonicity not only downshifts vibrational frequencies but also allows for addressing important effects on the experimental spectra (e.g., Fermi resonance).42 This has been found particularly important in the high-frequency range of vibrational spectra of 1-alkyl-3-methylimidazolium cations because this spectral range is prone to Fermi resonance between overtones or combination bands of ring vibrations with the C−H stretching modes.58,59 The harmonic model of expanding the potential energy function to quadratic terms in coordinates has also been proposed for an assembly of molecules forming a liquid. If liquid phase configurations are generated along a computer simulation, either by Monte Carlo (MC) or molecular dynamics (MD),60 the Hessian matrix can be evaluated for each configuration. Eigenvalues and eigenvectors resulting from the diagonalization of the Hessian matrix give frequencies and composition of normal coordinates for an instantaneous configuration of the liquid, so that these are called instantaneous normal modes (INM).61,62 Part of the INM has imaginary frequency because a liquid configuration of thousands of particles is not a minimum energy configuration. Nevertheless, the fraction of imaginary frequency INM also carries physical information; for instance, it has been related to ionic diffusion coefficients.63−65 The INM method was applied in MD simulations of (high temperature) molten salts (e.g. ZnCl2,66−68 BeCl2,69 and cryolitic NaF-AlF3 mixtures)70 but not yet for (room temperature) ionic liquids. In atomic molten salts, the positive frequency INM covering the low-frequency range are direct probes of the intermolecular dynamics as experimentally accessible by far-infrared and low-frequency

Raman spectroscopy (see section 5). The INM analysis of molten ZnCl2 was useful to disentangle collective soundlike modes and localized vibrations of a molecular-like structure of ZnCl4 tetrahedral.67,68 In the case of BeCl2 and NaF-AlF3 mixtures, INM of frequencies up to ∼1000 cm−1 are obtained because of relatively stable [BeCl2]n oligomers69 and AlFn(n−3)− polyhedrals.70 Dynamical effects are not included in a quantum chemistry calculation of vibrational spectrum once it is carried out in a minimum energy configuration of an isolated ion, an ionic pair, or a cluster of ions. In contrast, the theoretical framework of time correlation function71−74 is appropriated for calculating vibrational spectra by MD simulations of liquids. A time correlation function measures how a system property at given time, A(t), correlates with another property at a previous time, B(0). In an autocorrelation function, CA(t) = < A(0)·A(t)>, where indicates an average according to statistical mechanics, CA(t) starts from < |A|2> at time zero and reaches the long time value < |A|>2. Figure 7 illustrates the behavior of a time correlation function for the atomic velocities, Cv(t) = < vi(0)·vi(t)>, calculated by MD simulation of [C2C1im][NTf2]. The intra- and intermolecular potential function we used in this simulation is the CL&P model proposed by Lopes and Padua

Figure 7. Upper panel: power spectra, P(ω), calculated according to eq 3 by classical MD simulation of [C2C1im][NTf2] at 400 K and density 1.05 g cm−3. The MD simulation considered 500 ion pairs and the CL&P force field.75 The total P(ω) (black line) has been split into contributions of atoms belonging to anions (red line) and cations (green line). Intensity was normalized by the most intense band of each P(ω). The inset highlights the low-frequency range of the total P(ω). Bottom panel: the normalized time correlation functions of velocity, Cv(t), obtained from the Fourier transform of the corresponding power spectrum. The inset shows the total Cv(t) in a wider time range. G


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M(t) may be approximated at first order as the sum of individual molecular dipole moments, although induced interaction effects imply that M(t) is a collective property.86 An expression analogous to eq 4 follows for the Raman spectrum as the Fourier transform of the time correlation function of polarizability fluctuation. The alternative method of eq 3 also applies to the calculation of IR or Raman spectrum from the time-dependence of dipole moment or polarizability. Vibrational spectra resulting from time correlation functions of dipole moment or polarizability fluctuations calculated by ab initio MD simulations show reasonable agreement to experimental spectra of ionic liquids.33 This approach for calculating vibrational spectra of liquids has been implemented in the TRAVIS (Trajectory Analyzer and VISualizer) package,87 which includes several routines for analyzing trajectories generated by computer simulations. The time correlation function approach for vibrational spectroscopy allows for a direct comparison between simulation and experiment in terms of peak positions, band shapes, and relative intensities, but the ultimate goal of the normal mode assignment is not yet reached. One possibility is to take a few liquid configurations generated by the MD simulation for energy minimization using, for instance, the conjugate gradient method.88,89 In contrast to the INM analysis discussed above, this approach calculates the Hessian matrix and performs the normal-mode analysis for those quenched configurations. In the time correlation function approach, however, the actual nature of the molecular vibration responsible for a given band should be retrieved from the spectrum obtained from the Fourier transform. A generalized normal coordinates approach90,91 has been applied for an anharmonic Hamiltonian as it is the case in ab initio MD simulations of ionic liquids. The approach is based on a generalization of , which is a single particle time correlation function as it involves the property of a given particle i. The generalization accounts for calculating the collective counterpart of mass-weighted velocities, , whose Fourier transform gives a power spectrum Pij(ω), including cross-correlations between different particles. The matrix relating Cartesian to the generalized normal coordinates is obtained according to a recipe which minimizes off-diagonal terms of Pij(ω) for all of the frequencies. The procedure brings the collective power spectrum as close as possible to a diagonal form leading to a representation of vibrations in terms of normal modes. This approach for calculating normal modes from computer simulation includes anharmonicity and condensed phase effects, and it has been applied to assign vibrational spectra of [C2C1im][CH3COO] and its mixture with CO2 and water.33

for MD simulations of ionic liquids.75 Figure 7 shows Cv(t), including all of the atoms i, and also Cv(t) calculated separately for atoms belonging either to cations or anions. The Cv(t) is shown normalized by its initial value, decaying to the average value of velocity which is zero. Figure 7 also shows the so-called power spectrum, P(ω) (i.e., the density of vibrational states). Given any time-dependent property A(t), its power spectrum PA(ω) can be obtained as the Fourier transform of the corresponding time correlation function, CA(t) = < A(0)·A(t)>. An alternative method to calculate PA(ω) more efficiently follows from the spectral density of A(t):74,76−78 2

∞

PA(ω) =

∫

A (t ) e

−iωt

dt

−∞

(3)

Then, the CA(t) can be obtained by Fourier transforming the PA(ω). The Cv(t) shown in the inset of the bottom panel of Figure 7 exhibits an overall damped oscillatory decay within the picosecond time range, arising from rattling dynamics of ions within a temporary cage made of neighboring ions. This intermolecular dynamics is manifested in the P(ω) within the frequency range below ca. 100 cm−1 (see inset in the top panel of Figure 7), which is the range accessible by far-IR and lowfrequency Raman spectroscopy. The CL&P model considers flexible ions, so that the very fast oscillations in Cv(t) (main figure at the bottom panel of Figure 7) arise from the intramolecular vibrations. Accordingly, P(ω) shows highfrequency peaks assigned to cation and anion intramolecular vibrations. Vibrational frequencies in P(ω) manifest condensed phase and anharmonicity effects, as long as an anharmonic intramolecular potential function is included in the model. It is worth noting, however, that the CL&P model75 considers harmonic terms for stretching and bending motions. On the other hand, it is well-known that proper coupling between intra- and intermolecular degrees of freedom, and the consequent vibrational frequency shift and vibrational relaxation in the liquid with respect to the gas phase, is heavily dependent on anharmonic terms in the intramolecular potential.79−81 Calculations of P(ω) have been done by ab initio MD simulations,82,78 which do not rely on an empirical parametrized force field, instead the electronic structure is solved along the simulation run giving the forces that move the nuclei to the new configuration. Therefore, anharmonicity of vibrations are taken into account when P(ω) of ionic liquids are calculated by ab initio MD simulations. The power spectrum shown in Figure 7 is not a theoretical IR or Raman spectrum.78 P(ω) exhibits all of vibrations included in the model, and it has to be weighted by how much the intra- and intermolecular dynamics fluctuate the electric dipole moment or the polarizability in order to represent the IR or the Raman spectrum, respectively. In other words, IR and Raman activities are determined by the coupling between mechanical vibrations and electronic molecular properties. It is a formal result of nonequilibrium statistical mechanics that the IR spectrum, IIR(ω), is proportional to the Fourier transform of the time correlation function of fluctuations of the electric dipole moment of the whole system:74,77,83−85

4. VIBRATIONAL SPECTROSCOPY OF PURE IONIC LIQUIDS IN THE MID-FREQUENCY RANGE 4.1. Vibrational Frequencies of Anions

4.1.1. Small Symmetric Anions. Vibrational spectroscopy is of long usage for studying (high temperature) molten salts of polyatomic inorganic anions, some of which are common species in ionic liquids. Reliable assignment of vibrational frequencies is needed in order to use IR and Raman spectroscopies as useful tools for unravelling molecular conformations, intermolecular interactions, hydrogen bonds, etc. The highly symmetric structures of inorganic anions allow for more straightforward assignment of vibrational frequencies in comparison with the ionic liquids forming organic cations.

∞

IIR (ω) ∝

∫−∞ ⟨M(0)·M(t )⟩eiωt dt

(4) H


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Table 1. Fundamental Frequencies of Some Highly Symmetric Anions Commonly Used in Ionic Liquidsa [NO3]− IR

[BF4]− D3h

Raman

1346 vs 1041 wb 830 m 708 w

IR

νas (E′) νs (A1′) γ (A2″) δ (E′)

1041 vs 706 m

1062 vs 764 mb 522 m

[PF6]−

Raman

Td

764 vs 521 w 352 w

νas (F2) νs (A1) δ (F2) δ (E)

[SCN]−

IR

Oh

Raman

843 vs 741 mb 565 wb 558 s 470 wb [N(CN)2]−

864 740 568 560 471

νas (F1u) νs (A1g) ν (Eg) δ (F1u) δ (F2g)

b

w vs m wb m

IR

Raman

C∞v

IR

Raman

C2v

2056 vs 737 w 471 w

2054 s 738 m

ν(CN) (Σ+) ν(CS) (Σ+) δ (Π)

2192 s 2133 vs 1309 s 904 w

2192 s 2133 w

νs(CN) (A1) νas(CN) (B2) νas(N−C) (B2) νs(N−C) (A1) δ(CNC) (A1) γs(N−CN) (A2) γas(N−CN) (B1) δas(N−CN) (B2)

904 w 666 m

524 wb 508 w 496 w [C(CN)3]−

[B(CN)4]−

IR

Raman

D3h

2209 vwb 2165 vs 1257 m 647 wb 563 s

2209 vs 2163 s 1257 m 646 m 565 wb 483 w

νs(CN) (A1′) νas(CN) (E′) δ((CN)C(CN)) + ν(C−CN) (E′) νs(C−C) (A1′) δ(CCN) (A2″) δ(CCN) (E″)

IR

Raman

Td

2223 vs

νs(CN) (A1) νas(CN) (F2) ν(B−C) (F2) δ(BCN) (E) δ(BCN) (F2) ν(B(CN)4) (A1)

2223 m 938 s 521 w 496 m 483 m

Values (cm−1) correspond to frequencies observed in IR and Raman spectra at room temperature for ionic liquids with the [C4C1im]+ cation for [NO3]−, [BF4]−, and [PF6]− anions and the [C2C1im]+ cation for the cyanate anions. ν, stretch; δ, bend; γ, out-of-plane; s, symmetric; as, antisymmetric; w, weak; m, medium; s, strong; v, very. bInactive according to the symmetry of isolated species. a

Figure 8. IR (red, transmittance scale at right) and Raman (black) spectra of [C4C1im][BF4] (left panel) and [C4C1im][PF6] (right panel) at room temperature in the range of the totally symmetric stretching mode of the anion.

molten salts based on alkali cations. For instance, the vibrational frequency of the totally symmetric stretching mode of the nitrate anion, νs(NO3), in molten alkali nitrates (at T ∼ 400 °C) exhibits a significant downward shift from 1067 to 1043 cm−1 when the counterion is changed along the sequence of Li+, Na+, K+, Rb+, and Cs+.93 Accordingly, νs(NO3) is observed at 1041 cm−1 in molten [C4C1im][NO3] at 313 K. Analogous effect of cation polarizing power is found in the CN stretching mode of [SCN]−. In molten LiSCN, NaSCN, and KSCN, ν(CN) follows the trend 2083, 2074, and 2068 cm−1.43 Arguing beyond mechanical springs−and−balls models, Chabanel et al.94 claimed that strongly polarizing cations stabilize the anion σ orbitals. This effect of strengthening the CN bond is partially counterbalanced by the effect of resonance structures, −S−CN ↔ [SCN]−, that softens the CN bond and becomes predominant as larger is the cation. Accordingly, the ν(CN) mode is observed at 2054 cm−1 in

There is accumulated knowledge on how vibrational frequencies of anions depend on cation charge and size, solid−liquid phase transition, temperature, and dilution in solvents of different dielectric constants. Vibrational frequency of stretching motion of simple anion decreases when the cation is replaced by another with lower polarizing power (i.e., the ratio between charge and ionic radius of the cation).92 In terms of mechanical models of balls and springs,80,79 vibrational frequency shift in the condensed phase is a subtle balance between attractive and repulsive intermolecular forces contributing, respectively, to negative and positive shift in comparison with the vibrational frequency of the free species. The very fact that anion vibrational frequencies increase with strength of interaction with the cation points out the role played by short-range repulsive forces on the probe oscillator.92 As ionic liquids are usually made of bulky organic cations, the anion stretching mode is expected at lower frequency than I


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dipole or polarizability adds further mechanism of fluctuation of electrical properties beyond vibrational and rotational dynamics of the probe molecule.86 Being (o)μi(t) and (o)αi(t), respectively, dipole and polarizability of the isolated molecule i, where time dependence comes from vibration and rotation of the molecule, the actual dipole and polarizability of the molecule in the liquid have additional contributions, (I)μi(t) and (I)αi(t), induced by interactions with neighbor molecules. Let Qi be an IR inactive normal mode of molecule i (i.e., the transition dipole ∂(o)μi/∂Qi is zero for the isolated molecule) but Raman active (i.e., nonzero ∂(o)αi/∂Qi). If the molecule i is surrounded by dipolar molecules j, the dipole-induced dipole (DID) mechanism at this order is enough to account for an interaction-induced contribution to the dipole of i, (I)μi = Σj≠i (∂(o)αi/∂Qi)· T(rij)·(o)μj, where rij is the distance vector between the molecules, and the dipole−dipole tensor is T(r ij ) = (4πεo)−1(3rij·rij − rij2·I)r−5, where I is the unitary matrix. In general, the total dipole and polarizability must be obtained in a self-consistent way because the electric properties of all of the molecules contain permanent and interaction-induced parts.86 In the case of Raman spectra of ionic systems, Madden et al.100,101 showed that other mechanisms might contribute to the fluctuating polarizability besides the DID mechanism (e.g., distortion of a given ion by the Coulomb field of other ions and short-range overlap interactions). Liquid carbon disulfide is a well-known example of interaction−induced effect contributing to vibrational spectra:102−104 the doubly degenerate bending and the antisymmetric stretching modes, which are Raman inactive for an isolated CS2 molecule, become active in the Raman spectrum of the liquid phase. Additional signature of interaction−induced effect is an exponential long tail, e−ω/Δ, where Δ is a parameter, in a Raman band corresponding to a normal mode which is allowed by symmetry of the isolated molecule.102,103 Depolarized Raman spectra have been interpreted by assuming there is time separation between fast rattling dynamics of molecules within the cage of neighbors, resulting in interaction-induced effect as intermolecular distances are changed, and slow reorientational dynamics of the molecule as a whole. The short-time rattling dynamics contributes to the high-frequency tail of the band, whereas the relatively slow reorientational dynamics contributes to the center of the band. Unfortunately, there are overlaps of bands in vibrational spectra of ionic liquids, being difficult to address whether the high-frequency range of a Raman band exhibits exponential shape. A particular band that could be considered in this respect is the stretching mode ν(CN) of [SCN]− because it appears in a spectral range free of overlaps. Figure 9 shows the depolarized ν(CN) Raman band of [C2C1im][SCN] at room temperature. The ν(CN) Raman band indeed exhibits a long tail which extends far from the band-center. The inset of Figure 9 makes clear that the tail exhibits the exponential shape, strongly suggesting that interaction-induced mechanisms should not be ruled out. It would be interesting for future work attempts to disentangle symmetry reduction and interaction-induced effects in vibrational spectroscopy of ionic liquids. Classical intramolecular force fields for computing vibrational frequencies are available for some ionic liquid forming anions (e.g., [NO3]−,98 [SCN]−,43 and [C(CN)3]−).105 On the other hand, the complex molecular structures of the ions typically involved in ionic liquids prompt for quantum chemistry as the more appropriate approach to calculate vibrational frequencies. Hipps and Aplin105 accounted for the vibrations of [C(CN)3]−

the ionic liquid [C2C1im][SCN]. Since a vast literature is available reporting vibrational frequencies for simple anions with different counterions in solid or liquid phases,95 Table 1 gives for reference purpose the values actually observed for some highly symmetrical anions in ionic liquids based on the [C2C1im]+ or [C4C1im]+ cations. Symmetry considerations are more easily applied for vibrations of relatively rigid inorganic anions than for the complex cations of ionic liquids. Vibrational spectroscopy was a powerful tool to unravel the presence of complex anionic species in the early period of highly hygroscopic ionic liquids based on mixing AlCl3 and organic cations chloride. Using Raman spectroscopy, Takahashi et al.96 showed that in the basic solution of AlCl3/[C2C1im]Cl, x(AlCl3) = 0.54, the prevailing anionic species is [AlCl4]− belonging to the Td point group, whereas in acidic solution, x(AlCl3) = 0.67 and the prevailing species is [Al2Cl7]− belonging to the C2 point group (acidic melts have AlCl3:organic salt mole ratio greater than 1). Grondin et al.58 suggested that the octahedral symmetry of the [PF6]− anion is perturbed in the ionic liquid [C4C1im][PF6] since the totally symmetric stretching mode (A1g), which is IR inactive on the basis of the Oh point group of the isolated [PF6]−, is actually observed in the IR spectrum. Analogous effect has been found by Katsyuba et al.,54 who observed the [BF4]− band corresponding to the νs(A1) mode in the IR spectrum of [C2C1im][BF4]. These findings are shown in Figure 8, which compares IR and Raman spectra of [C4C1im][BF4] and [C4C1im][PF6] in the range where the anion totally symmetric mode is observed. Environmental effect on the anion symmetry is an issue in vibrational spectroscopy of molten salts,92 nitrate certainly being the most investigated anion.93,97,98 Experimental evidence of perturbation on the D3h symmetry of [NO3]− is the IR band of the νs(A1′) or the Raman band of the γ(A2″) mode and split of νas(E′) because of the degeneracy lift. The split of νas(E′) is seen in molten LiNO3 but not in molten NaNO3, KNO3, RbNO3, and CsNO3. These condensed phase effects in the [NO3]− vibrations might be understood on the basis of relatively strong ion-pairing leading to C2v symmetry when the cation has high polarizing power. For less polarizing cations, one considers that the D3h symmetry of [NO3]− is retained, but the environmental effects are enough to breakdown selection rules for the isolated nitrate. Accordingly, Table 1 indicates that the νs(A1′) mode is observed as a weak band in the IR spectrum of [C4C1im][NO3]. In the case of [C4C1im][PF6], even though νs(A1g) is observed in the IR spectrum, no lifting of degeneracy of anion bands has been observed in the IR and Raman spectra.58 Therefore, Grondin et al.58 proposed that [PF6]− preserves a quasi-octahedral symmetry in the ionic liquid. We found the νas(F2) mode of [BF4]− as a broad band with maximum at 1062 cm−1 in the IR spectrum of [C4C1im][BF4]. In contrast, Holomb et al.99 found that the νas(F2) mode splits into three peaks (1015, 1033, and 1045 cm−1) in the IR spectrum of [C4C1im][BF4]. We found in the Raman spectrum of [C4C1im][BF4] a band at 493 cm−1, which may be the result of splitting of the F2 bending mode of [BF4]−. This assignment is supported by ab initio calculations performed in this work (MP2 level of theory, aug-cc-pVDZ basis set) for a [BF4]− anion under the perturbation of an external positive charge. Symmetry reduction is not the only condensed phase effect leading to IR or Raman activity of an otherwise inactive vibration according to the point group of the isolated molecule. The so-called interaction-induced contribution to the molecular J


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opposite is true for [N(CN)2]− and [C(CN)3]−. The IR, polarized, and depolarized Raman spectra shown in Figure 10 for [C2C1im][N(CN)2] and [C2C1im][C(CN)3] in the spectral range covering νas(CN) and νs(CN) modes support the assignment. Crystallographic analysis108 of Li[N(CN) 2] showed that the N−C bond length (131 pm) is in between typical values for single and double bonds, whereas the CN bond length (116 pm) is in between double and triple bonds. Therefore, Reckeweg et al.108 concluded that the electronic structure is better represented as the resonance hybrid −N CN−CN ↔ NC−NCN−, rather than NC− N−−CN. On the basis of this resonance, it is reasonable that a vibration will be of lower energy when the two CN moieties oscillate antisymmetrically. The IR spectrum of [C2C1im][N(CN)2] exhibits another band at 2227 cm−1 (just out of the spectral window shown in Figure 10), which has been assigned109 to a combination band of νs(N−C) and νas(N− C) modes whose frequency is shifted and intensity is enhanced because of the Fermi resonance with the fundamental of the νas(CN) mode. In the case of [B(CN)4]−, the Raman active νs(CN) and the IR active νas(CN) are observed at the same frequency.110 4.1.2. More Complex Fluorinated Anions. The bis(trifluoromethanesulfonyl)imide, [NTf2]−, is one of the most popular anions, resulting in low melting point salts when combined with the majority of common organic cations. Assignment of vibrational frequencies of [NTf2]− has been an issue in the literature because the lithium salt of [NTf2]− is commonly used in polymer electrolytes. Rey et al.111 provided a detailed analysis of the [NTf2]− normal modes on the basis of quantum chemistry calculations for its C2 conformer. In a subsequent IR and Raman spectroscopic study, Herstedt et al.112 were able to distinguish bands characteristic of C2 and C1 conformers of [NTf2]−, commonly called transoid and cisoid, respectively, in solutions of Li[NTf2] in ethers CH3O(CH2CH2O)nCH3, where n = 1, 2, 3, and 4. A complete table of experimental versus calculated vibrational frequencies, internal force constants, and potential energy distribution of [NTf2]− normal modes can be found in these papers.111,112 Some of the bands assigned to [NTf2]− vibrations actually observed in IR and Raman spectra of a typical ionic liquid, [C2C1im][NTf2], are marked in Figure 11. Tables containing the [NTf2]− vibrational frequencies are available in many papers,111−123 but values strongly depend on the counterion, physical state, and solvent. Thus, Table 2 lists the actual values observed in [C2C1im][NTf2] at room temperature. The eigenvectors of [NTf2]− normal modes exhibit a complex pattern involving displacements of many atoms,112 so that the assignment in Table 2 is only a simplification of the potential

Figure 9. Depolarized Raman spectrum of the ionic liquid [C2C1im][SCN] at room temperature in the range of the ν(CN) normal mode. Intensity has been normalized by the maximum of the band. The inset shows the logarithm of the high frequency side of the band, where the red line is a linear fit highlighting the exponential tail, e−ω/Δ, with Δ = 36.7 cm−1.

in the potassium salt with a quadratic force field with force constants for internal coordinates of bond displacements and angles based on ab initio calculation. The carbon−carbon distance and force constant obtained for [C(CN)3]− are similar to values for benzene, indicating significant resonance stabilization in [C(CN)3]−.105 We have found bands at 1231 and 1230 cm−1 in IR and Raman spectra, respectively, of [C2C1im][C(CN)3]. In accordance with the assignment suggested by Hipps and Aplin for simpler [C(CN)3 ] − salts,105 this is a combination band intensified by Fermi resonance with the fundamental E′ mode observed at 1257 cm−1 in IR and Raman spectra of [C2C1im][C(CN)3]. The occurrence of this Fermi resonance was indeed confirmed by anharmonic ab initio calculations of an isolated [C(CN)3]− anion performed in this work (MP2/VPT2 level of theory, augcc-pVDZ basis set). The calculation showed that the 1230 cm−1 band is the combination of A1′ δ(CCN) and E′ νs(C−CN) modes calculated at 606 and 641 cm−1, respectively. The vibrational frequency of the totally symmetric CN stretching mode decreases along the sequence [B(CN)4]−, [C(CN)3]−, and [N(CN)2]− as the electronic delocalization increases by 2223, 2209, and 2192 cm−1, respectively, in ionic liquids with the same [C2C1im]+ cation (see Figure 10).106 In an IR spectroscopy study of solid [NH4][N(CN)2], Sprague et al.107 assigned the nontotally symmetric mode νas(CN) at higher frequency than the totally symmetric mode νs(CN). It is a rule of thumb in vibrational spectroscopy assigning the nontotally symmetric stretching at higher frequency than the totally symmetric stretching of a given moiety. However, the

Figure 10. IR (red, transmittance scale at right) and Raman spectra (black; full line, polarized; dashed line, depolarized) of ionic liquids [C2C1im][N(CN)2] (left), [C2C1im][C(CN)3] (middle), and [C2C1im][B(CN)4] (right) at room temperature. K


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cisoid [NTf2]− conformers in the normal liquid phase of ionic liquids. Ionic liquids in which [NTf2]− conformers have been found by vibrational spectroscopy include systems based on derivatives of imidazolium,119,3,124,125 pyrrolidinium126,127 (either protic or nonprotic derivatives of these cations),116,118 piperidinium,120 and ammonium.122,123,128 Quantum chemistry calculations indicate that the spectral ranges 260−370 cm−1 and 620−660 cm−1 are well-suited for finding bands that characterize the transoid or the cisoid conformation. These spectral ranges of the Raman spectrum of [C2C1im][NTf2] are highlighted in Figure 12. The antisymmetric SO2 out-of-plane bending at 623 cm−1 characterizes the transoid conformer, whereas the S−N−S bending at 650 cm−1 characterizes the cisoid conformer. These are very weak Raman bands so that the 260−370 cm−1 range is more appropriated for identifying the [NTf2]− conformation. In particular, the SO2 rocking at 326 cm−1 is characteristic of the cisoid conformer. The quantum chemistry assignments marked in Figure 12 are supported by remarkable spectral changes eventually observed in crystalline phases as bands belonging to a given conformer might be absent. Following phase transitions of ionic liquids by vibrational spectroscopy will be discussed in section 6. The most intense Raman band at 741 cm−1, corresponding to a normal mode in which the [NTf2]− anion breathes as a whole, exhibits an asymmetric band shape that has been also assigned to transoid and cisoid conformers, giving two components with a small difference of ∼3 cm−1.116 On the other hand, it will be

Figure 11. IR (red, transmittance scale at right) and Raman spectra (black) of [C2C1im][NTf2] at room temperature. Some bands assigned to the [NTf2]− normal modes are indicated by arrows.

energy distribution among the different internal coordinates given in ref 111. It is clear from Figure 11 that the Raman spectrum of [C2C1im][NTf2] is dominated by anion bands because of large polarizability fluctuation resulting from anion vibrations. The remaining bands assigned to the cation are easily identified by comparison with the spectrum of an atomic anion counterpart (e.g., [C2C1im]Cl). It has been a successful application of Raman spectroscopy unveiling the coexistence of transoid and

Table 2. Vibrational Frequencies of Some Fluorinated Anions Commonly Used in Ionic Liquidsa [NTf2]− IR

408 514 571 602 619 651 741 762 790 1058 1139 1195 1231 1333 1352

[N(SO2F)2]−

Raman

IR

120 165 278 297 313 326 340 351 397 404

t(CF3)

551 571 590 623 650 741 764 796

δs(SO2) δa(CF3) δa,ip(SO2) δa,op(SO2) δ(SNS) νs(SNS) νs(SNS) ν(CS)

1136

νs,ip(SO2)

1243 1337 1353

νs(CF3) νa,op(SO2)

ρ(CF3) ρ(SO2) ρ(SO2) τ(SO2) τ(SO2) ω(SO2) ω(SO2)

454 482 524 572 725 831 1218 1365 1383

Raman 291 326 359 456 482 524 568 727 831 1217 1361 1380 1388

τ(SO2F) δop(SO2F) δop(SO2F) δsci(SOF) + δ(OSNSO) δsci(SOF) + δop(SO2F) δsci(SO2) + ν(SF) δip(O2SNSO2) + ν(SF) + δip(SO2F) δsci(SO2) + ν(SF) + δsci(SNS) ν(SF) + ρ(SO2) + νs(SNS) νs(SO2) νas(SO2) νas(SO2) νas(SO2)

[CF3SO3]− IR

Raman

518 573 638 755 1031 1162 1225 1261 1281

210 312 348 518 573 640 755 1034 1167 1226 1259 1281

ρ(CF3) ρ(SO3) δas(SO3) δas(CF3) δs(SO3) δs(CF3) νs(SO3) νas(CF3) νs(SO3) νas(SO3) νas(SO3)

a Values (cm−1) correspond to frequencies observed in IR and Raman spectra at room temperature for ionic liquids with the [C2C1im]+ cation for [NTf2]− and [N(SO2F)2]− anions and the [C4C1im]+ cation for [CF3SO3]−. ν, stretch; δ, bend; δsci, scissoring; ω, wagging; τ, twisting; ρ, rocking; t, torsion; s, symmetric; as, antisymmetric; ip, in plane; and op, out-of-plane.

L


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Figure 12. Raman spectrum of [C2C1im][NTf2] in the spectral ranges covering the bands characteristics of [NTf2]− in transoid (marked #) and cisoid (marked *) conformations. Optimized structures of transoid and cisoid conformers of [NTf2]− are shown.

discussed that the 741 cm−1 band is very sensitive to coordination of [NTf2]− to small cations of high polarizing power (see section 7). The closely related bis(fluorosulfonyl)imide anion, [N(SO2F)2]−, also exhibits equilibrium between C1 and C2 conformers in ionic liquids. Fujii et al.129 discussed the signature of conformational equilibrium of [N(SO2F)2]− in vibrational spectra of ionic liquids containing the cations 1ethyl-3-methylimidazolium129 and N-methyl-N-propyl-pyrrolidium.130 Figure 13 shows vibrational spectra of [C2C1im][N-

bands of [N(SO2F)2]− in the range of 250−400 cm−1 are asymmetric because of the presence of both the [N(SO2F)2]− conformers. The bands seen in Figure 14 at 291, 326, and 359 cm−1 in the Raman spectrum of [C2C1im][N(SO2F)2] correspond to SO2F out-of-plane bending for cisoid conformer

Figure 13. IR (red, transmittance scale at right) and Raman spectra (black) of [C2C1im][N(SO2F)2] at room temperature. Some bands assigned to [N(SO2F)2]− normal modes are indicated by arrows.

(SO2F)2] with the anion bands indicated by arrows. Assignment of [N(SO2F)2]− normal modes has not been provided in refs 129 and 130, but it is available in the work of Matsumoto et al.131 concerning polymorphism in crystals of Na+, K+, and Cs+ salts of [N(SO2F)2]−. On the basis of quantum chemistry calculations at the DFT/ B3LYP level of theory, Fujii et al.129,130 proposed that Raman

Figure 14. Raman spectrum of [C2C1im][N(SO2F)2] at room temperature. Vibrational frequencies and relative intensities of Raman bands calculated by the DFT/B3LYP level of theory are indicated for [N(SO2F)2]− at C1 (cisoid, blue lines) and C2 (transoid, red lines) conformation. Optimized structures of transoid and cisoid conformers of [N(SO2F)2]− are shown. M


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or SO2F twisting for transoid conformer, SO2 rocking for cisoid conformer or SO2F out-of-plane bending for transoid conformer, and SO2F in-plane bending for cisoid conformer or SO2F out-of-plane bending for transoid conformer, respectively.131 The calculated frequencies for each [N(SO2F)2]− conformer are relatively close, so that the more strong experimental evidence in favor of conformational equilibrium in the ionic liquid is the temperature dependence of the spectral pattern.129,130,132 The relative intensities of components belonging to transoid conformer decrease with increasing temperature providing support to the calculations that transoid is the lower energy conformer between them.129,130 Giffin et al.133 prepared pyrrolidinium ionic liquids with an anion whose molecular structure lies in between [NTf2]− and [N(SO2F)2]−, namely, (fluorosulfonyl) (trifluoromethanesulfonyl)imide, [N(SO2F) (CF3SO2)]−. The most intense Raman band of [N(SO2F) (CF3SO2)]− at 730 cm−1 is the counterpart of the characteristic band due to the expansion and contraction modes of [NTf2]−. Giffin et al.133 emphasized that this band of [N(SO2F) (CF3SO2)]− is broader than analogous band of [NTf2]− proper to distribution of three rotamers of [N(SO2F) (CF3SO2)]−. The authors of ref 133 provided a table of experimental versus calculated vibrational frequencies and normal mode assignment for different conformers of [N(SO2F) (CF3SO2)]−, but overlap of bands in the spectral range 280−400 cm−1 implies that distinguishing the presence of [N(SO2F) (CF3SO2)]− conformers by Raman spectroscopy is a more challenging task than [NTf 2 ] − conformers. The trifluoromethanesulfonate (triflate) anion, [CF3SO3]−, was the subject of many vibrational spectroscopy studies because it has been extensively used in polymer electrolytes.134−137 One recurrent issue in these works is to unveil, from frequency shift and band split, ionic pairing between [CF3SO3]− and alkali metal cations. Here again vibrational frequencies of some normal modes are very dependent on the polarizing power of the cation. For instance, δs(CF3) and νs(SO3) are observed at 755 and 1034 cm−1, respectively, in the Raman spectrum of [C4C1im][CF3SO3]. These frequencies are close to values of “free” [CF3SO3]− in 2-methyltetrahydrofuran solution,138 whereas these modes are observed at 767 and 1053 cm−1 for [CF3SO3]− in aggregates with the strongly polarizing Li+ cation. In a quantum chemistry investigation of [CF3SO3]− coordinated to Li+, Gejji et al.139 calculated the potential energy distribution of normal modes among the internal coordinates of [CF3SO3]−. Huang et al.138 compared the nature of the normal modes of free and lithium-coordinated [CF3SO3]−. Potential energy distribution indicates less delocalized normal modes for [CF3SO3]− than [NTf2]− or [N(SO2F)2]−, as one would expect from the electronic structure of [CF3SO3]−. Since the [CF3SO3]− vibrational frequencies are very sensitive to the local environment experienced by the anion,140−143 we show in Figure 15 IR and Raman spectra of a common [CF3SO3]− based ionic liquid. Vibrational frequencies belonging to [CF3SO3]− normal modes actually observed in spectra of [C4C1im][CF3SO3] are listed in Table 2. Assignments are available from the previous works on polymer electrolytes and reconsidered by Schwenzer et al.144 within the context of ionic liquids. A distinctive feature of a study published by Akai et al.145 was recording IR spectrum of the ionic pair after evaporating [C2C1im][CF3SO3] and trapping in a cryogenic neon matrix. Of course the bands in the IR spectrum of matrixisolated ionic pair become much sharper than the normal liquid

Figure 15. IR (red, transmittance scale at right) and Raman spectra of [C4C1im][CF3SO3] (black) at room temperature. Some bands assigned to the [CF3SO3]− normal modes are indicated by arrows.

phase spectrum shown in Figure 15. This allowed a fine comparison between experimental and calculated frequencies at the DFT/B3LYP level of theory for two different arrangements of the [C2C1im]+−[CF3SO3]− pair.145 The experimental matrix-isolated IR spectrum of [C2C1im][CF3SO3] was more consistent with a local arrangement in which the anion SO3 group points toward the cation C2 atom with five anion−cation hydrogen bonds.145 The vibrations of tris(pentafluoroethyl)-trifluorophosphate, [FAP]−, in the ionic liquid [C2C1im][FAP] have been discussed in an IR and Raman study by Mao and Damodaran146 and in an IR study by Voroshylova et al.147 These two works provide a full list of observed frequencies of [C2C1im][FAP] and assignments based on the potential energy distribution of normal modes calculated for the ionic pair at the DFT/B3LYP level of theory. Concerning those bands belonging to [FAP]− normal modes, there are some disagreement on assignments proposed in these works. For example, the intense IR band at 1209 cm−1 was assigned to ν(CC) by Mao and Damodaran,146 but to νas(CF2) by Voroshylova et al.147 Two isomers of [FAP]− are possible, meridional and facial, each one with several conformers. The calculations of Voroshylova et al.147 indicated IR bands at ∼800 and ∼700 cm−1 as characteristic features of meridional and facial isomers, respectively, and mixture of anion conformers in [C2C1im][FAP]. 4.1.3. Alkylsulfates and Hydrogen Sulfate. Few works have discussed vibrational spectra of pure ionic liquids containing alkylsulfate anions, [R−O−SO3]−, in spite of the relevance of this class of anion forming ionic liquids. IR and Raman spectra of 1-ethyl-3-methylimidazolium ethylsulfate, [C2C1im][C2SO4], have been first discussed by Kiefer et al.,115 and in a subsequent paper by Dhumal et al.148 The IR frequencies listed in these two works agree with each other; however, there is systematic mismatching between them for the Raman frequencies of [C2C1im][C2SO4]. For instance, the most intense Raman band of [C2SO4]− is reported at 1072 cm−1 in ref 115, whereas it is reported at 1060 cm−1 in ref 148. The latter is most probably the correct value, being close to 1062 cm−1 as found in aqueous solution of Na[C2SO4].149 In light of disagreement of reported frequencies for [C2SO4]−, Figure 16 shows IR and Raman spectra obtained in this work for [C2C1im][C2SO4] at room temperature. Raman frequencies of [C2SO4]− we obtained agree with values given in ref 148. Dhumal et al.148 calculated vibrational frequencies at the DFT/B3LYP level of theory for isolated ions and the ionic pair. N


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spectroscopies and quantum chemistry calculations to characterize the conformations of ions in [C6C1im][HSO4], concluding that [HSO4]− occurs in the trans conformation. Kiefer and Pye155 found a group of bands which they assigned to sulfuric acid in the IR (903, 1158, and 2963 cm−1) and Raman (959, 1157, and 1364 cm−1) spectra of [C6C1im][HSO4]. They did not conclude whether the sulfuric acid originated from proton transfer, like the case of 1-alkyl-3methylimidazolium acetate, or from a residual of synthesis. On the other hand, sulfuric acid bands have not been found either in the IR spectrum of [C4C1im][HSO4] reported by Schwenzer et al.144 or in the Raman spectra of [C2C1im][HSO4] and [C4C1im][HSO4] reported by Ribeiro.156 The [HSO4]− Raman bands observed in the ionic liquids at 416 [δas(SO3)], 581[δs(SO3)], 845 [ν(S−OH)], and 1046 cm−1 [νs(S O)]156 nicely match the corresponding bands in the (high temperature) molten salt KHSO4.151 Another νs(SO) band at 1010 cm−1 has also been found, whose intensity increases as temperature decreases, and assigned to [HSO4]−, engaged in chains of hydrogen-bonded anions.156 Signature of anion− anion hydrogen bonding has been found in vibrational spectra of alkali bisulfate crystals.157−160 It had been proposed decades ago that simple [HSO4]− molten salts are very viscous because structures of hydrogen-bonded anions existing in the crystalline phase remain in the liquid phase just above the melting temperature.161 Therefore, it was proposed that the very high viscosity of [HSO4]− ionic liquids in comparison with alkylsulfates for a given 1-alkyl-3-methylimidazolium cation is due to anion−anion, rather than anion−cation, hydrogen bonding.156 4.1.4. Carboxilates. Vibrational spectroscopy studies of carboxylate-based ionic liquids are heavily linked to applications in gas absorption to be discussed in section 7. Here we focus on vibrational frequencies of acetate in pure ionic liquids based on 1-alkyl-3-methylimidazolium cations. IR and Raman spectra of [C2C1im][CH3COO] and [C4C1im][CH3COO] have been discussed by Thomas et al.33 and Cabaço et al.,162 respectively. Raman frequencies of [CH3COO]− reported in these works are listed in Table 3 showing some inconsistencies between them. Ito and Bernstein163 discussed IR and Raman spectra of aqueous solutions of formate, oxalate, and acetate; the vibrational frequencies of the latter are given in Table 3 for comparison purposes. Assignment of νs(COO) and νas(COO) modes in ionic liquids [CnC1im][CH3COO] is cumbersome because of overlaps with bands of imidazolium ring modes in the same spectral range. A large number of works address vibrational spectra of acetate in alkali salts and metal complexes.163−166 However, direct comparison with the frequencies observed in ionic liquids is not straightforward

Figure 16. IR (red, transmittance scale at right) and Raman (black) spectra of [C2C1im][C2SO4] at room temperature. Some bands assigned to [C2SO4]− normal modes are indicated by arrows.

Assignment of vibrational frequencies on the basis of comparison between calculated and experimental data is particularly difficult for this system because strong anion− cation interaction imply significant frequency shifts. For instance, the most intense and sharp Raman band at ∼1060 cm−1, which is assigned to S−O symmetric stretching of [C2SO4]− in ref 149, is assigned to C−O stretching in ref 148. In this work, we calculated vibrational frequencies of an ionic pair made of [C2SO4]− and tetramethylammonium cation at the DFT/B3LYP level of theory with a basis set 6-311+ +G(d,p). Our results are inline with the assignment of Dhumal et al.,148 that is, the Raman bands observed at 1061.5 and 959 cm−1 in Figure 16 are assigned to C−O and S−O stretching modes, respectively. We also found that the vibrational frequencies of the main bands of methylsulfate, [C1SO4]−, are essentially the same in the ionic liquid [C4C1im][C1SO4] (spectra not shown) since differences between [C2C1im][C2SO4] and [C4C1im][C1SO4] spectra arise from different alkyl chain lengths. Vibrational frequencies of stretching modes of C−H bonds of alkylsulfate anions overlap the same highfrequency range of corresponding vibrations of 1-alkyl-3methylimidazolium cations discussed in the next section. At first sight, one would expect more simple spectra for ionic liquids based on hydrogen sulfate (or bisulfate), [HSO4]−, since it could be considered the first of the [R−O−SO3]− series. However, hydrogen bonding implies nontrivial features in vibrational spectra of molten bisulfates. Bisulfate belongs to the class of relative simple polyatomic anions with a large number of spectroscopic studies concerning alkali cation molten salts150−152 or aqueous solution.153,154 Within the more recent context of ionic liquids, Kiefer and Pye155 used IR and Raman

Table 3. Fundamental Frequencies (cm−1) and Assignments Reported in the Literature for Raman Spectra of the Acetate Anion in Ionic Liquids. Raman Frequencies of Na[CH3COO] Aqueous Solution are Given for Comparison Purposesa Thomas et al.33 [C2C1im][CH3COO]

a

635 899 1334

δ(OCO)+ν(CC) δ(OCO)+ν(CC) νs(COO)

1567

νas(COO)

Cabaço et al.162 [C4C1im][CH3COO] ρ(COO) δ(OCO) ν(CC) δ(CH3) νs(COO) νas(COO) νs(CH3)

454 637 902 1326 1382 1582 2917

Ito and Bernstein163 Na[CH3COO](aq) 471 650 926 1344 1413 1556 2936

ρip(COO), B1 δ(OCO), A1 ν(CC), A1 δ(CH3), A1 νs(COO), A1 νas(COO), B1 νs(CH3), A1

ν, stretch; δ, bend; ρ, rocking; s, symmetric; as, antisymmetric; ip, in plane. O


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since [CH3COO]− frequencies are very sensitive to coordination. Furthermore, theoretical calculations for [CnC1im]+ and [CH3COO]− in the gas phase might lead to proton transfer and formation of the N-heterocyclic carbene. This is regarded as an important step along the process of CO2 absorption, but the ionic environment in the liquid phase stabilizes the ions.167 The theoretical tool employed by Thomas et al.33 was ab initio molecular dynamics simulation of 36 pairs of [C2C1im]+− [CH3COO]− ions, whereas Cabaço et al.162 performed DFT/ B3LYP calculations of a cluster made of two pairs of [C4C1im]+−[CH3COO]− ions. In order to identify the [CH3COO]− bands, it is useful to compare vibrational spectra of acetate-based and other ionic liquids with the same cation. Cabaço et al.152 compared IR and Raman spectra of [C4C1im][CH3COO], [C4C1im][BF4], and [C4C1im][PF6]. In Figure 17, we provide a comparison of IR and Raman spectra

Table 4. Assignment According Different Authors for Some Characteristic Imidazolium Ring Vibrations Numbered as Figure 18a #

IR

Raman

assignment

1

1021

1021

2

1337

1337

3

1378

1385

ref 58: ν(CNring) + δ(CNCring) + δ(CH) ref 53: νring,ip,s ref 54: breathing + ν (N−CH2) + ν (N−CH3) ref 58: νip(C(2)H3N) + νip(CNring) + w(CH2) ref 53: ν[CN CH3(N)] + ν[CN CH2(N)] + νring,ip,s ref 54: breathing + ν(N−CH2) + ν(N−CH3) ref 58: νas(C(2,4)N) + δ(CH) + δs(CH3) ref 53: δs[HCH CH3(N)] + ν[CN CH2(N)] + νring,ip,s ref 54: νas(C(2)N(1)C(5)) + δs(CH3) ref 58: νas(C(2,5)N) + δ(CH2) + δ(C(2,4,5)H)+ ν(C(2)N) ref 53: νring,ip,as + ν[CN CH2(N)]+ δs[HCH butyl] ref 54: νring + δ(CH2) ref 58.:ν(C(2)N) + δ(CH) + ν(CCring) ref 53: νring,ip,as + ν[CN CH2(N)] + ν[CN CH3(N)] ref54: νas(N(1)C(2)N(3)) + r(C(2)−H)

4

5

1416

1568

1564

v, stretching; δ, in plane bending; w, wagging; r, rocking; s, symmetric; as, antisymmetric; ip, in-phase. a

formate and propanoate ionic liquids are higher, whereas it is slightly smaller in butanoate than corresponding values in the sodium salts. The work of Tanzi et al.170 provided vibrational frequencies of these carboxylate anions and assignments on the basis of DFT calculations for isolated ions or ionic pairs and by ab initio MD simulations of clusters of ions. The sequence of calculations for isolated ions, ionic pairs, and clusters highlight the spectral signature of strong hydrogen bonds between carboxylate anions and the choline cation. The Raman band shape of the ν(CC) mode is also a probe of the coordinating structure of acetate in solution.171 Cabaço et al.162 considered the asymmetry of the ν(CC) Raman band of [C4C1im][CH3COO] at 902 cm−1, which exhibits a high frequency tail due to a low-intensity component at 909 cm−1, as an indication of a fraction of anions in the bidentate coordination. A jointed neutron diffraction and MD simulation study of [C2C1im][CH3COO] by Bowron et al.172 indeed suggested dominance of unidentate over bidentate coordination of [CH3COO]− to the hydrogen atoms bounded to the imidazolium ring of [C2C1im]+. It will be discussed in the next section that an anion hydrogen-bonded to 1-alkyl-3-methylimidazolium cations has significant effect on vibrational frequencies of stretching of C−H ring bonds of the cation. The Raman band at 2917 cm−1 assigned to the νs(CH3) mode of [CH3COO]− (see Table 3) overlaps the spectral range of C−H stretching modes of the butyl chain of [C4C1im]+ (see Figure 19 below). Vibrations of the trifluoroacetate anion have been discussed by Cabaço et al.173 These authors provided a list of vibrational frequencies and assignments for [CF3COO]− in [C4C1im][CF3COO] together with corresponding values for the simple salt Na[CF3COO] in aqueous solution.174 Some effects of replacing H by F atoms in going from [CH3COO]− to [CF3COO]− are worth noting on the most important vibrations of the anion. In the case of [C4C1im][CF3COO], νas(COO) and νs(COO) are observed at 1691 and 1406 cm−1, respectively.173 The difference between them (Δ = 285 cm−1), being significantly larger than in Na[CF3COO] (Δ = 246 cm−1),165 is also an indication of unidentate interaction

Figure 17. IR (red) and Raman (black) spectra of [C4C1im][CH3COO] at room temperature. For comparison purposes, IR (green) and Raman (blue) spectra of [C4C1im]Br in the liquid phase are shown. Arrows indicate bands assigned to [CH3COO]− normal modes.

of [C4C1im]Br and [C4C1im][CH3COO], where bands that Cabaço et al.162 assigned to [CH3COO]− are indicated by arrows. The Raman bands at 1334 and 1567 cm−1 assigned by Thomas et al.33 to the acetate anion actually belongs to imidazolium ring modes as discussed in the next section (see Table 4). Therefore, we follow Cabaço et al.162 assignment of [CH3COO]− bands. However, it is worth mentioning that we found the most intense [CH3COO]− Raman band at 911 cm−1 in the Raman spectrum of [C4C1im][CH3COO]. It is worth noting the difference Δ = νas(COO) − νs(COO) = 200 cm−1 in the ionic liquid [C4C1im][CH3COO]. In the context of coordination chemistry, the magnitude of Δ has been used as a signature of the coordinating structure of acetate to cations, 164−166,168 taking the value of Δ in Na[CH3COO]163,169 as a reference of purely ionic interaction. In the case of unidentate coordination (i.e., only one oxygen atom of [CH3COO]− coordinating the metal cation), the equivalence of oxygens is removed, implying large Δ, typically Δ > 200 cm−1. In contrast, bidentate coordination (i.e., chelating structure) implies small Δ, typically Δ < 150 cm−1.165 The value of Δ in [C4C1im][CH3COO] suggests dominance of unidentate interaction of acetate to imidazolium cation. The magnitude of Δ has also been commented by Tanzi et al.170 in an IR and Raman investigation carried out for ionic liquids with three different carboxylate anions (formate, propanoate, and butanoate) and a common cation (choline). The values of Δ in P


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between [CF3COO]− and the imidazolium cation. In contrast to the asymmetric Raman band shape of ν(CC) of [CH3COO]−, the ν(CC) mode of [CF3COO]− gives a single inhomogeneously broadened Gaussian profile at 824 cm−1 in the Raman spectrum of [C4C1im][CF3COO].173 Another worth mentioning difference between [C4C1im][CH3COO] and [C4C1im][CF3COO] is the Raman band at 1203 cm−1 in the latter due to the CF3 stretching mode.173 4.2. Vibrational Frequencies of Cations

4.2.1. Imidazolium. Vibrations of 1-alkyl-3-methylimidazolium cations have been discussed in more detail than other ionic liquid forming cations. Assignment of vibrational frequencies has been done with the help of quantum chemistry calculations for the isolated cation or ion pairs. Berg3 used MP2 level of theory to calculate vibrational frequencies of [C4C1im]+ with two different conformations of the butyl chain. A wellsuited level of theory to calculate vibrational frequencies is DFT/B3LYP with a typical basis set, say 6-31+G(d,p). DFT/ B3LYP has been used by Dhumal et al.119 for isolated ions and ionic pairs of [C2C1im]+ and [Tf2N]−, by Talaty et al.52 for ionic pairs of [CnC1im][PF6] and by Heimer et al.53 for [CnC1im][BF4], n = 2, 3, and 4, and by Katsyuba et al.54 for [C2C1im][BF4]. Once harmonic frequencies follow from these standard calculations, scaling factors are needed for agreement between calculated and experimental data. Grondin et al.58 calculated harmonic and also anharmonic frequencies of [C1C1im]+ and [C2C1im]+ using the second-order perturbative method proposed by Barone56,57 in order to avoid such scaling factors of harmonic frequencies. Long tables of vibrational frequencies calculated for 1-alkyl-3methylimidazolium cations are available in these papers. The composition of normal modes is naturally complicated because of the electronic structure of imidazolium cations. Furthermore, visual inspection of the eigenvectors on computer screen implies that description of the same normal coordinate may vary among different authors. Grondin et al.58 provided the distribution of potential energy allowing for more rigorous assignment of normal modes in terms of internal coordinates. In the following, we will distinguish three spectral ranges in vibrational spectra of 1-alkyl-3-methylimidazolium cations. The high-frequency range, 2800−3200 cm−1, exhibits a complex pattern of overlapped bands proper to several C−H stretching modes. The 800−1600 cm−1 range includes characteristic bands of imidazolium ring vibrations. The 400−800 cm−1 range, which also contains ring vibrations, is interesting because it provides insight on conformations of alkyl chains. (The lowfrequency range probing intermolecular dynamics is the issue of section 5.) Figure 18 shows IR and Raman spectra of [C4C1im]Br and [C6C1im]Br in the spectral range covering imidazolium ring vibrations. (There are also out-of-plane ring vibrations within 600−650 cm−1.) It is clear from this figure that vibrational spectra in this range are essentially the same irrespective of the length of the alkyl chain. Table 4 illustrates different ways that authors describe the nature of vibrations of the bands marked 1−5 in the Raman spectra of Figure 18. The stretching of C−H bonds of 1-alkyl-3-methylimidazolium cations can be separated into two groups of bands: 2800− 3000 cm−1, belonging to CH modes of alkyl chains attached to the imidazolium ring, and 3000−3200 cm−1, belonging to CH of the imidazolium ring. Figure 19 shows for [C4C1im][PF6] and [C4C1im][CF3SO3] such separation of two groups of

Figure 18. IR and Raman spectra in the 1000−1600 cm−1 range of [C4C1im]Br (green and blue) and [C6C1im]Br (red and black) in liquid phase at room temperature. Table 4 gives the assignment of Raman bands marked 1−5.

Figure 19. IR (red, transmittance scale at right) and Raman (black) spectra in the CH stretching range of [C4C1im][PF6] (full lines) and [C4C1im][CF3SO3] (dashed lines) at room temperature.

bands of CH stretching modes. The comparison made in Figure 19 illustrates the well-known finding that this spectral range is a signature of ionic interactions:54,119,175,176 the imidazolium ring CH stretching modes shift to lower wavenumbers for ionic liquids containing stronger interacting anions. Furthermore, the band at ∼3115 cm−1 in the IR spectrum of [C4C1im][CF3SO3] is much broader than [C4C1im][PF6], suggesting stronger hydrogen bonding in the former. Out-of-plane ring CH vibrations within 700−900 cm−1 are also sensitive to strength of anion−cation interactions, shifting to lower wavenumber with increasing anion basicity.58 Using these γ(CH) vibrations as probes of ionic interactions is better accomplished with IR than Raman spectroscopy because of higher IR activity. Figure 19 also illustrates stronger anion dependence for the ring CH than alkyl CH stretching modes, as expected from the preferred location of anions around the imidazolium polar head. Furthermore, the doubled-peaked feature observed within the 3100−3200 cm−1 range is commonly assigned to stretching of C(2)−H (the low frequency component) and C(4),(5)−H (the high frequency component). This interpretation is supported by quantum chemistry calculations of ionic pair54,119,53,175,177,48 putting the anion either in front of the C(2) or the C(4),(5) atoms of the imidazolium ring. These findings are considered as evidence in favor of anion−cation hydrogen bonding, the C(2)−H···A− arrangement being preferred over C(4),(5)−H···A−. Q


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The interpretation of this spectral range in terms of different strengths between C(2)−H···A− and C(4),(5)−H···A− hydrogen bonds has been disputed by Lassègues et al.59 These authors claim that the double-peaked feature above 3100 cm−1 is instead a Fermi resonance,42 resulting from mixing of vibrational states of fundamental CH ring vibrations with overtone and combination bands of ring modes whose fundamental transitions are observed in the 1550−1585 cm−1 range (see Figure 18). Lassègues et al.59 grounded their interpretation by isotopic substitution of each hydrogen atom bonded to the ring in order to clear out the spectral range of 3100−3200 cm−1, leaving the corresponding C−D stretching within the 2300−2400 cm−1 range. In accordance with their proposition, the band at ∼3170 cm−1 (see Figure 19) includes all C(2),(4),(5)−H vibrations, and the band at ∼3115 cm−1 is overtone and combination bands of ring modes enhanced by Fermi resonance. The alternative assignment put forward by Lassègues et al.59 roused a lively debate in the literature178,179 because it implies there is no need of anion−cation hydrogen bonding to explain the vibrational spectra nor C(2)−H being stronger interacting site than C(4),(5)−H. Wulf et al.178 strengthened the usual point of view with further quantum chemistry calculations showing that C(2)−H modes are always obtained at lower frequency than C(4),(5)−H modes in many different ion pairs. However, the cyanate-anions chosen by Wulf et al.178 are strongly interacting species, whereas Lassègues et al.179 provided a reminder that their interpretation concerned imidazolium ionic liquids containing less coordinating anions (e.g., [NTf2]−, [BF4]−, and [PF6]−). Nevertheless, there is consensus that strongly coordinated anions shift CH stretching modes to lower wavenumber,178,179,58 even though definitive assignment of this spectral range is an open issue in vibrational spectroscopy of imidazolium ionic liquids. Irrespective of the assignment of vibrational spectra, yet a more delicate issue is the nature of hydrogen bond between imidazolium cations and anions.180 Vibrational spectroscopy has been a powerful tool to reveal molecular conformations in ionic liquids. Studies in this direction followed after the discovery of crystal polymorphism in [C4C1im]Cl by Hayashi et al.181 and Holbrey et al.182 These authors concluded from X-ray diffraction that [C4C1im]Cl forms two crystalline phases differing in the conformation of the butyl chain. Hayashi et al.181 found that these polymorphs exhibit different patterns in the 500−800 cm−1 range of the Raman spectrum, where a group of bands (625, 730, an 790 cm−1) and (500, 600, an 700 cm−1) is characteristic of each crystal. All of these bands appear in the Raman spectrum of liquid phase of [C4C1im]Cl, so that one concludes there is a mixture of conformers in the ionic liquid.181,183,184 Hamaguchi and Ozawa185 reviewed their early Raman spectroscopy studies on conformational changes of [CnC1im]+ in ionic liquids with Cl−, Br−, I−, [BF4]−, and [PF6]−. In a previous review, Berg3 offered a detailed analysis on how alkyl chain conformation of [CnC1im]+ adds fingerprints in Raman spectra of ionic liquids. We also recommend a very detailed work recently published by Endo et al.186 concerning DFT calculations of conformational flexibility and vibrational frequencies of typical ionic liquid forming cations (imidazolium, pyridinium, pyrrolidinium, and piperidinium). It should be noted that this issue has been addressed mainly by Raman spectroscopy, since features indicating the [CnC1im]+ conformation are not so evident in the IR spectrum.99 Figure 20 illustrates the frequency range exhibiting bands that characterize anti−anti (AA, 620 and 735

Figure 20. Raman spectra of [C4C1im]Br (blue) and [C6C1im]Br (black) in the liquid phase at room temperature. For comparison purpose, Raman spectrum of [C2C1im]Cl (red) at T = 360 K is shown. Bands marked * (620 and 735 cm−1 in [C4C1im]Br) characterize the AA conformer; bands marked # (600 and 697 cm−1 in [C4C1im]Br) characterize the GA conformer. Optimized structures of AA and GA conformers are shown.

cm−1) and gauche−anti (GA, 600 and 697 cm−1) conformations of [C4C1im]+. Quantum chemistry calculations show that these bands belong to ring deformations coupled to CH2 rocking motions3,185 so that the actual vibrational frequency is sensitive to the butyl chain conformation. As emphasized by Berg,3,47 Figure 20 also shows a mixture of gauche and anti conformers in the longer alkyl chain [C6C1im]+ cation. Kiefer and Pye155 considered other bands as signatures of three conformers of [C6C1im]+ in [C6C1im][HSO4] (e.g., bending modes in 340−510 cm−1 and C−C stretching modes in 900− 1100 cm−1 ranges). Nine conformers of [C4C1im]+ have been the relative energies calculated by quantum chemistry methods.187,188 In a re-examination of the Raman spectrum of [C4C1im][BF4], Holomb et al.99 were able to discriminate four conformers (GG, GA, TA, and AA) coexisting in the liquid phase by considering additional bands covering wider spectral range (200−1200 cm−1). Further studies by Umebayashi et al.,188 Katayanagi et al.,189 and Singh et al.190 focused on the anion dependence of relative intensities of the 600 and 620 cm−1 bands. The ratio of intensities I600/I620 increases in the sequence [C4C1im]I, [C4C1im]Br, and [C4C1im]Cl, so that one concludes that gauche conformation is stabilized for stronger interacting halide anions. Umebayashi et al.49 and Lassègues et al.125 showed that [C2C1im]+ conformers with the planar or nonplanar ethyl chain can be distinguished by characteristic bands in the spectral range of 200−500 cm−1 of the Raman spectrum. Figure 21 shows this spectral range of the Raman spectrum of [C2C1im] Cl with the bands proposed by these authors as signatures of the [C2C1im]+ conformation. Endo and Nishikawa191 found two isomers (symmetric and asymmetric) of 1-isopropyl-3methylimidazolium cation in the ionic liquids [i-C3C1im]Br and [i-C3C1im]I. Raman bands considered as signatures of the symmetric conformation of [i-C3C1im]+ are found at ∼315 and R


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Figure 23. Atomic displacement of the two normal modes of [C4C1C1im]+ calculated at the DFT/B3LYP level of theory for the isolated cation. The harmonic vibrational frequencies calculated at 1533 and 1560 cm−1 correspond to experimental bands at 1515 and 1540 cm−1, respectively, of [C4C1C1im][NTf2] (bands marked by arrows in Figure 22).

Figure 21. Raman spectrum of [C2C1im]Cl at T = 360 K. Bands marked * belong to the planar conformer, whereas bands marked # belong to the nonplanar conformer.

level of theory for two normal modes given new spectral features in IR (1540 cm−1) and Raman (1515 cm−1) spectra of [C4C1C1im][NTf2]. The small mass of hydrogen implies large displacements of hydrogen atoms in the eigenvectors displayed in Figure 23. However, in terms of potential energy distribution calculated with the VEDA program,50 the [C4C1C1im]+ IR band at 1540 cm−1 involves mainly stretching of N(1)−C(2) and N(3)−C(2) bonds (29%) and bending of the methyl group and the C(5)−N(3)−C(2) angle (23%). The Raman band at 1515 cm−1 involves stretching of C(2)−CH3, C(4)−C(5), and N(1)− C(2) bonds (54%) and bending of the N(1)−C(2)−N(3) angle (13%). Hunt et al.197 showed that the electronic structure of imidazolium cations is better represented by the double bond C(4)C(5) and delocalization in the N(1)−C(2)−N(3) bonds of the ring.197 Therefore, the 1515 cm−1 Raman band of [C4C1C1im]+ is very intense because of vibrations of N(1)− C(2)−N(3) and C(2)−CH3 moieties with large polarizability fluctuation. Endo et al.198 discussed the effect of methylation of the C(2) position on the spectral ranges of C−H stretching modes and 550−800 cm−1. Raman frequencies of C(4),(5)−H stretching modes change when [C4C1im]+ is replaced by [C4C1C1im]+, shifting to lower wavenumber when the counterion is Cl−, Br−, or I−, but shifting to a higher wavenumber when the counterion is [BF4]− or [PF6]−. The relative intensities of bands that characterize AA and GA conformers (see previous Figure 20) change in going from [C4C1im]+ to [C4C1C1im]+. These findings were considered as the consequence of the local arrangement of anions around [C4C1C1im]+ being different from [C4C1im]+.198 Fewer works have been dedicated to detailed assignment of vibrational frequencies of protic imidazolium ionic liquids (i.e., monoalkylimidazolium with a free N−H bond). Moschovi et al.118 discussed IR and Raman spectra as a function of temperature of [C1im][NTf2], which melts at 325 K, where [C1im]+ is the 1-H-3-methylimidazolium cation. Moschovi et al.118 addressed the issue of vibrational signatures of [NTf2]− conformers and the effect of the hydrogen bond on the N−H and C−H vibrations of the imidazolium ring. This work provides full tables of observed IR and Raman frequencies and vibrational assignments for [C1im][NTf2]. An expected difference in comparison with 1,3-dialkylimidazolium cations is the occurrence of the N−H stretching mode of [C1im]+ at higher frequency than the C−H stretching modes. The ν(N−H) mode is observed at 3287 cm−1 in the Raman spectrum of solid [C1im][NTf2], becoming a very weak band at 3281 cm−1 upon melting. The ν(N−H) mode gives intense IR bands both in solid and liquid phases of [C1im][NTf2], 3271 and 3273 cm−1,

688 cm−1 and for the asymmetric conformation at ∼340 and 699 cm−1.191 The spectral pattern of Raman spectra of ionic liquids may become very different after phase transition when some bands characterizing mixture of conformers in the liquid phase are absent in the crystalline phase. Section 6 discusses the usage of vibrational spectroscopy for studying phase transition of ionic liquids. Methylation at the C(2) position of imidazolium cations, leading to [CnC1C1im]+, blocks the most important site for C− H···A− hydrogen bonding. Despite eliminating this site for hydrogen bonding, for a given anion the ionic liquid based on [CnC1C1im]+ is more viscous than the [CnC1im]+ counterpart. The reason for this effect is not fully understood, and different explanations have been proposed.192−195 Concerning vibrational spectroscopy, the most significant difference between [CnC1im]+ and [CnC1C1im]+ is seen in the spectral range of ring vibrations. Noack et al.196 discussed IR and Raman spectra of ionic liquids based on [CnC1im]+ and [CnC1C1im]+, n = 2 and 4, with the [NTf2]− anion. Figure 22 compares vibrational spectra of [C4C1im][NTf2] and [C4C1C1im][NTf2]. Taking the [NTf2]− band marked with asterisk in Figure 22 as the intensity pattern, it is remarkable how strong the [C4C1C1im]+ Raman band is at 1515 cm−1. Figure 23 shows atomic displacements calculated in this work at the DFT/B3LYP

Figure 22. IR and Raman spectra of [C4C1C1im][NTf2] (green and blue lines) and [C4C1im][NTf2] (red and black lines) at room temperature. The band marked with asterisk is assigned to an anion normal mode. Bands marked with arrows (Raman, 1515 cm−1; IR, 1540 cm−1) belonging to [C4C1C1im]+ have the atomic displacements of the corresponding normal modes shown in Figure 23. S


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respectively, although broader in the liquid.118 Moschovi et al.199 went further in reporting IR and Raman spectra for a series of protic ionic liquids [Cnim][NTf2], n = 0−12. Insights on conformation of the alkyl chain of [Cnim]+ cations have been obtained from the Raman bands in the 820−900 cm−1 range, corresponding to deformation motions of the CH3 group at the end of the chain.199 It has been found that vibrational frequencies of N−H and C−H stretching modes in protic [C nim][NTf 2 ], 199 and also C−H modes in nonprotic [CnC1im][NTf2],200 decrease as n increases, but these findings have not been considered as signatures of stronger anion− cation hydrogen bonding. These authors considered instead the role played by an intramolecular effect, that is, the positive charge on the imidazolium ring is diminished by the electron donor inductive effect of longer alkyl chain, so that the strength of anion−cation interaction does not increase even though polar/nonpolar segregation becomes better defined in ionic liquids with the imidazolium cation of the long chain. In a subsequent work, Moschovi et al.201 provided very detailed analyses and comparisons of vibrational spectra for nonprotic, [CnC1im][NTf2], and protic, [Cnim][NTf2], ionic liquids. Inserting a functional group in the side chain of 1-alkyl-3methylimidazolium cations might result in ionic liquids with improved properties for some specific tasks (e.g., gas absorption). Despite vibrational spectroscopy being a powerful tool to characterize molecular interactions in these solutions (see section 7), detailed spectroscopic analyses and assignments of vibrations for these imidazolium derivatives are more sparse than the usual 1-alkyl-3-methylimidazolium cations. Once the vibrational frequencies of [CnC1im]+ and anions are wellestablished, the most important band of the added functional group is easily identified. Figure 24 shows molecular structures of some derivatives whose characteristic vibrations we show in Figure 25.

Figure 25. Characteristics IR (red, transmittance scale at right) and Raman (black) bands of (A) 1-allyl-3-methylimidazolium dicyanamide, (B) 1-benzyl-3-methylimidazolium dicyanamide, (C) 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and (D) 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate at room temperature.

in [C1C1im][NTf2]. However, these works focused on the lowfrequency range obtained by femtosecond Raman-induced Kerr effect spectroscopy. Shirota et al.204 provided the observed vibrational frequencies for bands below 700 cm−1, and they distinguished the vibrations belonging to anions or cations. We show in Figure 25B part of the vibrational spectra, including the most characteristic new feature of 1-benzyl-3-methylimidazolium dicyanamide. The sharp band observed at 1003 cm−1 in the Raman spectrum of the ionic liquid is the counterpart to the totally symmetric breathing mode of benzene. There is still no detailed analysis of vibrational spectra of an ionic liquid based on CN-functionalized imidazolium cation. Figure 25C shows the characteristic band of the CN stretching vibration in 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [NC-C3C1im][NTf2], at room temperature. The vibrational frequency of the ν(CN) mode at 2251 cm−1 in [NC-C3C1im]+ lies at a significantly higher wavenumber than cyanate-anions (see Figure 10 and Table 1). The ν(CN) frequency in [NC-C3C1im]+ is indeed close to the value in the molecular liquid acetonitrile (2253 cm−1). Knorr et al.206,207 discussed temperature effect on the IR band corresponding to O−H stretching mode of 1-(2hydroxyethyl)-3-methylimidazolium tetrafluoroborate, [HOC2C1im][BF4]. The ν(OH) spectral range is shown in Figure 25D for [HO-C2C1im][BF4] at room temperature, where the strong feature at ∼3552 cm−1 in the IR spectrum belongs to the O−H stretching motion in O−H···F hydrogen bonded to [BF4]−. Knorr et al.206,207 found that the low-frequency tail in the IR band grows in intensity with decreasing temperature, becoming a well-resolved band at ∼3402 cm−1 at 233 K. With the support of quantum chemistry calculations of clusters of ions, this low-frequency component has been assigned to the O−H vibration engaged in O−H···O hydrogen bond with the neighbor cation.206,207 Katsyuba et al.208 observed a weak band at ∼3425 cm−1 in the IR spectrum of [HO-C2C1im][PF6], and they also assigned it to ν(OH) in hydrogen bonding with −OH groups of neighbor cations. Hydrogen bonding between like-

Figure 24. Molecular structures of some functionalized imidazolium cations.

Xuan et al.202 studied 1-allyl-3-methylimidazolium dicyanamide and 1-allyl-3-methylimidazolium chloride with assignments based on potential energy distributions of normal modes calculated for ion pairs at the DFT/B3LYP level of theory. Figure 25A shows IR and Raman spectra of 1-allyl-3methylimidazolium dicyanamide in the spectral range where the CC stretching mode is observed at 1647 cm−1 in the Raman spectrum. Xu et al.203 assigned a few IR bands of 1-allyl3-methylimidazolium bicarbonate in agreement with the more complete assignment of ref 202. The quantum chemistry calculations of Xuan et al.202 indicated three stable conformers of the 1-allyl-3-methylimidazolium cation, but no spectroscopic signatures have been proposed for these conformers. Shirota et al.204 discussed the vibrational spectra of some ionic liquids containing benzyl-substituted cations, and Xue et al.205 compared the spectra of 1-benzyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and a solution of benzene T


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It has not been addressed in these works on Nalkylpyridinium cations whether vibrational spectroscopy could be used to identify different conformation of the alkyl chain. Thus, we calculated harmonic vibrational frequencies at the DFT/B3LYP level of theory for gauche and anti conformers of an isolated [Py1,4]+ cation (see molecular structures in Figure 27). The calculations suggest there is indeed a doublet of

charge species has been also suggested for the choline cation from the analysis of the O−H stretching region of IR spectra of [Cho][NTf2].209 Knorr et al.206,207,209 pointed out that these are the first spectroscopic evidence of what Weinhold and Klein210 called an antielectrostatic hydrogen bond (i.e., a hydrogen bond between ions of the same charge). It is worth remembering, however, that signature of anion−anion hydrogen bond between [HSO4]−, previously found in IR and Raman spectra of crystals of the halide salts157−160 has also been found in Raman spectra of [C2C1im][HSO4] and [C4C1im][HSO4] as discussed in the previous section.156 4.2.2. Pyridinium. Gale et al. discussed Raman211 and IR212 spectra of N-butylpyridinium in the early period of room temperature molten salts based on mixtures of an organic cation chloride and AlCl3. Tait and Osteryoung213 found that the main difference in IR spectra between basic and acidic AlCl3/N-butylpyridinium chloride mixtures (acidic melts have AlCl3:organic salt mole ratio greater than 1) occurs in the C−H stretching region, in particular enhancement in the intensity of bands of aliphatic relative to aromatic C−H stretching modes, suggesting loss in ring aromaticity with decreasing acidity. Tables of experimental frequencies of cation vibrations are available;213 however, the most important issue of a vibrational spectroscopy study of those systems was identifying complex species AlCl4− and Al2Cl7− rather than a detailed analysis of cation normal modes.211,212 Figure 26 shows the fingerprint regions of IR and Raman spectra of 1-butyl-4-methylpyridinium bromide, [Py1,4][Br],

Figure 27. Raman spectra of liquid [Py1,4][BF4] (black line) and solid [Py1,4]Br (blue line) at room temperature in the region including bands assigned to different [Py 1,4 ] + conformers. Vibrational frequencies and relative intensities calculated for [Py1,4]+ are indicated by green (886 cm−1, gauche) and red (917 cm−1, anti) lines. Atomic displacement vectors calculated for these normal modes are shown for each conformer.

Raman bands (887 and 909 cm−1 in liquid [Py1,4][BF4]; 892 and 913 cm−1 in solid [Py1,4]Br) that can be used to discriminate each conformer. These bands correspond to Raman active modes with frequencies calculated at 886 and 917 cm−1, respectively, for gauche and anti [Py1,4]+ conformers. Figure 27 highlights this spectral range of the Raman spectra of [Py1,4][BF4] and [Py1,4]Br together with peak position and relative Raman intensities obtained from the DFT calculation. This figure indicates there is a mixture of gauche and anti [Py1,4]+ conformers in both solid [Py1,4]Br and liquid [Py1,4][BF4]. Recently, Endo et al.186 provided a detailed theoretical analysis by DFT calculations on the conformational flexibility and the corresponding vibrational frequencies of the most common organic cations forming ionic liquids. In the case of pyridinium derivatives, the calculations suggest that the 480−510 and 740−820 cm−1 regions of the Raman spectrum could also be used to identify different conformers. 4.2.3. Pyrrolidinium. The relationship between vibrational frequency and molecular conformation has been studied in more detail for 1,1-dialkylpyrrolidinium than pyridinium cations. Castriota et al.114 reported Raman spectra in the 250−1700 cm−1 range of ionic liquids based on 1-methyl-1propylpyrrolidinium, [Pyr1,3]+, with the anions [NTf2]− and I−. However, assignment of [Pyr1,3]+ vibrations was not given because the issue was coordination of Li+ by [NTf2]− in a 2:1 mixture of [Pyr1,3][NTf2] with Li[NTf2].114 The analogous effect of a strong polarizing cation on the [NTf2]− vibrations

Figure 26. IR (red) and Raman (black) spectra of 1-butyl-4methylpyridinium tetrafluoroborate, [Py1,4][BF4], at room temperature. Bands marked with asterisk belong to anion vibrations. For comparison purposes, IR and Raman spectra of solid [Py1,4]Br are shown by green and blue lines, respectively.

and 1-butyl-4-methylpyridinium tetrafluoroborate, [Py1,4][BF4], where the few [BF4]− bands are marked with asterisks. The vibrational modes of N-ethylpyridinium have been discussed by Zhao et al.214 for [BF4]− and [CF3COO]− salts, and by Zheng et al.215 for the Br− salt. Sashina et al.216 discussed IR and Raman spectra of halides salts of N-alkylpyridinium for the series from two to ten carbon atoms in the alkyl chain. These authors paid attention to the high-frequency range of 2800− 3200 cm−1 of C−H stretching modes. This spectral range for N-alkylpyridinium cations can be separated in stretching vibrations of C−H bonds of the alkyl chain (2800−3000 cm−1) and the ring (above 3000 cm−1). Frequency shift of C− H stretching of N-alkylpyridinium to a lower wavenumber can also be related to stronger anion−cation interaction.216 U


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components at 892, 905, and 930 cm−1 decreases with increasing temperature, while the intensity of the 883 cm−1 band remains the same, also indicating a mixture of eq and ax [Pyr1,4]+ conformers in [Pyr1,4][NTf2].218 Umebayashi et al.127 also concluded for mixtures of eq and ax conformers in [Pyr1,3][NTf2] and [Pyr1,4][NTf2] on the basis of the DFT/ B3LYP calculations and the temperature dependence of Raman spectra in the range shown in Figure 28. Fujii et al.130 paid attention to the 250−400 cm−1 range of the Raman spectrum of [Pyr1,3][N(SO2F)2], in which cation bands at 350 and 380 cm−1 again indicate a mixture of [Pyr1,3]+ conformers. There is an interesting contrast between these studies on pyrrolidinium218,127 and the above discussion of imidazolium ionic liquids: there is mixture of anti and gauche conformation of the butyl chain in [C4C1im]+ ionic liquids, but anti conformation of [Pyr1,4]+ predominates with negligible population of the gauche conformer. However, when [Pyr1,4]+ is functionalized upon insertion of an ether group function in the long alkyl chain then IR spectroscopy with support of DFT calculations indicate that the side chain acquires gauche geometry.219 A distinctive aspect of works by Vitucci et al.121 and Mao et al.117 is that these authors considered the IR spectrum, rather than the Raman spectrum, of [Pyr1,4][NTf2]. In order to assign the IR bands by DFT calculations, Vitucci et al.121 considered the isolated ion, whereas Mao et al.117 considered an ionic pair. Tables containing assignment of [Pyr1,4]+ frequencies are available.117,121 In particular, Figure 6 of ref 121 provides visualization of atomic displacements for the normal modes corresponding to the bands shown in Figure 28. 4.2.4. Piperidinium. Derivatives of piperidinium encompass an important class of ionic liquid forming cations, but assignment of vibrational spectra has been much less discussed for piperidinium than imidazolium or pyrrolidinium cations. Shukla et al.120 compared calculated and experimental IR and Raman spectra of N-butyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide, [Pip1,4][NTf2] and [Pip1,4]Br, the latter being a solid with a relatively high melting point (241 °C). Figure 29 shows IR and Raman spectra of [Pip1,4][NTf2]

has been discussed by Rocher et al.217 in [Pyr1,4][NTf2]/AlCl3 mixtures as a function of Al3+ concentration. The clear signature provided by Raman spectroscopy about coordination of [NTf2]− to Li+ will be discussed in section 7. The vibrational motions of [Pyr1,3]+ and [Pyr1,4]+ have been discussed in several works.122,3,127,130,217,218 Besides gauche or anti conformation of the butyl chain, additional issues concerning [Pyr1,4]+ include equatorial or axial position (eq or ax) of the butyl chain relative to ring carbon atoms, and envelope or twist conformation of the nonplanar ring. By comparing experimental Raman spectrum and DFT/B3LYP calculation within the whole 200−1600 cm−1 range, Fujimori et al.218 concluded that in [Pyr1,4][NTf2] the butyl chain and the ring are at all anti and envelope conformations, respectively, but there is mixture of eq and ax [Pyr1,4]+ conformers. It is difficult to characterize the conformers when many experimental and calculated frequencies have to be compared, instead of a small frequency range including a few characteristics bands. Comparison with related systems with a single conformer in crystalline phase (e.g., [Pyr 1,4 ]Br) helps identifying appropriate bands as the fingerprint of a given conformer.218 Bands observed at 486, 586, and 655 cm−1 in the Raman spectrum of solid [Pyr1,4]Br closely match the frequencies calculated for [Pyr1,4]+ in ax−anti-envelope conformation. Figure 28 shows IR and Raman spectra of

Figure 28. IR (red, transmittance scale at right) and Raman (black) spectra of [Pyr1,4][NTf2] at room temperature in the region used to characterize different conformers of [Pyr1,4]+. The asterisk marks the 883 cm−1 Raman band characteristic of the ax conformer. Optimized structures are shown for an isolated [Pyr1,4]+ cation with the butylchain in the equatorial (eq) or axial (ax) position in relation to the ring. In both the structures, the butyl chain is in anti and the ring in envelope conformations.

Figure 29. IR and Raman spectra of liquid [Pip1,4][NTf2] (red and black) and solid [Pip1,4]Br (green and blue) at room temperature.

[Pyr1,4][NTf2] in the range of 860−950 cm−1 used by Fujimori et al.218 in order to reveal coexisting eq and ax [Pyr1,4]+ conformers in the liquid phase. Although there is overlap of bands in this region, quantum chemistry calculations indicate that the 883 cm−1 Raman band is a signature of the ax conformer, whereas the 890−930 cm−1 range includes bands of both eq and ax conformers. Furthermore, the intensities of

and [Pip1,4]Br. As the [NTf2]− bands dominate the spectra of [Pip1,4][NTf2], we provide spectra for solid [Pip1,4]Br in order to highlight the [Pip1,4]+ bands. Conformations of piperidinium, like pyrrolidinium cations, include ring flexibility (chair, twist, or boat), eq or ax position of the butyl chain, and rotational isomers of the butyl chain. The DFT/B3LYP calculations of Shukla et al.120 indicated that the more stable V


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Figure 30. IR (red) and Raman (black) spectra of [C4C1C1C1N][NTf2] at room temperature in different regions covering cation bands.

ref 121. In a subsequent work, Vitucci et al.122 discussed spectral changes after crystallization of [C1C1C1C3N][NTf2] and [C1C1C1C6N][NTf2] at ∼250 K. The possibility of revealing alkyl chain conformation of tetraalkylammonium cations by combined usage of IR spectroscopy and quantum chemistry calculations is a more challenging task in comparison with the above discussion on 1-alkyl-3-methylimidazolium cations. Palumbo et al.223 used the DFT/B3LYP level of theory to calculate vibrational frequencies for six different conformers of [C1C1C1C3N]+. An appropriate spectral window for comparison between calculation and experiment is the 900−1070 cm−1 range of the IR spectrum of [C1C1C1C3N][NTf2]. The calculations indicated that the IR feature at 1000 cm−1 (a vibration mixing C−C, C−N stretching, and CH2 wagging) is a characteristic band of the lowest energy [C1C1C1C3N]+ conformer.223 Occurrence of several bands in this spectral range indicates a mixture of conformers in the liquid phase, whereas only the lowest-energy conformer is retained in the crystalline phase accompanied by concomitant change in the spectral pattern.223 The application of vibrational spectroscopy to investigate phase transitions of ionic liquids will be reviewed in section 6. Domańska and Bogel-L̷ ukasik,224 in a study focusing on thermodynamic properties of ammonium salts, provided the IR frequencies of solid phases of tetraalkylammonium bromide salts in which one of the alkyl chains has been functionalized with a terminal −OH group. Aparicio et al.225 used the highfrequency range of the IR spectrum, 3100−3600 cm−1, in order to monitor water uptake from the atmosphere by ionic liquids based on 2-hydroxyethyltrimethylammonium and tris(2hydroxyethyl)methylammonium cations, but detailed assignment of cation vibrations was not an issue. A discussion on the nature of vibrations was given by Arkas et al.226 for N,N-di(2hydroxyethyl)-N-methyl-N-alkylammonium bromides (alkyl chain from dodecyl to octadecyl), however, within the context of liquid-crystal phase transition. Concerning −OH-functionalized ammonium cations, more detailed spectroscopy studies have been reported for choline (i.e., hydroxyethyl-trimethylammonium, [Cho]+). In the case of nonprotic tetraalkylammonium ionic liquids, an indication of weak interaction comes from finding that simple juxtaposition of vibrational frequencies calculated for isolated anion and cation give reasonable agreement to experimental spectra.121 On the other hand, this is not the expectation for systems with the possibility of strong hydrogen bonds such as choline cation with carboxylate170 or amino acid anions.227 Vibrational frequencies of the choline cation have been discussed by Tanzi et al.170 in ionic liquids with formate, propanoate, or butanoate. By comparing quantum chemistry calculations for the isolated choline and the choline−anion pair, these authors found the expected hydrogen bond induced frequency shift:

conformation is chair for the piperidinium ring and gauche for the butyl chain. The characteristic Raman bands of cisoid and transoid conformations of [NTf2]− were discussed by Shukla et al.,120 but they did not identify vibrations that could be used as signatures of cation conformation. Shimizu et al.132 identified characteristic bands of [Pip1,4]+ conformers in the range of 700−800 cm−1 of the Raman spectrum of [Pip1,4][N(SO2F)2]. With the support of DFT/B3LYP calculations for the chair form of [Pip1,4]+, Shimizu et al.132 proposed that eq and ax conformations of [Pip1,4]+ can be identified by the Raman bands at 704 and 710 cm−1, respectively, in the crystalline phases of [Pip1,4][N(SO2F)2]. In the liquid phase, the Raman spectrum indicates a mixture of eq and ax conformers of [Pip1,4]+. 4.2.5. Ammonium. A large variety of ionic liquids can be prepared from mono-, bi-, tri-, and tetraalkylammonium cations, [C i NH 3 ] + , [C i C j NH 2 ] + , [C i C j C k NH] + , and [CiCjCkClN]+, respectively, with lengths of alkyl chains indicated by the number of carbon atoms i, j, k, and l. Vibrational frequencies of the prototype [NH4]+ of T d symmetry in halide salts were already reviewed by Herzberg,42 3033 (A1), 1685 (E), 3134 (F2), and 1397 (F2) cm−1, the actual values being dependent on the counterion and temperature.143,220,221 On the other hand, in the context of ionic liquid forming cations, carbon chains of different lengths result in asymmetric species, and the alkyl substituents give conformational flexibility. Those cations with one or more hydrogen atoms directly bounded to the nitrogen are called protic ionic liquids with properties dependent on relatively strong anion−cation hydrogen bonds,222 whereas those based on tetraalkylammonium cations are nonprotic ionic liquids. Let us focus first on tetraalkylammonium ionic liquids, for which detailed assignment of cation vibrations became available recently.121,122,223 Vitucci et al.121 discussed IR spectra of [C1C1C1C3N][NTf2] and [C1C1C1C6N][NTf2] with the help of quantum chemistry calculation at the DFT/B3LYP level of theory. The IR spectra exhibit of course the high-frequency bands of C−H stretching modes at 2800−3200 cm−1, but the fingerprint region is mainly dominated by the [NTf2]− bands. In accordance with Figure 11, spectral windows appropriate to characterize the cations without [NTf2]− bands include the ranges of 800−1050 cm−1 and 1350−1550 cm−1. The work of Vitucci et al.121 concerned the IR spectrum, so that we show in Figure 30 for completeness these spectral ranges for both IR and Raman spectra for a closely related system, [C4C1C1C1N][NTf2]. The range of 1350−1550 cm−1 corresponds to C−H bending modes, and the range of 800−1050 cm−1 corresponds to normal modes with displacements of many atoms of the cation. The complex patterns of atomic displacements calculated for the normal modes of [C1C1C1C3N]+ and [C1C1C1C6N]+ can be found in W


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ν(OH) shifts to a lower wavenumber, whereas HOC bending and CO stretching and torsion modes shift to a higher wavenumber.170 The more powerful tool to calculate vibrational spectra of these strongly hydrogen-bonded ionic liquids is ab initio MD simulation, since the method takes into account many different ionic arrangements resulting from the liquid dynamics.170,227 Tanzi et al.170 showed that Raman spectra of choline carboxilates exhibit a large variability of O−H stretching frequencies within 3200−3400 cm−1, whereas the less perturbed C−H stretching modes of choline cover narrower range centered at ∼2985 cm−1. This spectral separation between O−H and C−H stretching modes is clear in IR spectra reported by Harmon et al.228 for the simple salts choline bromide and choline iodide. The O−H stretching band in the IR spectrum of [Cho][NTf2] exhibits a tail with components at 3431 and 3474 cm−1 which have been assigned as the signature of hydrogen bonding between neighboring choline cations.209 On the other hand, the IR spectrum reported by Campetella et al.227 for choline with the amino acid anion alanine, where there is also overlap with the alanine N−H stretching modes, shows a broad band covering the whole 2500−3500 cm−1 range. The environmental-induced modifications of IR spectra have also been discussed by Perkins et al.229 for the eutectic mixture of choline chloride and urea (1:2 molar ratio). The ν(OH) IR band in pure choline chloride is a relatively sharp band at 3256 cm−1, which overlaps the N−H stretching modes of urea upon formation of the mixture. It is difficult to correlate vibrational frequencies in the spectral range of 700−1700 cm−1 for simple choline halides228 with the frequencies observed in ionic liquids with strongly interacting carboxilate and amino acid anions. For instance, the IR band at 1088 cm−1 for choline carboxylate,170 which agrees with the value for choline halides,228 was assigned to the choline CO stretching mode in ref 170, but the corresponding band at 1091 cm−1 in choline alanine was assigned to CH rock and twisting.227 Thus, it would be interesting to mark the choline vibrational frequencies in IR and Raman spectra of an ionic liquid with a less interacting anion. Once the [NTf2]− frequencies were already discussed in the previous section, then we show IR and Raman spectra of [Cho][NTf2] in Figure 31 with the choline bands indicated by arrows. Harmon et al.228 assigned the choline normal modes within 700−1200 cm−1, assuming a C3v point group for the (CH3)3N−CH2− moiety. Some of the IR frequencies reported for choline halides228 have

a correspondent feature in the spectra shown in Figure 31 for [Cho][NTf2]. The first example of the series of protic alkylammonium ionic liquids is monomethylammonium, [C1NH3]+. Raman spectra and X-ray powder diffraction as a function of temperature have been reported by Bodo et al.230 for [C1NH3][NO3], which melts at 381 K. Calculation of vibrational spectra have been done by first using classical MD simulation of clusters of ion pairs, ([C1NH3][NO3])n, n up to 8, in order to generate reasonable configurations, which were then optimized by quantum chemistry calculation at the DFT/B3LYP level of theory. This approach aims for a better representation of the hydrogen bond network and the condensed phase spectrum. The high-frequency range of the Raman spectrum of liquid [C1NH3][NO3] exhibits relatively sharp bands of C−H stretching modes in the range of 2800−3050 cm−1 and N−H stretching modes seen as broad bands at 3100−3300 cm−1 because of a distribution of cation−anion hydrogen bonds of different strengths.230 The intense [NO3]− Raman bands dominate the spectrum in the 400−1700 cm−1 range, where few bands (C−N stretching and CH2 bending) have been assigned to [C1NH3]+.230 This approach of comparing experimental spectra with DFT calculations of clusters of ions was continued by Bodo et al.231 for the sequence [C2NH3][NO3], [C3NH3][NO3], and [C4NH3][NO3]. Figure 32 shows IR and Raman spectra of

Figure 32. IR (red, transmittance scale at right) and Raman (black) spectra of [C3NH3][NO3] at room temperature. Bands indicated by blue arrows at 828 and 868 cm−1 characterize gauche and anti conformers. Optimized structures of propylammonium in gauche and anti conformations are shown.

[C3NH3][NO3]. Several groups of Raman bands were assigned to [CnNH3]+ vibrations:231 Cn−N bending (416−450 cm−1), Cn−N symmetric stretching (870−890 cm−1), CH2 and CH3 wagging (∼990 cm−1), asymmetric C−C and C−N stretching (∼950 cm−1), CHn rocking (1175−1195 cm−1), CHn scissoring (∼1300 cm−1), and CH2 and CH3 bending (1450−1460 cm−1). A specific comment concerns the band located at 870−890 cm−1, which Henderson et al.232 assigned instead to C−C stretching and found at 875 cm−1 in the Raman spectrum of [C2NH3][NO3]. This assignment is in line with a detailed

Figure 31. IR (red, transmittance scale at right) and Raman (black) spectra of [Cho][NTf2] at room temperature. Some bands assigned to choline are indicated by arrows. X


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analysis performed by Hagemann and Bill233 of the Raman spectra of the simpler salts [C2NH3]Cl and [C2NH3]Br. These authors provided a full list of Raman frequencies and assignments for ethylammonium halides, including the complicated pattern of bands intensified by Fermi resonance in the high-frequency range of C−H and N−H stretching modes. It is worth mentioning that the ν(CC) mode at 1047 cm−1 in [C3NH3]Cl is an useful probe of local structure along the phase transition,232,234 but it is hidden by the intense anion band νs(NO3) in [CnNH3][NO3]. Nevertheless, there is nice agreement between experiment and theory,231 in particular the finding that cation frequencies within 400−1700 cm−1 depend on the alkyl chain length of [CnNH3]+. Faria et al.235 calculated vibrational frequencies for different conformers of [C3NH3]+ at the DFT/B3LYP level of theory. Temperature and pressure induced crystallization of [C3NH3][NO3] helped Faria et al.235 to propose that Raman bands at 828 and 868 cm−1 characterize, respectively, gauche and anti conformers of [C3NH3]+ (i.e., there is a mixture of conformers in the liquid phase of [C3NH3][NO3]) (see Figure 32). The high-frequency spectral range, which is separated into bands belonging to C−H and N−H stretching modes, is barely affected by the alkyl chain length because hydrogen bonds remain essentially the same between the polar head of [CnNH3]+ and the nitrate anion.231 Bodo et al.236 have also compared the experimental Raman spectrum of liquid [C2NH3][NO3] with the power spectrum obtained from the Fourier transform of the time correlation function of atomic velocities calculated by ab initio MD simulation. IR frequencies have been reported by Luo et al.237 for derivatives containing longer alkyl chains (trioctylammonium) and the more complex triphenylammonium (and also the trialkylphosphonium counterpart) triflate ionic liquids. 4.2.6. Other Cations. In Berg’s review on Raman spectroscopy of ionic liquids,3 he provided preliminary results for a system based on the tetramethylguanidinium cation, [TMGH]+. More detailed analysis of vibrations of [TMGH]+ have been performed by Berg et al.238 and Horikawa et al.239 for [TMGH][NTf2], and by Berg et al. for [TMGH]Cl240 and [TMGH]Br.241 These works provide lists of vibrational frequencies experimentally observed and calculated by quantum chemistry methods. Carper et al.242 reported IR and Raman spectra of trimethylsulfonium dicyanamide, [C1C1C1S][N(CN)2]. The authors provided vibrational assignment based on quantum chemistry calculations for an ionic pair and a dimer ([C1C1C1S][N(CN)2])2.242 We show in Figure 33 the fingerprint range of IR and Raman spectra of other alkylsulfonium ionic liquid, [C2C2C2S][NTf2], with some cation bands indicated by arrows.

Figure 33. IR (red, transmittance scale at right) and Raman (black) spectra of [C2C2C2S][NTf2] at room temperature. Some bands assigned to [C2C2C2S]+ are indicated by arrows.

vibrational contribution, the knowledge of vibrational frequencies allows for the calculation of thermodynamic functions according to fundamental expressions of statistical mechanics (e.g., for the heat capacity of harmonic oscillators):74,243 C V,vib Nk

3N − 6

=

∑ j=1

⎛ hvj ⎞2 e hvj / kT ⎜ ⎟ hv / kT ⎝ kT ⎠ (e j − 1)2

(5)

where k is the Boltzmann constant, h is the Planck constant, T is temperature, N is the number of atoms, and the summation extends to the 3N − 6 normal modes (3N − 5 for linear molecules) each one with vibrational frequency νj. Paulechka et al.244 calculated heat capacity, energy, and entropy for the ideal gas phase of [C4C1im][PF6]. Blokhin et al.245 calculated these thermodynamic properties for [C4C1im][NTf2], and Paulechka et al.244 extended the calculations for the series [CnC1im][NTf2], n = 2, 4, 6, and 8. Following a combined usage of experimental IR frequencies and calculated by quantum chemistry methods, these works give full lists of vibrational frequencies νj that have actually been considered in the thermodynamic calculations. Some low frequencies are not included in the summation because they correspond to internal rotations, which are commonly represented by an empirical cosine potential function for hindered rotations.74,243 Rotational contributions to thermodynamic properties take into account the moments of inertia given in these works.244−246 The quantum chemistry calculations giving the full set of vibrational frequencies needed in evaluating thermodynamic properties have been done for a cation−anion pair because it is considered that the gas phase of ionic liquids is made of ion pairs rather than isolated ions.247,248 Dong et al.249 were able to obtain in situ IR spectrum of [C2C1im][NTf2] in the gas phase after vacuum distillation of the liquid at 150 °C. Relative intensities of some bands change in the gas phase in comparison with the liquid phase spectrum, a finding reproduced by calculations of IR spectra of clusters of different sizes.249 Obi et al.250 and Cooper et al.251 also concluded for vaporization of [C2C1im][NTf2] as ion pairs, but these workers obtained the IR spectrum after trapping the vapor in helium nanodroplets at very low temperature250 or supersonic jetcooled in helium carrier gas.251 The role played by methylation of the imidazolium ring on the strength of interactions of the anion to the imidazolium cation was addressed by Fournier et al.252 by comparing IR spectra of clusters of [C4C1im][BF4] and [C4C1C1im][BF4]. Both trapping techniques of helium

4.3. Applications

The discussion of vibrational frequencies and normal modes assignments for important anions and cations in the previous sections already stated some of the applications of vibrational spectroscopy in studies of ionic liquids. This section aims an overall view of these and further applications of vibrational spectroscopy of pure ionic liquids. One of the most fundamental applications of molecular spectroscopy42 is the calculation of thermodynamic properties of the ideal polyatomic gas model, for which exact expressions exist for the partition function of translational, rotational, and vibrational degrees of freedom. Focusing here on the Y


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nanodroplets and matrix isolation were used by Hanke et al.253 to reveal individual ion contributions to the IR spectra of [C1C1C1C4N][NTf2]. Booth et al.254 identified in the gas phase several conformers of [C2C1im][NTf2] ion pairs with different strengths of interaction with the anion according to the magnitude of vibrational frequency shift of the C(2)−H stretching mode. Berg et al.255 obtained in situ gas phase Raman spectrum of a protic ionic liquid, 1-methylimidazolium acetate. Comparison between spectra of the gas phase ionic liquid and 1-methylimizadole and acetic acid indicated that proton transfer takes place upon vaporization, so that the gas phase of this protic ionic liquid is made of the starting neutral molecules.255 However, this is not a general rule for protic ionic liquids as demonstrated by Horikawa et al.,239 who obtained IR spectra of several protic ionic liquids after vaporization and trapping in a cryogenic neon matrix. Comparison between spectra of the evaporated ionic liquid and the parent acids and bases leads to the conclusion that vaporization as an ionic pair is preferred as larger is the difference in pKa of the parent acids and bases.239 Returning to the issue of calculating thermodynamic properties of the ideal gas phase of nonprotic imidazolium ionic liquids,245,246 the agreement between calculation under the assumption of ionic pairs and experimental data gives further support for the physical picture that nonprotic ionic liquids in the gas phase are not made of isolated anions and cations. Vibrational spectroscopy is a complementary tool for thermal analysis and mass spectrometry256 to reveal whether thermal decomposition of ionic liquids takes place and the nature of decomposition products. Berg et al.241 proposed that the Raman spectrum of 1,1,3,3-tetramethylguanidinium chloride, [TMGH]Cl, in the vapor phase at 225 °C was consistent with the presence of [TMGH]+Cl− ion pairs rather than the neutral molecules 1,1,3,3-tetramethylguanidine and HCl. A Raman band at 2229 cm−1 was assigned to the N−H stretching mode experiencing significant lower frequency shift because of the N−H···Cl− hydrogen bond. In a subsequent work, however, Berg et al.241 corrected this interpretation since this band was observed in exactly the same position in the Raman spectrum of vapor phase of [TMGH]Br. Thus, it was proposed instead that this band belongs to CN stretching mode of dimethylcyanamide, (CH3)2N−CN, resulting from thermal decomposition of [TMGH]Cl and [TMGH]Br.241 In the temperature-jump experiments of Chambreau et al.,257 the IR spectrum is recorded for the vapor phase generated after a discharge on a filament within a small sample of the ionic liquid. For instance, characteristic IR bands of CH3NCS indicated formation of this species upon heating of [C4C1im][SCN], and Chambreau et al.257 proposed reaction mechanisms of thermal decomposition for several ionic liquids based on 1-alkyl-3-methylimidazolium cations and cyanate-anions. Liaw et al.258 obtained IR spectra of [C2C1im][C2SO4], [C6C1im]Cl, and [C4C1im][NTf2] before and after the flash point and after ignition. Changes on the spectral pattern after ignition were considered as evidence of thermal decomposition,258 so that these authors claimed that flammability of these ionic liquids is related to thermal decomposition rather than vaporization of the liquid. Vibrational assignments discussed in previous sections indicated there are some frequencies that characterize molecular conformations, whose difference of conformer energies according to quantum chemistry calculations might be relatively small in comparison with thermal energy available at room temperature. Vibrational spectroscopy has been

extensively used to estimate the relative population of conformers and to obtain the thermodynamic functions characterizing conformational changes in ionic liquids. The procedure considers equilibrium between conformers, conf1 ⇆ conf2, with equilibrium constant K = [conf2]/[conf1], and takes Raman intensities, Iconf1 and Iconf2 (i.e., areas of appropriate Raman bands), as proportional to the concentration of conformers, Iconf1 = Jconf1[conf1] and Iconf 2 = Jconf2[conf2], where Jconf1 and Jconf2 are the Raman scattering cross sections corresponding to the vibration of each conformer. From the experimental intensity ratio Iconf2/Iconf1, one obtains the relative population of conformers as long as one takes into account the ratio of Raman scattering cross sections Jconf2/Jconf2, which can be estimated by quantum chemistry methods. By recording Raman spectra as a function of temperature, one obtains the temperature dependence of ln(Iconf2/Iconf1) and the enthalpy of conformational change by traditional application of the van’t d ln K ΔH o Hoff equation, 1 = − R , with the assumption that Raman d

(T )

scattering cross sections Jconf1 and Jconf2 do not depend on temperature. The ratio Jconf2/Jconf1 must be known in order to obtain the entropy of conformational change because the intercept of an Arrhenius plot of Raman intensities depends on both ΔSo and Jconf2/Jconf1: ln

J Iconf2 ΔH o ΔS o =− + + ln conf2 Iconf1 RT R Jconf1

(6)

Umebayashi et al.49 considered Raman bands of [C2C1im][BF4] and [C2C1im][CF3SO3] in the spectral range shown in Figure 21 and obtained ΔHo ≈ 2 kJ mol−1 for the nonplanar ⇆ planar equilibrium of [C2C1im]+ (i.e., the nonplanar is slightly more stable than the planar conformer). This figure of ΔHo obtained from Raman spectroscopy was practically the same as calculated by quantum chemistry methods for the two [C2C1im]+ conformers.49 In the case of [C4C1im]+, Holomb et al.99 considered the intensities of Raman bands at 808, 825, 883, and 905 cm−1 as characteristics of four conformers gauche−gauche, gauche−anti, anti−gauche, and anti-anti, respectively, and obtained the relative populations 0.07:0.48:0.17:0.28 in the ionic liquid [C4 C1im][BF4 ]. Thermodynamic functions of conformational changes of [C4C1im]+ were obtained by Umebayashi et al.188 from the temperature-dependence of Raman intensities of bands in the spectral range shown in Figure 20. These authors calculated ΔHo, ΔSo, and ΔGo by considering an equilibrium between only two [C4C1im]+ conformers, anti ⇆ gauche, for a sequence of ionic liquids containing chloride, bromide, and iodide. Umebayashi et al.188 found that both the conformers coexist in comparable quantities (i.e., ΔGo close to zero), even though ΔHo drives the preference for the gauche [C4C1im]+ conformer as the halide anion gets smaller. This spectroscopic approach for obtaining thermodynamic properties of conformational changes has been used for other cations (e.g., 1-isopropyl-3methylimidazolium,191 N-alkyl-N-methylpyrrolidinium,126,127 and [C1C2C2C3N]+).223 Figure 12 showed Raman bands that are good markers of different conformers of [NTf2]−, so that Raman spectroscopy has been used to quantify thermodynamic properties related to conformational changes of this anion. Assuming the [NTf2]− equilibrium as transoid ⇆ cisoid, Fujii et al.124 used the 398 and 407 cm−1 Raman bands as markers of transoid and cisoid conformers, respectively. These authors obtained ΔHo = 3.5 kJ Z


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mol−1 from the Raman spectra of [C2C1im][NTf2] in close agreement with ab initio calculations of the isolated anion, 2.2− 3.1 kJ mol−1, depending on the level of theory. Lassègues et al.125 considered the spectral range of 260−360 cm−1 shown in Figure 12 and found that the relative population of transoid [NTf2]− conformer is 75% in [C2C1im][NTf2] at room temperature but a slightly higher value for the enthalpy, ΔHo = 4.5 kJ mol−1. Essentially the same ΔHo for conformational change of [NTf2]− was obtained in N-alkyl-N-methylpyrrolidinium ionic liquids.126,127 A distinctive feature of a Raman spectroscopic study performed by Martinelli et al.116 for some nonprotic and protic ionic liquids was that ΔHo of [NTf2]− conformational change was also obtained from the intense Raman band at 740 cm−1. This band exhibits a slight asymmetric shape toward the low-wavenumber side, so that two components at 738 and 741 cm−1 resulting from the curve fit have been assigned to cisoid and transoid [NTf2] − conformers, respectively. The analysis of the temperature dependence of the band shape gave ΔHo in reasonable agreement with the value obtained from the analysis of the 260−360 cm−1 spectral range.116 Plots ln(Itrans/Icis) versus T−1 for bands that characterize transoid and cisoid conformers also offer spectroscopic signatures of the glass transition of glassforming ionic liquids. Palumbo et al.259 considered IR bands in the range of 500−650 cm−1 to characterize [NTf2]− conformers in an ammonium-based ionic liquid: the linear Arrhenius plot above the glass transition temperature (Tg ∼ 210 K) changes slope at Tg and becomes constant indicating that the relative concentration of [NTf2]− conformers does not change any longer in the glassy phase. Moschovi et al.118 paid attention to appropriate corrections on the raw spectral data prior the analysis of equilibrium by Raman spectroscopy. As already warned by Papatheodorou et al.2 in their review on molten salts, these corrections include accounting for polarization by using the isotropic component of the Raman spectrum, the excitation wavelength dependence, and the Boltzmann population factor. In the liquid phase of protic [C1im][NTf2], which melts at Tm ∼ 325 K, Moschovi et al.118 found significantly higher enthalpy for conformational change of [NTf2]− when these corrections are taken into account, 8.5 kJmol−1, in comparison with the analysis performed using raw spectra (6.0 kJ mol−1). Taking a sequence of protic ionic liquids [Cnim][NTf2] with increasing length of the alkyl chain (n up to 12), Moschovi et al.199 claimed that the population of [NTf2]− conformers and the corresponding ΔHo are signatures of the relative importance of anion−cation interaction by Coulombic forces in comparison with other contributions to intermolecular forces (hydrogen bond, van der Waals, and dispersion due to π−π interaction). These authors proposed that as the alkyl chain of [Cnim]+ increases, leading to segregation of nonpolar and polar domains, then [NTf2]− experiences a local environment in which the transoid conformation is favored. The population of cisoid conformer increases with temperature proper to the positive ΔHo value, which however becomes less positive the longer the [Cnim]+ alkyl chain.199 On the other hand, the way that the transoid over cisoid ratio Itrans/Icis increases with the chain length is different for protic and nonprotic ionic liquids: Itrans/Icis reaches a plateau when n = 4 in [Cnim][NTf2], whereas it steadily increases up to n = 12 in [CnC1im][NTf2].201 In the case of [N(SO2F)2]−, Fujii et al.129 obtained essentially the same enthalpy of conformational change as [NTf2]− from the analysis of the temperature dependence of

the Raman bands of [C2C1im][N(SO2F)2] within 280−380 cm−1 (see this spectral range in Figure 14). If the [N(SO2F)2]− anion is in the ionic liquid based on the N-methyl-N-propylpyrrolidinium cation, the ΔHo of the [N(SO2F)2]− conformational change is slightly higher, 6.8 kJ mol−1.130 Investigating molecular conformations in condensed phase is a common issue for molecular dynamics (MD) simulations of ionic liquids. Classical MD simulation relies on an assumed potential energy function including intermolecular interactions, which are normally described by Lennard-Jones potential and Coulombic interactions between partial atomic charges and an intramolecular force field accounting for molecular vibrations.260 The validation of a proposed model is accomplished by comparing thermodynamic, structural, and dynamical properties calculated by MD simulations and experimental data.261 Nevertheless, vibrational spectra are the source of data for reasonable parameters of force constants needed for the intramolecular terms of the potential function. The well-known force field of Canongia Lopes and Pádua (CL&P)75 has been tested taking into account information on conformers distribution available from vibrational spectroscopy. Focusing here on the intramolecular part of the CL&P model, Vintra, this includes bond stretching, r, angle bending, θ, and torsion of dihedral angles, ψ: Vintra =

∑ bonds

kr k (r − req)2 + ∑ θ (θ − θeq)2 + 2 2 angles 4

∑ ∑ dihedrals m = 1

k ψ ,m 2

[1 + (− 1)m cos(mψ )] (7)

where re and θeq are equilibrium bond length and angle. Force constants of stretching and bending, kr and kθ, used in MD simulations of ionic liquids are usually taken from previous force fields (e.g., AMBER and OPLS-AA). It is worth noting that due to the harmonic oscillator model for stretching and bending modes, the condensed phase effect on vibrational frequency shift is not the issue being addressed by this kind of model. In fact, proper coupling between intra- and intermolecular degrees of freedom is very dependent on anharmonicity of the probe oscillator.80,79 On the other hand, special attention was given in the CL&P model for kψ,m parameters. The dihedral angle terms should give potential energy profiles of torsion consistent with quantum chemistry calculations and the distribution of conformers resulting from analyses of Raman spectra. For instance, in the case of 1-alkyl-3methylimidazolium cations,262 the relative population of gauche and anti conformers calculated by MD simulations was compared with the analysis of the 600−700 cm−1 spectral range, which includes the Raman bands that characterize cation conformers. Understanding macroscopic properties of ionic liquids on the basis of intermolecular interactions is a major aim that vibrational spectroscopy shares with other experimental techniques and theoretical methods. IR spectroscopy provides one of the most characteristic experimental evidence of the X− H···Y hydrogen bond: the vibrational frequency of the X−H stretching mode shifts to lower wavenumber (red shift) in comparison with the isolated X−H oscillator.263−267 Other spectroscopic features usually accompanying the ν(X−H) red shift on hydrogen bond formation include the enhancement of IR intensity and broadening of this band and the increase of vibrational frequency (blue-shift) of the bending mode δ(X− AA


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intermolecular anion−cation vibration, rather than the interaction energy of an ionic pair, that determines the melting temperature of ionic liquids. Their conclusion was based on quantum chemistry calculation of the potential energy curve as a function of anion−cation distance. Direct evidence of the intermolecular anion−cation vibration is the realm of far-IR and low-frequency Raman spectroscopies to be discussed in the next section. Some works went further on quantum chemistry calculations of vibrational spectra by considering clusters of ions instead of a single ionic pair. Dong et al.269 calculated the IR spectrum of [C2C1im][BF4] at the DFT/B3LYP level of theory for clusters of ions ([C2C1im][BF4])n with increasing size, n = 2, 3, 4, and 5. The overall appearance of theoretical IR spectrum (peak positions, band shapes, and relative intensities, including the FIR range) gets closer to the experimental spectrum of liquid [C2C1im][BF4] with increasing cluster size.269 Thus, there is an incipient hydrogen bond network making the five ion pairs cluster a microscopic model of bulk [C2C1im][BF4]. The need for considering ion clusters in order to the full appearance of vibrational spectra being recovered by ab initio calculations is particularly demanding for protic ionic liquids. Bodo et al.230 calculated the Raman spectrum of methylammonium nitrate by considering clusters whose structures were first generated by classical MD simulations and then optimized at the DFT/ B3LYP level of theory. It is worth remembering that such calculations of clusters still concern harmonic vibrational frequencies, so that scaling factors are needed for better agreement between calculation and experiment. Nevertheless, the Raman spectra calculated for clusters ([C1NH3][NO3])n, n = 2, 4, 6, and 8, exhibit distribution of frequencies that become broader as the cluster size increases.230 In a subsequent work, Bodo et al.231 extended the calculations of Raman spectra for the series ethyl-, propyl-, and butylammonium nitrate. Thus, the ab initio calculation of reasonable structures of clusters of ions captures some key features of ionic interactions that are important for the calculation of vibrational frequencies, in particular the strength distribution of anion−cation hydrogen bonds. The nitrate anion can be used as an example of taking an anion vibration, νs(NO3), rather than C−H, N−H, or O−H stretching modes, as a probe of the difference in width in the distribution of hydrogen bond strengths between protic and nonprotic ionic liquids. Figure 34 shows the νs(NO3) Raman band in [C4C1im][NO3] and [C3NH3][NO3]. Stronger anion− cation interaction in [C3NH3][NO3] in comparison with [C4C1im][NO3] implies +3.5 cm−1 shift of νs(NO3) and a remarkable broadening and asymmetry of the band because of the disturbed hydrogen bond network in the protic ionic liquid. Full width at half height (fwhh) of the νs(NO3) Raman band in [C4C1im][NO3] and [C3NH3][NO3] are 5.0 and 11.0 cm−1, respectively. For comparison purposes, fwhh is ∼15.0 cm−1 in molten NaNO3 at 600 K and ∼6.0 cm−1 in [NO3]− aqueous solution at room temperature.270 DFT calculation performed by Faria et al.235 showed that νs(NO3) vibrational frequencies of anions in a cluster ([C3NH3][NO3])4 indeed cover the spectral range of bandwidth of the experimental spectrum. Ionic liquids whose vibrational spectra have been interpreted with the help of ab initio MD simulations include those based on 1-alkyl-3-methylimidazolium with cyanate anions or acetate,33 alkylammonium with nitrate236 or bromide,89 and choline with carboxylates170 or amino acid anions.227 Ab initio MD simulations of ionic liquids go further than calculations of

H). Applying vibrational spectroscopy for studying pure ionic liquids aims insights on the strength and atomic sites involved in hydrogen bonds and, within a more general viewpoint, on the overall nature of anion−cation interactions. Katsyuba et al.48 provided a detailed comparison between experimental frequencies and ab initio calculations for different ion pair configurations of 1-alkyl-3-methylimidazolium cations and [BF4]− or [PF6]−. Frequencies related to C−H stretching and out-of-plane vibrations of the imidazolium ring and ν(B−F) orν(P−F) are sensitive to ion pair formation. Katsyuba et al.48 advocate that IR bands belonging to ν(C(4)−H) and ν(C(5)− H) modes (∼3160 cm−1) are separated from the red-shifted ν(C(2)−H) mode (∼3120 cm−1) because the C(2)−H···F interaction is stronger than C(4),(5)−H···F interactions. This assignment of C−H stretching modes,54,119,53,175,177,48 and the alternative interpretation that the doublet of bands in the high-frequency range of 1-alkyl-3-methylimidazolium vibrations is due to Fermi resonance,52,168,169 have already been addressed in section 4.2.1. In a subsequent work, Katsyuba et al.208 preferred to call these bands asν(Caromatic− H) being aware of the role played by Fermi resonance in the spectral range of C−H stretching modes. These authors investigated imidazolium ionic liquids with a terminal −OH group in the side chain [HO-C2C1im]+ (see Figure 25) and different fluorinated anions. Vibrational frequencies of both ν(Caromatic−H) and ν(O−H) decrease, and the bands become broader, as the anion basicity increases.208 It is worth noting that the intensity of the lower frequency ν(Caromatic−H) component (3100−3130 cm−1) increases in relation to the higher frequency component of the doublet (3150−3170 cm−1) with increasing strength of anion−cation hydrogen bond.208 Taking for granted the assignment of these bands asν(C(2)−H) and ν(C(4),(5)−H), respectively, these findings have been considered signatures of stronger H-bonding through the more acid C(2)−H site of dialkyl-substituted imidazolium cations. Some empirical relationships between vibrational frequency shift, Δν = ν(XHfree) − ν(XHbounded), and the enthalpy of hydrogen bond formation, − ΔHHB, are wellknown.265,266 Katsyuba et al.208 estimated −ΔHHB for hydrogen bonding between [HO-C2C1im]+ and fluorinated anions from both ν(Caromatic−H) and ν(O−H) modes, covering a rather large range depending on the anion basicity, 1.6−10.0 kJ mol−1. In contrast, Moschovi et al.199 kept the same [NTf2]− anion, while investigating how the hydrogen bond strength depends on the alkyl chain length of protic imidazolium cations, [Cnim]+. They found that vibrational frequencies of both ν(Caromatic−H) and ν(N−H) modes of [Cnim]+ are red-shifted as n increases.199 Garaga et al.200 obtained the same results for ν(Caromatic−H) modes in nonprotic [CnC1im][NTf2] ionic liquids. However, these findings were assigned to the intramolecular inductive effect of longer alkyl chain rather than an effect on the strength of the anion−cation hydrogen bond. The works of Katsyuba et al.48,208 and Moschovi et al.199,201 mentioned above illustrate the application of IR and Raman spectroscopies to infer about structural features both in nonpolar and polar domains of ionic liquids. In fact, the combined usage of vibrational spectroscopy and quantum chemistry calculations covers a large literature of ionic liquids,99,117,120,146,148,52,53,202,242,268 part of it already considered in the previous sections. On the other hand, in an attempt to correlate with melting temperature, Katsyuba et al.48 put forward the proposition that it is the anharmonicity of the AB


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5. VIBRATIONAL SPECTROSCOPY OF PURE IONIC LIQUIDS IN THE LOW-FREQUENCY RANGE In the previous section, insights on anion−cation interactions relied on the analysis of vibrational frequency shifts and changes in intensities and band shapes in mid-infrared and Raman spectra. Attempt to obtain data more directly related to intermolecular vibrations is the realm of far-infrared (FIR) spectroscopy, 10 < ω < 400 cm−1.271 It is natural that a smaller number of studies on ionic liquids has been reported using FIR spectroscopy because of more demanding components (e.g., beamsplitter and detector for this spectral range).272 Nevertheless, interesting results have been obtained by FIR spectroscopy, in particular, evidence in the spectral range below 150 cm−1 of anion−cation hydrogen bonds. In the case of Raman spectroscopy, the range ω < 150 cm−1 is usually called the low-frequency Raman spectrum. Raman spectrometers with double or triple monochromator, or else single stage spectrometer with special bandpass filters, are needed to achieve the frequency range close to the exciting laser line. However, the intermolecular vibrational dynamics is hidden under the very intense quasi-elastic scattering which dominates the low-frequency Raman spectrum of a liquid. On the other hand, the quasi-elastic intensity decreases as the liquid is cooled, so that intermolecular vibrations become apparent in Raman spectra of supercooled or glassy phases of ionic liquids. The revision of vibrational spectroscopy studies on crystallization and glass transition of ionic liquids is the issue of the next section. Fumino et al.273 reported the first FIR spectroscopic study showing evidence of +C−H···A− hydrogen bonding for a series of ionic liquids with the [C2C1im]+ cation and different anions. An overview of similarities and differences in FIR spectra of aprotic and protic ionic liquids is provided by Figure 35 taken from a subsequent work by Fumino et al.274 (We recommend the review published by Fumino and Ludwig275 focusing on their FIR spectroscopy studies of ionic liquids.) Assignments of the featureless bands in FIR spectra demand quantum chemistry calculations for clusters of ions in order to disentangle the intermolecular anion−cation vibrations from intramolecular modes eventually covering the same frequency range. Low-frequency vibrations expected for anion−cation hydrogen bond are the stretching of one ion against the other, ν(C− H···A), and the bending, δ(C−H···A). The ν(C−H···A) in imidazolium ionic liquids has been assigned to a FIR band around 100 cm−1, depending on the anion. Stronger anion− cation hydrogen bonding, according to the magnitude of redshift of high-frequency C−H stretching modes (see previous section), has the counterpart in the FIR spectrum as the frequency of the intermolecular vibration shifts to higher wavenumber.274 Curve fit by Voigt functions showed that the low-frequency tail of this broad FIR band overlaps the band assigned to δ(C−H···A) at 50−60 cm−1.275 On the other hand, Buffeteau et al.15 refrained from assigning the FIR band of imidazolium ionic liquids to a specific anion−cation vibration. Instead, these authors called the spectral feature in the FIR spectra as the density of states by analogy to low-frequency spectra of glass forming liquids (see eq 10 below). Schwenzer et al.144 assigned anion−cation hydrogen bond vibrations in FIR spectra of [C4C1im][CF3SO3] and [C4C1im][HSO4] to bands observed at 80 and 103 cm−1, respectively. A distinctive feature in the FIR spectrum of [C4C1im][HSO4] are additional bands

Figure 34. Raman spectra in the range of the νs(NO3) mode of [C3NH3][NO3] at room temperature (red) and the molten phase of [C4C1im][NO3] at 313 K (black). Intensities of Raman spectra have been normalized.

minimum energy structures by taking into account the liquid phase dynamics and anharmonicity of vibrations. Wendler et al.82 found that the calculation of liquid phase IR spectra of imidazolium ionic liquids exhibit a spectral pattern significantly different from the single ion pair calculation. The calculation of ion pair strongly favors the anion pointing toward the C(2)−H bond of the imidazolium ring, whereas ab initio MD simulation explores a distribution of anions around cations. Extreme examples of this situation is [C2C1im][CH3COO], for which quantum chemistry calculation of a single pair leads to the formation of hydrogen-bonded carbene−acetic acid with an IR band at 2167 cm−1, which is however not seen in the experimental spectrum nor in the spectrum calculated by ab initio MD simulation of the liquid phase.33 Wendler et al.82 found that including eight ionic pairs in the calculations was enough to capture the most important dynamics on the point of view of vibrational spectroscopy (i.e., the short-time dynamics of ions rattling inside the cages made by the neighbors). Bodo et al.236 calculated the vibrational frequencies of [C2NH3][NO3] by ab initio MD simulation of clusters of 6 or 24 ionic pairs. The experimental Raman spectrum of [C2NH3][NO3] was compared to the power spectrum obtained by Fourier transforming the autocorrelation function of atomic velocities so that Raman intensities were not reproduced in the fingerprint region. In this region there are different kinds of vibrations with very different Raman activities, so that they should be properly weighted by polarizability fluctuations. In the case of choline alanine, Campetella et al.227 found reasonable agreement between experiment and calculation in the fingerprint region of the IR spectrum since they calculated the Fourier transform of the autocorrelation function of molecular dipole moments. On the other hand, the power spectrum calculated for [C2NH3][NO3] reproduces the spectral pattern in the region of C−H and N−H stretching modes (see Figure 5 in ref 236). In other words, the high-frequency range of the Raman spectrum of [C2NH3][NO3] essentially reflects the density of vibrational states: ν(N−H) contributes with a broad background band between 3000−3250 cm−1 proper to hydrogen bonding of various strengths, while ν(C−H) give relatively sharp peaks on this background.236 AC


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square root of reduced mass for the vibration of anion and imidazolium ring. However, Fumino et al.274,279−281 compared the position of the FIR band for a large set of ionic liquids and showed that only the mass effect does not account for the vibrational frequency shift. These authors performed quantum chemistry calculations for clusters of ionic pairs and correlated FIR frequencies with binding energies, thus supporting the interpretation of frequency shifts as the result of different anion−cation force constants.273,274,281−283,280,277,284 Furthermore, Fumino et al.282 showed that the vibrational frequency of the main spectral feature in the FIR spectrum correlates with the enthalpy of vaporization of the ionic liquid. A step forward along the combined usage of FIR spectroscopy and DFT calculations was the attempt at separating the relative contributions of hydrogen bonding and dispersion forces out of the total energy, which is predominantly Coulombic energy.284−286 Fumino et al.284 measured FIR spectra as a function of temperature (303−353 K) for protic ammonium ionic liquids with appropriately chosen alkyl chain lengths and anions in order to play with the relative strengths of hydrogen bonding and dispersion forces. Bands assigned to δ(C−H···A) and ν(C−H···A) were seen in the FIR spectrum of [C6C6C6NH][CF3SO3] as two resolved peaks around 70 and 130 cm−1, respectively, as temperature decreases. When temperature increases, the FIR spectrum is dominated by a single broad band with maximum at 100 cm−1, which has been assigned to the anion interacting with the cation alkyl chains (i.e., an ion pair determined by dispersion forces). The temperature dependence of relative intensities of FIR bands assigned to local arrangements dominated by hydrogen bonds and dispersion forces allowed for a van’t Hoff analysis of the equilibrium between these two kinds of ion pairs.284 The van’t Hoff plot indicated that the hydrogen-bonded ion pair is favored by ∼34 kJ mol−1 over the dispersion-interaction dominated structure. This experimental finding pointed out the need for proper consideration of dispersion corrected methods in DFT calculations of ion pair energies.284−286 Fumino et al.193 and Buffeteau et al.287 addressed the effect of methylation of the C(2)−H position of the imidazolium ring by comparing the FIR spectra of [C 2 C 1 im][NTf 2 ] and [C2C1C1im][NTf2]. FIR bands of [C4C1im][NTf2] and [C4C1C1im][NTf2] were found respectively at 83.5 and 79.0 cm−1 by Fumino et al.193 and 82.5 and 73 cm−1 by Buffeteau et al.287 Analogous effect of methylation of the imidazolium ring is seen in the FIR spectra of [C4C1im][BF4] and [C4C1C1im][BF4] shown in Figure 35. In addition to the red shift of vibrational frequency, Fumino et al.193 found that the intensity of the FIR band decreases because of weakening of the anion− cation hydrogen bonding as the stronger binding site C(2)−H is switched off in [C2C1C1im][NTf2]. In contrast, Buffeteau et al.287 have not found significant difference in the FIR intensity between [C4C1im][NTf2] and [C4C1C1im][NTf2] after proper normalization by the intensities of high-frequency intramolecular bands. Buffeteau et al.287 also compared FIR spectra of [C4C1im][NTf2] and [C4C1C1C1N][NTf2] and they found essentially the same intensity and vibrational frequency for both the systems. Therefore, Buffeteau et al.287 reached the opposite conclusion, claiming that the characteristic broad band in FIR spectra should not be assigned exclusively to stretching mode of the anion−cation hydrogen bond. These authors preferred instead assigning the density of states revealed by the FIR spectrum as a kind of envelope over the solidlike lattice vibrations. This physical picture is alike the one resulting from

Figure 35. FIR spectra of some aprotic ionic liquids based on imidazolium cations with [BF4]− or [NO3]− and the protic ionic liquid propylammonium nitrate. The arrow on the top of the low-frequency feature indicates the band assigned to the stretching mode of anion− cation hydrogen-bonded. Bands in the gray area belong to intramolecular normal modes. Reproduced with permission from ref 274. Copyright 2009 PCCP Owner Societies.

that Schwenzer et al.,144 with the support of MD simulations and NMR spectroscopy, assigned to hydrogen-bonded [HSO4]− anions. The occurrence of anion−anion hydrogen bonding in [HSO4]− ionic liquids is in line with the proposition of Ribeiro156 based on the analysis of high-frequency Raman bands of [HSO4]−. Fumino et al.276,277 argued that strong hydrogen bonds in the protic ionic liquids alkylammonium nitrate imply higher frequencies for anion−cation intermolecular vibrations. The similarity between FIR spectra of alkylammonium nitrate and water led Fumino et al.276 to the proposition of a threedimensional hydrogen bond network in these protic ionic liquids. In the case of imidazolium ionic liquids, Fumino et al.193 proposed that [C2C1im][NTf2] is less viscous than [C2C1C1im][NTf2], even though the former has the C(2)−H site available for stronger hydrogen bond because hydrogen bonds act like defects perturbing the charge symmetry of the Coulomb network. However, it is yet an unsettled issue why methylation of the most acidic hydrogen of the imidazolium ring results in more viscous ionic liquid. It has also been proposed that methylation of the C(2) site of the imidazolium ring restricts ionic mobility because of loss of variation of anion−cation arrangements,192 higher potential energy barrier,194 or reduced free volume.195 In order to use the vibrational frequency shift seen in Figure 35 as a signature of the strength of +C−H···A− hydrogen bond, one must rule out that it is not a trivial effect of changing the reduced mass among different ion pairs. Yamada et al.278 found that the maximum of the FIR band for [CnC1im]X, n = 3, 4, and 6, does not depend on the alkyl chain length and follows the trend 72.5, 92, and 122 cm−1 for the halide anions I−, Br−, and Cl−, respectively. Yamada et al.278 argued that these frequencies are on the same proportion of the inverse of the AD


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low-frequency Raman spectroscopy studies of phase transitions of ionic liquids to be discussed in the next section. Wulf et al.288 compared FIR to low-frequency Raman spectra, and also Terahertz (THz) spectra, for the same set of ionic liquids ([C 2C 1im][SCN], [C 2C 1 im][N(CN) 2 ], [C2C1im][C2SO4], and [C2C1im][NTf2]) previously studied by Fumino et al.273 It has been found that the spectral pattern and optical constants obtained from FIR spectra of imidazolium ionic liquids agree with data obtained by THz spectroscopy.278,283,287,288 However, the experimental setup of Wulf et al.288 did not allow obtaining reliable Raman spectra below 100 cm−1 so that the comparison between FIR and low-frequency Raman was restricted to intramolecular bending modes of cations or anions observed in the range of 150−300 cm−1. Low-frequency Raman spectroscopy investigations of intermolecular dynamics encompass a large literature with a special focus on glass-forming liquids289−293 and among them several (high temperature) molten salts (e.g., alkali halides,294−296 ZnCl2,297,298 BiCl3,298,299 mixtures ZnCl2−AlCl3,300 etc.).1,2 An early study of the low-frequency range was reported by Ribeiro et al.301 for the low-temperature molten salt tetra(n-butyl)ammonium croconate, [C4C4C4C4N]2[C5O5]·4H2O, which is a glass-forming liquid with Tg = 293 K. The first low-frequency Raman spectroscopy studies concerning nowadays typical ionic liquids were reported by Ribeiro302 for [C4C1im][PF6] as a function of temperature and by Iwata et al.8 for [C4C1im][BF4], [C4C1im][PF6], [C6C1im][PF6], and [C8C1im][PF6] at room temperature. In these works, the low-frequency Raman spectrum is conveniently reported after discounting a thermal population factor from the raw experimental spectrum, I(ω), in the socalled susceptibility representation, χ″(ω): χ ″(ω) =

I(ω) n(ω) + 1

Figure 36. Low-frequency Raman spectra of [C4C1im][NTf2]. The top panel shows the raw spectrum, I(ω), and the inset shows the susceptibility representation, χ″(ω) (eq 8), at room temperature. The bottom panel shows I(ω) as a function of temperature. Note different intensity scales between top and bottom panels.

the low-frequency bands are slightly shifted to higher frequencies in the χ″(ω) representation.306 In order to highlight that the low-frequency bands are not artifact of the χ″(ω) representation, the bottom panel of Figure 36 shows the raw Raman spectra, I(ω), of [C4C1im][NTf2] as a function of temperature. Low-frequency Raman band assigned to intermolecular vibration is clearly seen in the raw spectrum of [C4C1im][NTf2] as the glass transition temperature, Tg = 181 K, is achieved from above. The quasi-elastic component is due to anharmonicity and fast relaxation processes, so that its intensity decreases significantly with decreasing temperature.290−292,298,300,307−310 In fact, the plot of the quasi-elastic intensity versus temperature exhibits break of slope at the glasstransition temperature for several ionic liquids.302,311,312 The optical Kerr effect (OKE) spectrum is equivalent to the depolarized Raman spectrum, the results of these techniques differing by the population factor.313 It has been found that the depolarization ratio is essentially constant at 0.75 in the lowfrequency range of the Raman spectrum of [C4C1im][PF6].302 In other words, essentially the same band shape is obtained in the low-frequency range for polarized, depolarized, or without polarization selection of the scattered radiation. It is worth noting that the frequency-dependent depolarization ratio has been found in Raman spectra of high temperature molten salts, especially when complex molecular-like structures occur in the melt.298,299,314 In the case of ionic liquids, the intermolecular vibrational dynamics has been much more investigated by OKE315−320 than low-frequency Raman spectroscopy. Both OKE and Raman spectroscopies probe polarizability fluctuations resulting from the molecular dynamics of the liquid, so that the spectral pattern of χ″(ω) is the same as the OKE spectra of ionic liquids. The OKE spectrum encompasses a temperature-dependent relaxation contribution within the GHz range (the structural or α-relaxation) not accessible in the Raman spectrum and the THz range of intermolecular

(8)

where

n(ω) =

1 e

ℏω / kT

−1

(9)

The motivation behind the reduction of the low-frequency Raman spectrum is the relation between I(ω) and the density of vibrational states g(ω):303,304 I(ω) =

n(ω) + 1 C(ω)g (ω) ω

(10)

where C(ω) is the light-vibration coupling. The spectral pattern of g(ω) corresponds to the power spectrum alluded for in section 3, and C(ω) accounts for how the low-frequency vibrations modulate the material polarizability. Figure 36 shows the raw low-frequency Raman spectrum of [C4C1im][NTf2] at room temperature, and the resulting χ″(ω) (inset in the top panel). The relatively sharp bands above 100 cm−1 belongs to intramolecular modes of the [NTf2]− anion (see Figure 11 and Table 2). If the frequency-dependence of C(ω) were linear then χ″(ω) would give directly the density of vibrational states. The actual frequency dependence of C(ω) is experimentally accessible by comparing the Raman spectrum with g(ω) obtained by neutron scattering spectroscopy.298,305 Another common representation of the low-frequency spectrum is the reduced Raman intensity, Ired(ω) = χ″(ω)/ω. Since the factor [n(ω)+1]−1 depends linearly with ω at lowfrequencies, Ired(ω) is similar to the experimental I(ω), whereas AE


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Figure 37. Low-frequency Raman spectra in the susceptibility representation of ionic liquids at room temperature. Left panel: [C4C1im][NTf2] (black) and [C4C1C1C1N][NTf2] (red); right panel: [C4C1im][BF4] (black) and [C3NH3][NO3] (red). Intensities of χ″(ω) spectra have been normalized.

shown in Figure 35. The significant frequency shift in the FIR spectrum when moving from the nonprotic [C4C1im][BF4] to the protic [C3NH3][NO3] (see Figure 35) is not manifest in the low-frequency Raman spectra. The χ″(ω) spectra of [C4C1im][NTf2] and [C4C1im][BF4] (black lines in the right and left panels of Figure 37) are similar, except for the [NTf2]− normal mode at 120 cm−1, which is evidently absent in the spectrum of the latter. On the other hand, the spectral patterns of χ″(ω) for the two ammonium ionic liquids shown by red lines in the right and left panels of Figure 37 differ in relative intensities of the components encompassing the band shape. The intense component with maximum at ∼70 cm−1 in the χ″(ω) spectrum of [C3NH3][NO3] has also been found by Sonnleitner et al. 323 in OKE spectra of ethyl- and propylammonium nitrate. This spectral feature is close to the FIR band at ∼60 cm−1, which Fumino et al.276 assigned to bending of the anion−cation hydrogen bond. Krüger et al.324 followed the Fumino et al.276 assignment for the corresponding feature observed at ∼40 cm−1 in the THz spectrum of ethylammonium nitrate. However, Sonnleitner et al.323 assigned the band at ∼70 cm−1 to [NO3]− libration (i.e., restricted outof-plane rotation about the C2 axes of [NO3]−) because OKE spectroscopy is primarily sensitive to rotational motions and to the large polarizability anisotropy of the [NO3]− anion. It is worth mentioning that the additional band assigned to libration of aromatic ring is also found at 60 cm−1 in the OKE spectrum when a phenyl group is attached to the longer chain of the imidazolium cation.322 The long tail extending up to 250 cm−1 in the χ″(ω) spectrum of [C3NH3][NO3] is absent in the [C4C1C1C1N][NTf2] spectrum (compare red lines in right and left panels of Figure 37). On the basis of curve fitting, Sonnleitner et al.323 proposed that the high-frequency tail in OKE spectra of [C2NH3][NO3] and [C3NH3][NO3] includes unresolved bands due to hydrogen bond stretching and cation librational motions. It is worth mentioning, however, early Raman spectroscopic studies on molten alkali nitrates in which broad bands assigned to [NO3]− libration have been found at relatively high wavenumber (maxima at ∼90, ∼115, and ∼140 cm−1 in molten KNO3, NaNO3, and LiNO3, respectively).325 Thus, the high-frequency tail in the χ″(ω) spectrum of [C3NH3][NO3] might also manifest an inhomogeneous distribution of librations in this protic ionic liquid, being the low-frequency counterpart of the previous discussion (see Figure 34) concerning the Raman band shape of νs(NO3). Despite similarities in positions of the components after curve fit of these featureless bands, low-frequency Raman and IR active intermolecular modes are not necessarily the same.

vibrations common to Raman spectroscopy (1 THz = 33.3 cm−1).321 OKE spectroscopy is a time-resolved spectroscopy technique, which is beyond the scope of this review, and the reader is recommended previous review focusing on OKE spectroscopy of ionic liquids.322 Low-frequency χ″(ω) and OKE spectra, like FIR spectra of ionic liquids, demand curve fit in order to grasp any trend of the components when changing ionic species or thermodynamic state. Different numbers and functional forms of bands (e.g., damped harmonic oscillator) antisymmetrized Gaussian and log-normal functions have been used in a curve fitting the low-frequency range of OKE and Raman spectra.315−320,322 Iwata et al.8 considered two components in the fit of lowfrequency Raman spectra of imidazolium ionic liquids, but most probably the broad spectral pattern encompasses three components around 20, 60, and 90 cm−1 assigned to intermolecular vibrations.312,322 (An example of this curve fit is shown below in Figure 43 for the χ″(ω) spectrum of [C2C1im][NTf2].) In line with assignments in aromatic molecular liquids,321 the 90 cm−1 component has been assigned to librational motion (i.e., hindered rotation) of the imidazolium ring. The left panel of Figure 37 illustrates this point by comparing χ″(ω) of [C 4 C 1 im][NTf 2 ] and [C4C1C1C1N][NTf2] at room temperature. When the aromatic cation is replaced by a nonaromatic one, the χ″(ω) spectrum is less broad because of the missing 90 cm−1 component, and the intramolecular [NTf2]− mode at 120 cm−1 becomes a welldefined band in the [C4C1C1C1N][NTf2] spectrum. It is also clear from the asymmetric band shape of the χ″(ω) spectrum of [C4C1C1C1N][NTf2] that there is at least two components at ∼20 and ∼60 cm−1, and these seem to be common spectral features whatever the combination of anion and cation. In FIR spectra, Buffeteau et al.287 found the same intensity of the intermolecular band at ∼84 cm−1 for [C4C1im][NTf2] and [C4C1C1C1N][NTf2]. In contrast, when Raman intensities are normalized by the high-frequency bands of [NTf2]−, rather than normalization by the low-frequency band as in Figure 37, low-frequency Raman spectra are more intense for imidazolium than ammonium ionic liquids,312 as one expects from larger fluctuation of polarizability in a system containing an aromatic ring. The anion−cation hydrogen bond vibrations are not expected to give as intense signals in Raman as in FIR spectra. The distinctive features of low-frequency Raman spectra are made clearer in the right panel of Figure 37 showing χ″(ω) spectra for [C4C1im][BF4] and [C3NH3][NO3], which are two systems corresponding to the extreme cases of FIR spectra AF


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case of protic trialkylammonium triflate ionic liquids, N−H···O stretching modes calculated around 160 cm−1 agree with FIR spectra.343 However, the peaks in the theoretical density of vibrational states are found at significantly lower frequencies than in the experimental FIR spectra.88,343 The density of states calculated by MD simulation is more like the low-frequency Raman than the FIR spectrum. In other words, even though polarizability fluctuations are not being taken into account in the density of states, it resembles the susceptibility representation of low-frequency Raman spectra of ionic liquids. Nevertheless, the calculations88,89,343 point out that the vibrational frequency of intermolecular dynamics is dominated by short-range interactions, and the peak position is indeed modulated by the strength of anion−cation hydrogen bond.275 However, the actual mode composition involves a large number of atoms rather than being localized only in the C−H···A moiety.88 Furthermore, the density of states calculated by MD simulation of the liquid phase of [C4C1im][PF6] resembles an envelope over the corresponding density of states of the crystalline phase,88 a finding in line with the previous proposition of Buffeteau et al.287 in their FIR study of ionic liquids. At this point we reach the application of vibrational spectroscopy in studying phase transitions of ionic liquids, an issue to be discussed in the next section. The MD simulations of Balasubramanian et al.88,89,343 aimed the analysis of atomic displacements of low-frequency vibrations by the calculation of time correlation function of velocities, rather than time correlation functions of dipole moment or polarizability fluctuation. In contrast, Hu et al.344 calculated time correlation function of polarizability by MD simulation of 1-methoxyethylpyridinium dicyanamide aiming a direct comparison with a previous OKE spectroscopy study by Shirota and Castner.345 The calculations of Hu et al.344 considered polarizability fluctuations arising from reorientational dynamics and interaction-induced mechanism according to the dipole−induced dipole (DID) model in analogy with the large body of literature on MD simulations of properties of spectroscopic interest in molecular liquids.86,103,346 It is worth noting that depolarization ratio as low as 0.1 in Raman spectra of high temperature molten salts1,347 prompted Madden et al.100,101 to propose other interaction-induced mechanisms resulting in polarizability fluctuation (e.g., short-range overlap, field, and field gradient). In the case of ionic liquids, including reorientation and DID mechanism is consistent with the depolarization ratio of 0.75 found in low-frequency Raman spectra.302 Reorientational and DID mechanisms in low-viscous molecular liquids account for long- and short-time parts, respectively, of the time correlation function of polarizability fluctuation.86 In other words, DID mechanism is expected to manifest higher-frequency dynamics of molecules rattling in the cage of neighbors. It is known that a clear time or frequency separation between reorientational and interaction-induced dynamics might not be strictly valid even in molecular liquids (e.g., CS2).86,103 Accordingly, Hu et al.344 found that interaction-induced contribution extends to a relatively longtime of a hundred picoseconds because the cage effect and nondiffusive dynamics take longer in ionic liquids. On the other hand, cross-correlation between different mechanisms contributing to dipole and polarizability fluctuations and strong coupling between reorientational and translational motions lead to the warning put forward by some authors323,320 that, irrespective of number and functional forms chosen in curve fitting of FIR, low-frequency Raman and OKE spectra of ionic

Within the context of large literature about intermolecular vibrational dynamics of glass-forming liquids, the band at ∼20 cm−1 seen in low-frequency Raman spectra of the glassy phase of ionic liquids (see Figure 36) is the so-called boson peak.291−293,298−300,306,308−310 It has been pointed out that the boson peak vibrations remain in the liquid state even though the peak is hidden under the intense quasi-elastic scattering of the low-frequency Raman spectrum.326−329 In an OKE spectroscopy study of 1-ethyl-3-methylimidazolium tosylate, Li et al.330 assigned to the boson peak dynamics the oscillatory component with period of ∼2.0 ps (i.e., ∼17 cm−1) seen in the time domain data a few degrees above the glass transition temperature (Tg = 201 K). The partial nature of longitudinal and transverse acoustic modes of the intermolecular dynamics in the spatial range of wavevectors 0.1 < k < 1.0 Å−1 and frequency range 0 < ω < 100 cm−1 in viscous glassforming liquids has been established in many studies by inelastic X-ray scattering spectroscopy and MD simulations.331−334 Although the exact origin of the boson peak is yet a lively debated issue, several studies have been pointing to the nature of transverse acoustic vibrations of high wave-vectors (i.e., soundlike modes of short wavelength).335−338 It is worth stressing that projecting into soundlike modes recovers only part of the intermolecular vibrations of liquids because the other part is random phase motion66,67,339,340 and eventually molecular-like normal modes in high temperature molten salts in which complexes survive for a relatively long time.1,2 The assignment of FIR spectra by Fumino et al.275 focused on local motions related to stretching and bending modes of ion pairs, whereas assignment of low-frequency Raman and OKE spectra emphasized librational and cage-rattling motions or the many body nature of the intermolecular dynamics. Shirota322 considered the average frequency (i.e., the first moment M1 = ∫ ωI(ω)dω/∫ I(ω) dω) as a single quantitative parameter for the whole low-frequency range of the OKE spectrum. It has been found that M1 obtained from OKE spectrum of nonaromatic ionic liquids correlates with (γ/ρ)1/2, where γ is the surface tension and ρ is the density.322 The correlation between M1 and (γ/ρ)1/2 is analogous to a simple harmonic oscillator result for the dependence of vibrational frequency with force constant and reduced mass (k/μ)1/2, however, involving the surface tension as a bulk property related to the strength of intermolecular forces. Comparison between Figures 35 and 37 indicates that the overall spectral pattern is different for FIR and low-frequency Raman (and OKE) spectra for a given ionic liquid. Molecular dynamics simulations have shown that the density of vibrational states resembles low-frequency Raman and OKE spectra of ionic liquids.341,342 The group of Balasubramanian attempted a theoretical approach for understanding the low-frequency vibrations of nonprotic imidazolium88 and protic ammonium89,343 ionic liquids. These authors used quantum chemistry calculations of ion pairs and computer simulations of liquid and crystalline phases by classical (force field based) and ab initio MD simulations. The vibrational analysis by MD simulations has been performed by a normal-mode analysis after diagonalization of the Hessian matrix obtained from quenched configurations and also by Fourier transforming the time correlation function of atomic velocities. The density of vibrational states resulting from MD simulations exhibits the trend of higher frequency shift with increasing strength of anion−cation interactions in imidazolium88 or the number of hydrogen-bonding sites in ammonium89 ionic liquids. In the AG


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vibrational probes the Raman bands at 1055, 1065, and 1119 cm−1, related to νs(CC) and νas(CC) modes, they concluded that the hexadecyl chain is in the anti conformation for all of the dihedrals in the crystalline phase and the crystal melts with formation of the mesophase and occurrence of gauche conformers. There are interesting questions concerning ionic liquid crystals,356 but we will focus in this review on phase transitions of medium size alkyl-chain ionic liquids, which usually have low melting points. A recurrent issue addressed by vibrational spectroscopy studies of ionic liquid phase transitions is eventual conformational changes indicated by the characteristic bands of different conformers. The possibility for the ions acquiring different conformations can lead to crystal polymorphism and solid− solid transitions due to interchange between conformers. Hayashi et al.181 characterized two crystal polymorphs of [C4C1im]Cl by a combined analysis of X-ray powder patterns and Raman spectra. Soon after this work, Ozawa et al.184 used Raman spectroscopy and DFT calculations to show that rotational isomerism of the butyl chain of [C4C1im]+ implies crystal polymorphism, one phase composed by anti−anti and the other one composed by gauche−anti conformers (see Figure 20 for the characteristic Raman bands of [CnC1im]+ conformers). In a chapter of a recently published book,4 Hamaguchi and coauthors reviewed their Raman spectroscopic studies on structure and phase transitions of imidazolium-based ionic liquids. Endo et al.198,357−359 extended the analysis of phase transitions to [C4C1im]+ based ionic liquids with different anions using Raman spectroscopy along other experimental techniques, such as NMR and differential scanning calorimetry (DSC). They reported the formation of three crystalline phases with [C4C1im]+ having anti−anti, gauche−anti, and gauche′−anti conformations. Raman spectra and DFT calculations reported by Endo et al.191 indicated the occurrence of two conformers, asymmetric and symmetric, for 1-isopropyl-3-methylimidazolium, [i-C3C1im]+, in the liquid phase. However, crystalline phases of [i-C3C1im]Br and [iC3C1im]I contain only the asymmetric conformer.198 Endo et al.191 also found similar behavior of solid−solid transitions and cation conformational changes when methylation in the C(2) atom of the imidazolium ring lead [C 4 C 1 im] + to [C4C1C1im]+based ionic liquids. Other systems for which temperature-dependent Raman spectroscopy was used to reveal conformational changes accompanying crystal polymorphism and solid−solid transition included piperidinium132 and ammonium230,232,235,360,123,128 ionic liquids. Turning now to conformational changes of the anion, Raman and IR spectra have been used to characterize the [NTf2]− conformation along phase transitions of [NTf2]− based ionic liquids. It is worth mentioning that before ionic liquids becoming a broadly investigated class of compounds, vibrational spectroscopy has been extensively used for studying [NTf2]− conformation in polymer electrolytes.113,361 Crystalline phases of typical ionic liquids have been found with [NTf2]− in cisoid or transoid conformations (see Figure 12 for the characteristic Raman bands of [NTf2]− conformers). Castriota et al.114 reported one of the first vibrational studies on temperature-dependent phase transition of a [NTf2]− based ionic liquid, namely [Pyr1,4][NTf2], and its mixture with Li[NTf 2]. Raman spectroscopy provided insight about interactions between [NTf2]− and [Pyr1,4]+ and Li+ cations in crystal and liquid phases, but [NTf2]− conformation was not an issue. Herstedt et al.360 performed a study of cation and anion

liquids, each component might not necessarily be related in a one-to-one basis to specific intermolecular dynamics.

6. VIBRATIONAL SPECTROSCOPY FOR STUDYING IONIC LIQUID PHASE TRANSITIONS Ionic liquids exhibit a rich phenomenology of different phases (e.g., crystalline, glassy, and supercooled or superpressed liquid) which the same system may achieve depending on the rate that the thermodynamic state is changed. Vibrational spectroscopy has been instrumental in revealing structural modifications accompanying phase transitions. Most of these studies were performed with variable temperature at atmospheric pressure or variable pressure at room temperature. The temperature and pressure dependence of vibrational frequencies, band shapes, and relative intensities in Raman and IR spectra provide insights on ion conformation or the local environment experienced by the ions in different phases. Early vibrational spectroscopic studies took advantage of the crystalline structure to infer about liquid phase structure before X-ray and neutron scattering experiments were used to study directly ionic liquids in the normal liquid phase.348−351 In this context, Dupont352 considered the C−H stretching modes in IR spectra of [C4C1im]+ based ionic liquids with different anions and emphasized structural similarities between solid and liquid states. The group of Hamaguchi181,183−185 compared Raman spectra of different crystal polymorphs and liquid phase of [C4C1im]Cl and [C4C1im]Br, concluding that the local structure in the liquid is similar to the one in the solid despite an ionic conformation being eventually selected in the crystalline phase. We illustrate this point in Figure 38 with

Figure 38. Raman spectra of [C4C1im][CF3SO3] in liquid and low temperature crystalline phases. Raman intensities have been normalized by the band marked with the asterisk. The inset highlights the range of 585−635 cm−1.

Raman spectra of [C4C1im][CF3SO3] in liquid and low temperature crystalline phases. There is indeed reasonable match between crystal and liquid spectra, the gauche conformation of [C4C1im]+ being selected in the crystal according to the occurrence of a single Raman band at 599 cm−1 (see inset of Figure 38). Ionic liquid mesophases (i.e., liquid crystal) are formed with increasing length of the alkyl chain of cation or anion.350,353,354 Among the first vibrational spectroscopy studies concerning ionic liquid phase transitions, De Roche et al.355 used Raman spectroscopy to analyze the crystal at room temperature and the smectic phase at 353 K of [C16C1im][PF6]. By using as AH


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in the crystal formed by slow cooling or cold crystallization, respectively. Many ionic liquids are easily supercooled, exhibiting glass transition typically around 190−210 K, so that it might be difficult to obtain the low-temperature crystal.366,367 Crystalline phase has never been obtained for some ionic liquids along cooling, or it has been obtained only by the cold crystallization process. The ionic liquid can also become partially crystallized forming the so-called glacial state (i.e., microcrystallites immersed in a matrix of supercooled liquid), for instance, [C4C1C1C1N][NTf2].123,128 A large number of studies using DSC, NMR, and Raman spectroscopy have investigated the slow dynamics during the melting process of ionic liquids.185,191,368−373 Hamaguchi and Ozawa185 followed the melting of [C4C1im]Cl crystal by the time dependence of Raman bands which characterize different conformations of the butyl chain. They observed an unusually long time for equilibration of anti/gauche conformers in the liquid phase, concluding that conformational interconversion happens along slow collective transformation of ensembles of [C4C1im]+ cations. In a subsequent paper, the melting process of [C4C1im]Cl was investigated by Okajima and Hamaguchi372 using simultaneously the lattice vibration bands in the lowfrequency range and the Raman bands of butyl chain conformers. Figure 40 taken from their work shows Raman spectra of [C4C1im]Cl during the melting process and Raman intensities as a function of time for the lattice vibration at 111 cm−1 and the characteristic bands of butyl chain conformation. In this experiment, a small piece of [C4C1im]Cl single crystal was rapidly heated to 363 K, that is 30 K higher than the

conformations in different solid phases of [C2C2C2C2N][NTf2], whose melting temperature is 377 K. The relative amount of [NTf2]− conformers in three different solid phases were evaluated on the basis of characteristic [NTf2]− Raman bands. The analysis helped understanding the disorder caused by [NTf2]− anion in the solid, thus explaining the formation of plastic crystals and, in part, accounting for the low melting point of [NTf2]− ionic liquids.362 Martinelli et al.116,363 used Raman spectroscopy to follow the evolution of [NTf2]− conformation as a function of temperature in bulk and nanoconfined ionic liquids with protic and aprotic cations. Some studies also considered IR spectra as a function of temperature to follow [NTf2]− conformational changes during phase transitions.118,122,259,364 Vitucci et al.122 suggested that the longer alkyl chain in ammonium ionic liquids stabilizes the lower energy cisoid [NTf2]− conformer in solid phase because of stronger interactions between [NTf2]− and the polar head of the cation. It became apparent from vibrational spectroscopy studies that structural rearrangements in polar and nonpolar domains during phase transitions may be independent of each other.235,123,128,365 The [N(SO2F)2]− is another anion for which conformational analysis in low temperature phase behavior was investigated by Raman spectroscopy. Shimizu et al.132 found two different crystalline phases for [Pip1,4][N(SO2F)2] with [N(SO2F)2]− either in the cisoid or transoid conformation (see Figure 14 for the characteristic Raman bands of [N(SO2F)2]− conformers). Kinetic effects play a central role in determining the complex phase behavior of ionic liquids. Lassègues et al.125 reported Raman spectra of [C2C1im][NTf2] in the crystal phase at 113 K obtained after slow cooling (∼1 K min−1) and glassy phase at 113 K obtained after fast cooling (∼20 K min−1). These authors found that the crystal phase contained the cisoid [NTf2]− conformer, whereas the glassy phase contained mainly the transoid conformer. The cooling-rate dependence of [NTf2]− conformation illustrates how the thermal history determines the ionic liquid phase behavior. In fact, Raman spectroscopy indicates that [NTf2]− achieves different conformations when partial crystallization of [C4C1C1C1N][NTf2] is obtained by slow cooling or by cold crystallization (i.e., crystallization by heating the glassy phase).123,128 The Raman spectra of [C4C1C1C1N][NTf2] shown in Figure 39 indicate there is mixture of [NTf2]− conformers in the normal liquid phase, whereas the cisoid or transoid conformation is obtained

Figure 40. Top panel: Raman spectra from the low-frequency region up to ∼700 cm−1 of a [C4C1im]Cl crystal during its melting process. The lattice vibration at 111 cm−1 and anti/gauche conformer bands (625 and 603 cm−1) are indicated by dashed lines. Time starts counting when melting begins. Bottom panel: time-dependence of intensity of the band at 111 cm−1 and the intensity ratio of anti/gauche bands during the melting process. Arrows indicate 48, 54, and 57 s. Reproduced with permission from ref 372. Copyright 2011 The Chemical Society of Japan.

Figure 39. Raman spectra of [C4C1C1C1N][NTf2] in liquid (298 K, black line) and crystalline phases after slow cooling (240 K, blue line) and cold crystallization (230 K, red line). AI


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melting point.372 The low-frequency Raman spectrum of liquid [C4C1im]Cl has the typical quasi-elastic scattering (see section 5), whereas the crystal phase spectrum has peaks characteristic of lattice modes. Lattice modes gradually disappear during melting, while the quasi-elastic scattering intensity increases. However, the bottom panel of Figure 40 indicates time lag between the disappearance of lattice modes and the equilibration of anti/gauche population in the liquid phase. Thus, conformers remain in local structures, and only after conversion of those arrangements as a whole is the melting process completed.185,372 Endo and Nishikawa191 have also observed that conformational changes are linked to the melting process of [i-C3C1im]I, concluding that the premelting region observed in DSC as broad peaks corresponds to an equilibrium state distinct from liquid and crystalline states. Altogether, the kinetics of ionic liquids during crystallization or melting processes has been assigned to complex conformational changes depending on cooperative rearrangements and a sluggish collective dynamics. Vibrational spectroscopy has been used more recently to investigate crystallization, solid−solid, and glass transition of ionic liquids under high pressure. In analogy with temperaturedependent studies, Raman and IR spectra provide insights on conformational changes accompanying high pressure phase transitions. These studies concerned mainly anti/gauche conformations of 1-alkyl-3-methylimidazolium cations,365,374−390 but some studies also analyzed conformations of ammonium cations 128,235, 391,392 and the [NTf 2 ] − anion.128,364,385,393,394 Many of these papers report the glass transition pressure at room temperature, Pg, according to the method of measuring the pressure dependence of the bandwidth of ruby fluorescence spectrum.395 Stress relaxation is no longer achieved when the glass transition takes place, so that the ruby acts like a microscopic probe of stress heterogeneity in the sample. Thus, the plot of the bandwidth of ruby fluorescence spectrum as a function of pressure exhibits a noticeable change in slope at Pg. The Pg of 1-alkyl-3methylimidazolium-based ionic liquids depends on the alkyl chain length. For instance, Pg ranges from ca. 3.0 to 2.0 GPa along the series [CnC1im][BF4], as n increases from 2 to 8.386 The anion also plays a role in determining the glass transition under high pressure, for instance, Pg is 1.6 and 2.1 GPa for [C8C1im][PF6] and [C8C1im][BF4], respectively.396 Interestingly, plots of the ruby emission bandwidth eventually suggests that other transitions take place for pressures higher than Pg.379,386 Yoshimura et al.386 proposed the formation of other densified structures most probably related to dynamic heterogeneity with the intrinsic hierarchy of structures relaxing at different time scales. However, this is an open issue which deserves further studies. Some ionic liquids in glassy phase at high pressure experience crystallization when decompressed, this finding being the pressure counterpart of cold crystallization when the lowtemperature glass is heated.386,397 Kinetic of high pressure phase transition also exhibits similarities with low-temperature transition. It has been found that [C2C1im][CF3SO3] and [C4C1im][CF3SO3] may become a glass or a crystal depending on the compression rate.387,388 Moreover, large hysteresis of vibrational frequency and bandwidth of the high-pressure [C4C1im][CF3SO3] crystal was found when the pressure was released stepwise back to the atmospheric pressure.387 Raman spectroscopy has been used to show that high pressure crystallization of the protic ionic liquid [C3NH3]-

[NO3] may result in a microscopically heterogeneous sample.235,392 Figure 41A shows two spectral patterns recorded

Figure 41. (A) Spectral patterns in the range of the νs(NO3) Raman band in two different regions of the crystalline sample of [C3NH3][NO3] inside the DAC at ca. 1.5 GPa. Raman intensities have been normalized. (B) Photograph of the DAC sample chamber showing the [C3NH3][NO3] crystal. (C) Micro-Raman imaging using the spectral region indicated by the blue square in (A). Spectra in red and green of (A) correspond to different regions of the mapping in (C). Adapted from ref 235. Copyright 2013 American Chemical Society.

by focusing the laser beam in different regions of the same sample inside the DAC after a quick increase of pressure to ca. 1.5 GPa. (Compare with the νs(NO3) Raman band of [C3NH3][NO3] in the normal liquid phase shown in Figure 34.) A photograph of the sample chamber with the [C3NH3][NO3] crystal is shown in Figure 41B. The distorted arrangement of hydrogen-bonded ions resulting in a distribution of νs(NO3) vibrational frequencies in liquid [C3NH3][NO3] was reproduced by DFT calculation of a cluster of four ionic pairs.235 Thus, distinct local structures can be arrested in isles of microscopic heterogeneity under high pressure, resulting in the anomalous crystallization of [C3NH3][NO3]. The micro-Raman imaging in Figure 41C illustrates the spatial distribution of microscopic heterogeneity of the high-pressure [C3NH3][NO3] crystal. It is worth mentioning that stepwise increase of pressure to 1.5 GPa also generates the microscopic heterogeneity, and the actual νs(NO3) band shape depends on the rate of increasing pressure.235,392 Concerning crystal growth under high pressure, it is usually verified heterogeneous nucleation starting up from the gasket wall in the DAC sample chamber. We provide as a movie showing the crystallization process of [C2C1im][NTf2] inside the DAC at ca. 1.0 GPa. Magnetic ionic liquids are composed of metal-containing anions with magnetic behavior, [FeCl4]− being the most ́ common. Garcia-Saiz et al.398 performed magnetization and Raman spectroscopy studies of [C2C1im][FeCl4] under high pressure. They found that the relatively low applied pressure of 0.34 GPa is enough to modify magnetic interactions and to induce transition from antiferromagnetic to ferromagnetic ordering. Furthermore, [C2C1im][FeCl4] exhibits magnetic hysteresis linked to liquid−solid phase transition due to the alignment of [FeCl4]− anions. Vibrational spectroscopy under high pressure allows one to obtain the volume of conformational change, ΔV, in analogy to the temperature-dependent studies discussed in section 4.3. One obtains ΔV from the pressure dependence of intensities of bands that characterize each conformer, Iconf1 and Iconf2. If one AJ


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obtained when pressure is stepwise increased. [C4C1im][CF3SO3] always crystallizes at low temperature under usual cooling rates,387 and the spectral pattern of the crystal at 0.1 MPa and 230 K does not resemble the pattern of the highpressure crystal. However, a direct qualitative interpretation of different crystal structures based only on the low-frequency vibrational spectrum has to be done with care. Chen et al.399 obtained single-crystal X-ray diffraction data for a series of nitrile functionalized ionic liquids and analyzed low-frequency Raman spectra taking into account the crystal structures. They found that ionic liquids having the same space group exhibit indeed similar low-frequency Raman spectra, but they also warned that the analysis based only on the similarity of Raman spectra might lead to erroneous conclusions and it should be complemented by X-ray diffraction measurements.399 Few works concerning ionic liquid phase transitions have been published using the low-frequency range of vibrational spectra. Roth et al.283 discussed the effects of hydrogen bonds in FIR spectra of [NTf2]− based systems with a series of imidazolium cations with the ring hydrogen atoms substituted by methyl groups, including the solid phase of 1,2,3,4,5pentamethylimidazolium derivative, whose melting point is 391 K. Low-frequency Raman spectroscopy has been used by Okajima and Hamaguchi372 in order to follow the melting process of [C4C1im]Cl (see Figure 40) and by Faria et al.235 to distinguish crystalline phases of [C3NH3][NO3] at different conditions of temperature and pressure. The low-frequency range of the Raman spectrum of [C4C1C1C1N][NTf2] also indicated the glacial state123 (i.e., mixture of microcrystals and supercooled liquid), according to the occurrence of sharp bands of lattice modes on top of the broad band characteristic of amorphous phase. The ionic liquids [C4C1C1C1N][NTf2] and [C1C4C4C4N][NTf2] do not crystallize under high-pressure at room temperature, instead they undergo glass transition at 1.1 and 1.3 GPa, respectively.128 Accordingly, the low-frequency Raman spectrum under high-pressure exhibits low intensity of quasi-elastic scattering and the characteristic boson peak due to intermolecular dynamics as expected for a glassy phase.128 Penna et al.400 have found a correspondence between the position of intermolecular vibrational modes in the liquid state and the spectral features observed after partial crystallization of samples at low temperature or high pressure. This point is illustrated in Figure 43 with the susceptibility representation of the Raman spectra of [C2C1im][NTf2] in supercooled liquid (250 K, ●) and partially crystallized (230 K, black line) phases.400 The components (blue lines) used in the curve fit (red line) of the liquid spectrum seems to correspond to broadening of sharp bands of the crystal spectrum. This finding strongly suggests that there is some kind of mesoscopic order in the supercooled ionic liquid beyond the well-known nanoscale heterogeneity of polar/nonpolar domains.400 However, it is worth noting that in many cases, e.g. [C4C1im]Cl, the broad bands in the low-frequency Raman spectrum of the glassy phase barely indicate any direct correspondence to the sharp peaks in the crystal parent spectrum. It is natural that there are several open questions related to phase behavior of ionic liquids proper to the complexity of structural and kinetic aspects of the transitions under low temperature or high pressure. Studies simultaneously changing temperature and pressure are on demand for covering a wider region of the phase diagram of ionic liquids. These investigations would go beyond the scope of ionic liquids by shedding light on more general issues, such as the interplay

assumes that Raman scattering cross sections for conformers 1 and 2 do not depend on pressure then ΔV is given by ⎡ ∂ln(Iconf2/Iconf1) ⎤ ΔV = −RT ⎢ ⎥ ⎣ ⎦T ∂P

(11)

where R, T, and P are the gas constant, temperature, and pressure, respectively. Takekiyo et al. 378 determined ΔV(planar→nonplanar) = +1.6 cm3/mol for [C2C1im][BF4] and ΔV(anti→gauche) = −0.7 cm3/mol for [C4C1im][BF4]. Capitani et al.364,394 obtained ΔV(transoid→cisoid) for the [NTf2]− anion in [Pyr41][NTf2] and [N1116][NTf2], respectively, − 0.34 and −0.41 cm3/mol. In the case of [N1116][NTf2], Capitani et al.364 obtained ΔV(transoid→cisoid) from IR spectra, and they also obtained ΔV(transoid→cisoid) = +0.7 cm3/mol in a linear regime for pressures higher than Pg (i.e., above 2 GPa). These authors also obtained ΔH for [N1116][NTf2] by temperature-dependent spectra, and they discussed the competition between anion− cation interactions, relative energy of conformers, and the anion volume. The low-frequency range probing lattice dynamics is of course expected to be very sensitive to ionic liquid phase transitions. As discussed in section 5, the low-frequency Raman spectra of amorphous phases exhibit the intense quasi-elastic scattering and the intermolecular vibrations appear as a broad band, the so-called boson peak, in contrast to the sharp peaks of crystal lattice modes. Figure 42 illustrates low-frequency Raman

Figure 42. Low-frequency Raman spectra of liquid, glassy, and crystalline phases of [C4C1im][CF3SO3] obtained under different conditions of temperature and pressure as indicated in the figure. Raman spectra have been shifted vertically to aid in visualization.

spectra at different temperature and pressure conditions for [C4C1im][CF3SO3] as it undergoes glass transition or crystallization. The quasi-elastic scattering dominates the Raman spectrum for the normal liquid phase at 0.1 MPa and 298 K. The Raman spectrum of the glassy phase obtained by sudden increase of pressure (2.4 GPa, 298 K) exhibits the boson peak at ∼21 cm−1 and the imidazolium ring librational mode at ∼140 cm−1. The high-pressure crystal (1.3 GPa, 298 K), with consequent sharp bands in the Raman spectrum, is AK


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word of caution is in order since not all multivariate analysis methods will provide meaningful information in the first approach, a careful choice being fundamental for the employed method and proper data pre and post processing.408 Hydrogen bonding in equimolar mixtures of ionic liquids has been studied by Fumino et al.409 using FIR spectroscopy. These authors performed an investigation of imidazolium-based ionic liquids with systematic methylation in the imidazolium ring while keeping the same [NTf2]− anion409 and also the mixture of ionic liquids with the same triethylammonium cation and methylsulfate and triflate anions.410 The underlying assumption in this approach is the additivity of spectra of different ionic liquids, an issue which has been discussed mainly using optical heterodyne-detected Raman-induced Kerr effect spectroscopy,411,319,412 and also using far409,413 and mid414 infrared spectroscopies. Cha and Kim414 considered the IR bands belonging to γ(CH), ν(C(2)−H), and νas(C(4,5)−H) modes to study hydrogen-bonding effects due to anion coordination in mixtures of regular or C(2)-deuterated [C4C1im]+ cation with different anions, Cl−, I−, [BF4]−, and [NTf2]−. If additivity were valid, the spectrum of the mixture would be the sum of spectra of the neat liquids weighted by the concentration. The agreement between the actual spectrum of the mixture and the concentration-simulated spectrum for [C4C1im]I/[C4C1im] Cl and [C4C1im]Cl/[C4C1im]BF4 indicated that cation−anion interactions were not significantly changed upon mixture. The same conclusion is valid for [C4C1im][BF4]/[C8C1im][BF4] mixture.414 Aparicio and Atilhan415 also concluded for additivity of IR spectra in mixtures of 1-butyl-3-methylpyridinium, [Py1,4]+, and 1-octyl-3-methylpiridinium, [Py1,8]+, with the [BF4]− anion. In contrast, in deuterated [C4C1im]+ mixtures of [NTf2]− with either Cl− or I−, the hydrogen bond imposed by the more coordinating halide anion caused red shift of vibrational frequency of the ν(C(2)-D) mode and change in dipole moment derivative as inferred by the dependence of band area with concentration.414 Analogous effect of mixing anions with different coordination strength has been found by Fumino et al.410 in mixtures of protic imidazolium ionic liquids. Aparicio and Atilhan415 found nonlinear concentration dependence of vibrational frequencies of C−H stretching modes in the range of 2800−3200 cm−1 for [Py1,4][BF4]/[Py1,4][N(CN)2] mixtures. This finding has been attributed to structural changes and dominance of [N(CN)2]− interaction, a conclusion being corroborated by molecular dynamics simulations. Miran et al.416 found anion effects in IR spectra of mixtures of protic trimethylammonium ionic liquids with [NTf2]− and [HSO4]−. Therefore, one finds significant spectral signatures in mixtures of ionic liquids, in particular involving different anions, when there is large difference in coordinating and hydrogen bonding abilities of the ions. Furthermore, the different coordination capacity of anions results in different chemical environments,416 as also suggested by the NMR measurements of the work of Cha and Kim.414 Even though mixtures of cations with different lengths of the alkyl chain (e.g., 1-alkyl-3-methylimidazolium cations), but the same anion, implies only small effects on vibrational spectra in comparison with spectra of the parent neat liquids, the transport coefficients, excess thermodynamic properties, and phase transitions may be greatly altered.417−420 In order to record Raman spectra of glassy [C2C1im][NTf2] at low temperature, Penna et al.400 added a small amount (10% mol) of [C6 C1 im][NTf2] to prevent crystallization of [C2C1im][NTf2]. This allowed studying the characteristic

Figure 43. Low-frequency Raman spectra in the susceptibility representation of [C2C1im][NTf2] in supercooled liquid phase (250 K, ●) and after crystallization (230 K, black line). The curve fit (red line) and the individual components of fit (blue lines) of the supercooled liquid spectrum are shown. Reproduced with permission from ref 400. Copyright 2013 American Institute of Physics.

between glass transition and crystallization, nucleation and crystal growth processes, amorphous−amorphous transitions, and the melting process. Moreover, the mixture of ionic liquid with other compounds can produce hybrid materials with interesting properties and phase transitions. For example, Abe et al.401 used Raman spectroscopy in studying confinement of water in an ionic liquid and the concentration dependence of phase behavior. Furthermore, crystallization of ionic liquid from different solvents might be an efficient method for purification and to generate new crystalline phases, as shown by Li et al.383 for an ionic liquid-methanol solution under high pressure. IR and Raman spectroscopies are powerful tools for addressing many of these issues.

7. VIBRATIONAL SPECTROSCOPY OF IONIC LIQUID SOLUTIONS An appealing feature of the ionic liquid chemistry is the possibility of fine-tuning the properties by proper combination of cations and anions or mixing different ionic liquids.402−404 From the point of view of vibrational spectroscopy, mixtures of ionic liquids with other ionic liquids, molecular solvents, ions, polymers, etc. can serve as model systems for understanding the complex balance of intermolecular interactions determining solvent properties and the liquid structure. A large number of systems has been studied using vibrational spectroscopy, but in this section we limit our discussion to mixtures of ionic liquids with other ionic liquids, water, or other molecular solvents, gases, and salts. The examples given in this section show that attempts to get insights on intermolecular interactions from vibrational spectroscopy of ionic liquids rely on frequency shift, band broadening, intensities variation, etc. take place in the mixtures. The approach of assigning physically meaningful interpretation to spectral changes in Raman excess spectroscopy usually demands a reference spectrum. In Raman excess spectroscopy, or other excess spectroscopies, the deviation of the experimental spectrum of the mixture from the ideal counterpart, which is the weighted average of pure samples spectra, may provide chemical information about interactions in the mixture.405,406 However, this approach is undermined when the reference spectrum cannot be obtained. In this context, Koch et al.407 discussed a multivariate approach for quaternary mixtures of [C2C1im][C2SO4], water, and D- and L-glucose. A AL


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that the population of gauche conformer increases upon the anti conformer when a [C4C1im]+ based ionic liquid is diluted in water.190 Jeon et al.30 also observed that the population of gauche increases with respect to the anti conformer as the water content increases, but the conformers population ratio decreases again beyond ca. 45 mol L−1. These authors attributed the increase in vibrational frequencies of νs(CH3) and νas(CH3) modes of the alkyl chain to interactions, which are mainly repulsive in nature, with the oxygen atoms of water. In line with this finding, Bodo et al.441 also observed a slight blue shift of vibrational modes related to the alkyl chain and the polar head in the Raman spectra of [C4NH3][NO3] protic ionic liquid in mixtures with water. Danten et al.442 carried out a systematic study of [CnC1im]+ with increasing length of the alkyl chain (n = 1, 2, 4, 8), with [BF4]− and [PF6]−, for water content below the limit of solubility in these ionic liquids. These authors used IR and Raman spectroscopies in combination with ab initio calculations for a nearly symmetrical complex made of one water molecule and two anions. The experimental difference of ca. 70−80 cm−1 between symmetric and antisymmetric OH stretching modes was reproduced by the calculations of such complexes, but they did not find any significant effect of the alkyl chain length on the vibrational spectra of the mixtures. Cammarata et al.29 studied the effect of imidazolium ring methylation, [C4C1im]+, [C4C1C1im]+, and 1-butyl-2,3,4,5tetramethylimidazolium, and the anions, [PF6]−, [SbF6]−, [BF 4 ] − , [NTf 2 ] − , [ClO 4 ] − , [CF 3 SO 3 ] − , [NO 3 ] − , and [CF3CO2]−, on IR spectra in ATR and transmission modes with the amount of water ranging from 2540 to 33090 ppm. The enthalpy of vaporization of water molecules from the bulk of the ionic liquid was estimated from the vibrational frequency shift of the antisymmetric stretching mode and resulted in the following order for the water−anion interaction strength: [PF6]− < [SbF6]− < [BF4]− < [NTf2]− < [ClO4]− < [CF3SO3]− < [NO3]− < [CF3CO2]−. The enhancement of the asymmetric stretching mode intensity of water molecule in ionic liquids and in some electrolytic solutions has been found.442,443 Danten et al.443 evaluated the anion dependence of this spectral feature in [C4C1im]+ ionic liquids, keeping the water content below the respective solubility. Using IR, Raman, and ab initio calculations, they showed that the “interaction hierarchy” for anions with water is [PF6]− < [BF4]− < [NTf2]− < [CF3SO3]−. The trend in interaction energy has the correspondence in distances between water and the fluorine atoms on the calculated clusters, 1.84 and 1.72 Å in [PF6]− and [BF4]−, respectively, or the oxygen atoms of the anions, 2.0 and 1.9 Å in [NTf2]− and [CF3SO3]−, respectively.442,443 Dahi et al.444 shared analogous conclusions concerning the anion effect on water uptake and miscibility, resulting in a “hierarchy” similar to one proposed by Danten et al.443 and Tran et al.434 while keeping the same cation and water activity (ca. 0.80). Dahi et al.444 studied the water sorption isotherms and ATR spectra of several ionic liquids with the protic cations [C2NH3]+, [C2im]+, and [C4im]+ and the nonprotic [C4C1im]+ and [C6C1im]+ with anions [PF6]−, [BF4]−, [CF3SO3]−, dibutylphosphate, and bis(2-ethylhexyl)phosphate. These authors found a small effect of the alkyl chain length of the cation for the same anion and whether the cation is protic or not. They also show that characteristic IR bands of free water do not appear in the spectrum of [C 2 NH3 ][CF 3 SO3 ], suggesting that water molecules are part of a hydrogen bond network even at low

boson peak in the low-frequency Raman spectra of glassy [C2C1im][NTf2], [C4C1im][NTf2], and [C6C1im][NTf2]. The boson peak frequency does not depend on the chain length, instead the frequency depends on the strength of the anion− cation interaction when the anion is changed while keeping the same cation.400 Mixtures of ionic liquids with molecular solvents have been used to probe intermolecular interactions and structure, transport properties, phase transitions, and excess thermodynamic properties.401,421−424 Vibrational spectroscopy may address structural features such as nanoscale segregation,422,425,426 solvent properties and ionic liquid polarity,427−429 and intermolecular interactions (e.g., the role played by hydrogen bonding).401,30,430,406,431,432 Methanol, ethanol, ethylene glycol, dimethyl sulfoxide, and water are among the most commonly investigated molecular solvents in ionic liquids solutions. As pointed out in section 2, water might be a problem while handling ionic liquids for spectroscopic studies, but the absorption of water may provide interesting information about structure and intermolecular interactions in ionic liquids. Andanson433 used water as a molecular probe of the effects on the liquid structure when mixing different anions in ternary mixtures of [C4C1im]+ based ionic liquids with [PF6]−, Cl−, Br−, and [NTf2]−. These authors performed ab initio calculations of vibrational frequency of water’s symmetric and antisymmetric stretching modes when the water molecule is bound to one or two anions in the presence of a cation. The relative population of water molecules in different environments is then obtained by adjusting the calculation as a weighted sum of Gaussian band shapes to the experimental data. They found there was no significant segregation in the mixtures and essentially the same affinity of both the anions for water molecules in concentrations as high as 1% mol of water.433 In contrast, Tran et al.434 showed the anion effect on the amount of water absorbed by the ionic liquid by studying the near-infrared region 1400−2000 nm (ca. 5000−7142 cm−1), the absorption coefficient of water at ca. 1419 nm being the most appropriate one to quantify water content. They found that [C4C1im][BF4] absorbs more water than [C4C1im][PF6] or [C4C1im][NTf2] because of stronger hydrogen bonding between water and [BF4]−. The view of stronger [BF4]−−water hydrogen bonding, at least in the comparison between [BF4]− and [PF6]−, was shared by Dominguez-Vidal et al.435 along a FIR spectroscopy study of [C4C1im]+ based ionic liquids. It is worth mentioning that Fadeeva et al.28 did not find different molar absorptivities of water in near-infrared spectra of [Pyr1,4]+ ionic liquids with [CF3SO3]− or [NTf2]−. Furthermore, hydrogen bond strength can be assessed by the shift of vibrational frequencies of ν(C(2)−H) and ν(C(4,5)−H) modes of imidazolium cations in the mid-infrared region with increasing water content.176,423,406,436−439 There are other spectral changes when ionic liquids are mixed with water besides those modes involving the hydrogen atoms of the imidazolium ring. Saha and Hamaguchi440 found all anti conformation of the butyl chains in [CN-C4C1im]Cl and [CN-C4C1im]I crystals, but the intensities of Raman bands belonging to the gauche conformer increase when the crystals were exposed to water. Proper to the appearance of a Raman band at 3300 cm−1 assigned to OH stretching mode, they also suggested that water molecules make a tight hydrogen bond network with the anions displacing them from their original equilibrium position.440 Raman spectroscopy indeed indicates AM


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higher wavenumber, ∼150 cm−1, and the intensity ratio between these two bands as a function of temperature allows for a van’t Hoff analysis of the equilibrium between contact and solvent-separated ion pairs.448 Jiang et al.449 considered IR spectra of deuterated DMSO−[C4C1im][BF4] mixtures and found that the occurrence of polar domains is particularly affected by increasing of pressure. Rodrigues and Santos450 used the CO stretching mode of dimethylformamide as a probe of the alkyl chain length effect in 1-alkyl-3-methylimidazolium bromide ionic liquids. These authors found frequency shift and change in band shape of the CO mode proportional to the length of the alkyl chain. Wang et al.451 used IR spectroscopy and ab initio calculations to study hydrogen bonding of 1butylpiridinium tetrafluoroborate with D2O and deuterated DMSO. The C−H stretching modes of the cation alkyl chain exhibit a blue shift in D2O, but a red shift in DMSO, while the C−D stretching modes of deuterated DMSO exhibit a blue shift in the mixture. These findings were assigned to the behavior of the C−D bonds of deuterated DMSO as electronic density acceptors and the alkyl chain atoms of the cations as electronic density donors. Other molecular solvents carrying the OH group may be used as a probe for anion−cation interactions, hydrogen bonds, and structural features of ionic liquids. Noack et al.423 compared the extent of conventional and unconventional hydrogen bonds in vibrational frequencies of ν(C(2)−H) and νas(C(4,5)−H) modes in the Raman spectra of [C2C1im][C2SO4] in mixtures of methanol and ethanol as cosolvents for water. Conventional and unconventional hydrogen bonds lead to red and blue shifts in vibrational frequencies of modes involving the hydrogen atom, respectively, this behavior being related to changes in both the C−H bond length and the electronic density.423,452 The authors correlated the effects on frequency shift with the competition between dispersion and electrostatic interactions. They also linked the balance between these interactions with several macroscopic properties, such as excess thermodynamic properties and excess transport coefficients.423 Abe et al.422 used Raman spectroscopy, among several other techniques, to probe the relevance of length scales associated with both the alkyl chain size of dialkylimidazolium cations and the one imposed when adding different alcohols. The authors studied 1-alkyl-3-methylimidazolium-based ionic liquids, ranging from ethyl to decyl, with primary (propanol, nbutanol), secondary (2-butanol), and tertiary alcohols (2methyl-2-propanol). Besides consequences on liquid structure and phase transitions, Abe et al.422 found that the cisoid to transoid ratio of [NTf2]− conformers, as revealed by the fingerprint region of the Raman spectrum, exhibits instability close to a critical length of the cation alkyl chain in mixtures with n-butanol, therefore, being a signature of effects of liquid− liquid equilibrium because of interplay between two length scales. Singh et al.427 studied the effects of different cations and anions in IR spectra of [C4C1im][BF4], [C8C1im][BF4], and [C8C1im][C8SO4] in mixtures with ethylene glycol. They assigned the splitting into three bands of the O−H mode of ethylene glycol to different chemical environments, and they also found frequency shifts of ν(C−H) modes of the imidazolium ring, specifically the νas(C(4,5)−H), in line with similar results obtained by Pal et al.452 The νas(S−O) and νs(C(2)−H) modes also exhibit frequency shifts in the [C8C1im][C8SO4]−ethylene glycol mixture.427 In the case of ionic liquids containing [BF4]−, it has been proposed that

concentrations so that vibrational frequencies of symmetric and asymmetric modes are the same as in liquid water.444 There is strong dependence of the νas(SO3) mode of [CF3SO3]− with the water activity, in contrast to [PF6]−, [BF4]−, and [NTf2]−, whose vibrational frequencies are only weakly concentrationdependent. Strong dependence of anion vibrational frequencies with the amount of water has also been reported for [CH 3 COO] − , 34 [CF 3 COO] − , 430 [NO 3 ] − , 441 and [C(CN)3]−.445 The ab initio calculations at the DFT/B3LYP level of theory performed by Danten et al.443 for the water-[C4C1im][NTf2] system considered a 1:2 complex with the [NTf2]− in the cisoid conformation. The anion−water conformation and stoichiometry they used, however, were contrary to previous results. Yaghini et al.438 found that the anion conformation does not change upon addition of water (recall that in pure [C2C1im][NTf2] and [C4C1im][NTf2] the transoid conformation is preferred, see section 4.3). Moreover, Wulf et al.429 pointed out that [NTf2]− is unlikely to form 1:2 complex with water due to steric reasons. When the 1:2 water−anion complexes are considered in order to represent solutions of low-water concentration,429,433,442,443,386 the calculations should also include two cations to take into account five body nonadditive interactions.442,443 High concentration of water seems to disrupt the nanoscale segregation of polar/apolar domains in ionic liquids.441 In the case of intermediate water concentration, some authors claim that the system is homogeneous without phase segregation, but others argue in favor of formation of water clusters. If the water content in N,N-diethyl-N-methyl-N-2-methoxyethylammonium tetrafluoroborate is below 80% (mol), it has been proposed that water is mostly confined since Raman frequencies of symmetric and asymmetric modes are close to the values of free water molecules.401,446 The limit value of 80% is related to the ability of forming 1:4 [BF4]−−water complexes, and beyond that, water starts to interact with the more electronegative part of the cation. The OH stretching modes appear in the spectral range characteristic of bulk water when the amount of water in the ionic liquid is above 90% (mol).401,446 These three regimes of concentration (up to 80%, 80−90%, and above 90%) has been also identified by Yoshimura et al.424 in the plot of vibrational frequency of the νs(BF4) mode versus water content in [C4C1im][BF4]. On the other hand, Fumino et al.276 proposed that the pure protic ionic liquid [C2NH3][NO3] has a threedimensional hydrogen-bonded network, and Bodo et al.441 pointed out that in [C4NH3][NO3] the H2O molecules will take part of the extended hydrogen bond network at any concentration of water. Vibrational spectroscopy has been used to investigate several others molecular solvent−ionic liquid mixtures. Fumino et al.447 combined FIR spectroscopy and ab initio calculations to study the equilibrium between contact and solvent-shared ionic pairs of [C3C3C3NH][CF3SO3] dissolved in different molecular solvents over a wide range of concentration. In the case of solvents of a low dielectric constant (chloroform and tetrahydrofuran), there is larger concentration of contact ionic pairs over solvent-separated ionic pairs, whereas for high dielectric constant (DMSO), solvent-separated ion pairs dominate. FIR spectra of the protic system [C2C2C2NH]I reveal that the IR band at 106 cm−1, which characterizes the contact ion pair of hydrogen bonded cation−anion, remains as the protic ionic liquid is diluted in molecular solvents. The IR band assigned to solvent-separated ion pair is found at a slightly AN


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ionic liquids with different anions, they showed that polarity of [NTf2]− is smaller than [SCN]− based ionic liquids, in line with the conclusions drawn by Wülf et al.429 using water as a probe. The ν(CO) frequency of Fe(CO)5 allowed the authors to propose a series of decreasing polarity as the alkyl chain length increases in [CnC1im][BF4], 3 ≤ n ≤ 10, and [CnC1im][PF6], 3 ≤ n ≤ 8. Garcia et al.461 used frequency shifts of ν(CN) and νs(CD3) modes of normal and deuterated acetonitrile to estimate acceptor (AN) and donor (DN) numbers458 of ionic liquids on the basis of extensive data available for acetonitrile in common molecular solvents. It has been found consistent AN and DN values estimated from either ν(CN) or νs(CD3) modes of acetonitrile. The observed trend is that fluorination of anions implies more acidic local environments (higher AN), while nonfluorinated anions implies more basic environments (higher DN).461 Summing up, the results of vibrational spectroscopy investigations of ionic liquids polarity indicate that the anion plays the dominant role, whereas the effect of changing the length of the alkyl chain of dialkylimidazolium cations is less important. A well-known application of ionic liquids concerns their capacity to absorb gases. This can be a selective process aiming sample purification or gas capture to remove greenhouse gases from the atmosphere.403,404 Vibrational spectroscopy has been used as an analytical tool for quantifying the amount of absorbed gas462 or for studying the mechanism and eventual reactions between the gas molecules and ionic species.32,463 Among the gas−ionic liquid solutions most widely investigated by vibrational spectroscopy, SO2 and CO2 are the gases which draw more attention proper to their impact on the environment. Zeng et al.464 carried out a systematic IR spectroscopy study of SO2 absorption by pyridinium-based ionic liquids with different lengths of the alkyl chain, while keeping the same [BF4]− anion, or different anions, [SCN]−, [BF4]−, and [NTf2]−, while keeping the same [Py4]+ cation. In the case of [Py4][NTf2], the authors did not observe the SO2 bands after gas absorption because of overlapping with the [NTf2]− bands at 1139 and 1352 cm−1. The symmetric and antisymmetric S− O stretching modes of the SO2 molecule, νs(SO) and νa(SO), are observed at 1149 and 1332 cm−1 in [Py4][BF4], and 1124 and 1299 cm−1 in [Py4][SCN], respectively. The vibrational frequencies of νs(SO) and νas(SO) in pure liquid SO2 are 1144 and 1336 cm−1, respectively.42 Although Zeng et al.464 did not address the issue of vibrational frequency shift between pure liquid SO2 and SO2-ionic liquid solutions, it is worth noting that νs(SO) and νa(SO) exhibit shifts to opposite directions when SO2 is dissolved in [Py4][BF4] and a much larger red shift of both νs(SO) and νa(SO) in [Py4][SCN]. Variation of the alkyl chain length of the pyridinium cation, while keeping the same [BF4]− anion, has no effect on vibrational frequencies of SO2 modes. Shang et al.465 and Huang et al.466 studied by IR spectroscopy the SO2 absorption in ionic liquids containing tetramethylguanidinium cations and different anions. Huang et al.446 claimed there is no chemical absorption but only physical absorption when SO2 is absorbed by [TMGH][BF4] or [TMGH][NTf2], as the only spectral feature is occurrence of two νa(SO) modes, 1376 and 1360 cm−1, observed in both ionic liquids. Shang et al.,465 dealing with the same [TMGH]+ cation, but the imidazolate, phenolate, and 2,2,2-trifluoroetanoate anions, argued that new bands at ca. 1410 and 954 cm−1 are due to S−O and S−O−H groups, respectively. The latter

regions rich in either ethylene glycol or ionic liquid are formed. Furthermore, Singh et al.427 used the blue shift of ν(OH) in order to make qualitative inference about the extent of disturbance of the hydrogen bond network of ethylene glycol. Comparatively, the effect is larger for [C8C1im][C8SO4], and in the case of [BF4]− based ionic liquids, the disturbance of the hydrogen bond network of ethylene glycol is larger as the alkyl chain is longer. Pal et al.452 suggested that the interaction that causes frequency shift of νas(C(4,5)−H) is of electrostatic nature, rather than hydrogen bonding between hydrogen atoms of the imidazolium ring and ethylene glycol. On the other hand, the fact that aggregation is not observed in the alkylsulfate-based ionic liquid might be assigned to the strong anion-ethylene glycol hydrogen bond. This finding is in line with conclusions from the IR study by Pal et al.,425 who used 1,2-propanediol as a probe in mixtures with [C4C1im]+ ionic liquids with [NTf2]−, [C1SO4]−, or [BF4]−. They pointed out from ATR measurements that [C1SO4]− disturbs the most the hydrogen bond network of the alcohol, followed by [NTf2]− and [BF4]−. It is worth noting that such order of “strength” of disturbance is somewhat similar to the interaction strength series proposed by Cammarata et al.29 and the polarity series obtained by Wulf et al.429 Shirota et al.453 and Shimomura et al.454 used IR spectroscopy, among other techniques, to study mixtures of benzene in imidazolium ionic liquids by following the effect of the ions on the vibrational frequency of the out-of-plane C−H bending mode, δop(CH), of benzene. They found a significant blue shift of δop(CH), and supported by structural data and quantum chemistry calculations, they suggested that the interaction between benzene and the imidazolium ring is stronger than interactions between benzene rings (π···π interactions) or between benzene and hydrogen atoms (C−H···π interactions). Along the comparison between [C8C1im][BF4] and [C8C1im][NTf2], Shirota et al.453 assigned the larger frequency shift in the latter to weaker interaction between [NTf2]− and the cation, allowing for stronger interaction between the probe molecule (benzene) and the cation. One could argue from the results of Shimomura et al.454 that the alkyl chain length plays a less important role because the vibrational frequency shift of δop(CH) is almost the same in the [C12C1im][NTf2]−benzene mixture. It is worth mentioning that benzene−ionic liquid mixtures have been studied more often by OKE spectroscopy.205,453−457 Vibrational spectra of different solutes have been used to probe the solvent ability of ionic liquids. Wülf et al.429 used water as a probe of solvent polarity of imidazolium ionic liquids with the anions [SCN]−, [N(CN)2]−, [C2SO4]−, and [NTf2]−. Their method relies on measuring the redshift of vibrational frequencies of stretching modes of water, whose magnitude is then related to dielectric constant and different polarity parameters (e.g., the Kamlet−Taft solvatochromic parameters).458,459 The anion increases the ionic liquid polarity in the order: [NTf2]− < [C2SO4]− < [N(CN)2]− < [SCN]−, while the length of the alkyl chain of imidazolium cations has negligible effect on polarity of the investigated ionic liquids. Other molecules besides water have been used to probe ionic liquid polarity using vibrational spectroscopy. Tao et al.460 considered the (CO) mode of acetone, N,N-dimethylformamide, and Fe(CO)5 as polarity probes. The authors did not provide parameters to quantify the polarity, but they showed qualitatively the correlation between the polarity and the vibrational frequency shift of (CO). Using acetone as a probe in AO


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1665 cm−1) and Raman (ca. 794, 1323, and 1672 cm−1) spectra. Figure 44 taken from the work of Cabaço et al.162

band was also observed when SO2 was dissolved in another imidazolate ionic liquid with the 1-(N,N-diethylaminoethyl)-3methylimidazolium cation,467 so that the authors claimed that the mechanism of SO2 uptake in these ionic liquids involves chemical absorption. Ando et al.468 and Siqueira et al.469 studied both the high- and the low-frequency spectral features of Raman spectra of [C4C1im]Br after SO2 absorption. Solid [C4C1im]Br at room temperature melts upon absorption of SO2. When SO2 is absorbed by [C4C1im]Br, the relative intensities of Raman bands at ca. 600 and 620 cm−1, which characterize gauche and anti conformers (see Figure 20), become more similar to that found in [C4C1im]I. Furthermore, vibrational frequency shifts of νs(SO) and νa(SO) modes were attributed to specific charge transfer interactions between SO2 and Br−. The proposed physical picture of shielding the cation−anion interactions after uptake of SO2 was supported by molecular dynamics simulations.468,469 The authors found good agreement between the density of states calculated by the Fourier transform of the autocorrelation function of velocity (see section 3) and the low-frequency Raman spectra at 100 K. Kazarian et al.470 obtained ATR spectra of subcritical CO2 dissolved in [C4C1im][BF4] and [C4C1im][PF6] at 40 °C and 6.8 MPa. Splitting of the IR band belonging to the bending mode of CO2 has been found. Such splitting of the band is due to the lifting of degeneracy, with the expectation that the splitting would be larger for solvents with more pronounced Lewis base character. Since the splitting was more pronounced in [C4C1im][BF4] than [C4C1im][PF6], Kazarian et al.470 concluded that [BF4]− is a stronger Lewis base than [PF6]−. Andanson et al.471 characterized CO2 solutions in [C4C1im][PF6] under different CO2 pressure at 40 °C using ATR spectroscopy. The authors were able to estimate the swelling of the ionic liquid with increasing CO2 pressure, the CO2 diffusion within the ionic liquid, and solubility in [C4C1im][PF6]. The vibrational spectrum of [C4C1im][PF6] suggested only minor structural modifications after CO2 uptake. The most significant effects of CO2 absorption on the IR spectrum were the change in relative intensities of bands at 600 and 625 cm−1, which characterize the relative proportion of gauche and anti [C4C1im]+ conformers and frequency shift of the νs(PF) mode of [PF6]−.471 Seki et al.472 studied supercritical CO2 dissolved in [C4C1im]+ based ionic liquids with [PF6]−, [BF4]−, and [NTf2]−, and [Py4][BF4], using ATR spectroscopy at 50 °C and 12 MPa. In contrast to Kazarian et al.,470 Seki et al.472 have not found splitting of the CO2 bending mode in the CO2 solution in [C4C1im][BF4], arguing that the doublet convolutes into a single band under higher CO2 pressure. In the case of CO2 solution in [Py4][BF4], no frequency shift of [Py4]+ modes was observed, whereas in [C4C1im][BF4] there were frequency shifts of the cation νs(C(4,5)−H) mode and the anion νs(BF) mode. Seki et al.472 claimed that CO2 interacts mainly with the [BF4]− and that the new species formed CO2-[BF4]− is a stronger base than the single [BF4]−. Vibrational spectroscopy has been extensively used to study CO2 absorption in imidazolium ionic liquids with the [CH3COO]− anion.32,162,473,474,462 The mechanism of chemical absorption of CO2 in [C4C1im][CH3COO] based on the reaction between [CH3COO]− and the C(2)−H acidic hydrogen atom of the imidazolium ring, leading to the formation of a carbene complex with CO2, 1-butyl-3methylimidazolium-2-carboxylate, was verified through IR and Raman spectroscopies.32,162 Formation of such carboxylate species leads to new bands in both the IR (ca. 792, 1323, and

Figure 44. Comparison of Raman (top) and IR (bottom) spectra of [C4C1im][CH3COO] and its mixtures with CO2 (red) and 13CO2 (blue) (mole fraction less than ca. 0.3). The arrows pinpoint the three new bands assigned to 1-butyl-3-methylimidazolium-2-carboxylate. Reproduced with permission from ref 162. Copyright 2012 American Chemical Society.

indicates by arrows these IR and Raman appearing in the spectra of [C4C1im][CH3COO] containing CO2 or 13CO2. Cabaço et al.162 draws attention to the fact that, at molar fractions smaller than 0.3 (or CO2 pressures of the order of 6 MPa), the spectra do not exhibit the Fermi dyad [i.e., the overtone of the CO2 bending mode in Fermi resonance with the νs(CO)].475 This indicates that the CO2 symmetry has been altered in [C4C1im][CH3COO], whereas the Fermi dyad appears when CO2 is dissolved in ionic liquids containing anions other than [CH3COO]−. Besides [CH3COO]− based ionic liquids, there are reports of other anions with carboxylate474,476 or phenolate477 groups which also favor mechanism of chemical absorption of CO2. Furthermore, the vibrational dynamics of CO2 in ionic liquids has also been studied by time-resolved IR spectroscopy.478 Other important contexts for ionic liquid applications include electrochemistry and catalysis, in which they can be used as solvents for electrodeposition (or electroplating) of materials, electrolytes for batteries, etc. Dissolution of precursors or catalysts (e.g., TaCl5, AlCl3, and NbCl5) or ionic species (e.g., Li + and Na + ) may be studied using IR and Raman spectroscopies for a deeper understanding of solvation and structural modification of the ionic liquid upon dissolution of small ions. In analogy to the previously discussed AlCl3 mixtures with imidazolium- and pyrrolidinium-based ionic liquids (see section 4.1), in which speciation of aluminum as [AlCl4]− or [Al2Cl7]− was inferred by IR and Raman spectroscopies,96,217,479 spectroscopic studies have been carried out for other metals, for instance, tantalum and niobium, whose deposition process cannot be done in water.480,481 Babushkina480 studied TaCl5-[Py1,4]Cl mixtures using IR spectroscopy over a wide range of compositions at room temperature. It has been found that all of the vibrational modes of [Py1,4]+ are disturbed upon dissolution of the tantalum salt, the ring breathing vibrations within 890−930 cm−1 being the most affected modes exhibiting change on relative intensities and frequency shift depending on the TaCl5 concentration.480 Using Raman spectroscopy, Alves et al.481 studied NbCl5AP


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Lithium cation solutions in ionic liquids have been widely studied because of the application as electrolyte in lithium batteries. Ionic liquids based on the [NTf2]− anion show remarkable structural and dynamical changes when mixed with the lithium salt. For example, it was shown by NMR measurements492 that in Li[NTf2]−[C4C1im][NTf2] mixtures the Li+ self-diffusion coefficient drops by almost a third when the concentration ranges from 2% to 22% of Li+. Nicolau et al.493 using OKE spectroscopy estimated that the viscosity of [C 4 C 1 im][NTf2 ] increases by almost ten times when approximately 40% mole fraction of Li+ is added. The overall observed trend for different systems which have been studied is that increasing Li+ content results in increase of viscosity and density and decrease of self-diffusion coefficient and conductivity.494−497 These effects on transport coefficients upon addition of Li+ are accompanied by conformational changes of the [NTf2]− anion as revealed by vibrational spectroscopy. Lassègues et al.498,499 and Duluard et al.500 used Raman spectroscopy to study mixtures of [C2C1im][NTf2] and [C4C1im][NTf2] with Li[NTf2] over a wide range of concentration. The authors deconvoluted the anion Raman band with the maximum at ∼745 cm−1 into two components, the lower wavenumber component being assigned to uncoordinated (“free”) [NTf2]− and the higher wavenumber component to coordinated (“bounded”) [NTf2]− to Li+. These authors500,498,499 evaluated the coordination number of Li+ equal to two and then inferring for the formation of the [Li(NTf 2 ) 2 ] − species in line with the proposition of Umebayashi et al.501−503 According to Lassègues et al.498 and Hardwick et al.,504 such complexes are weakly bounded, being promptly destabilized in the presence of other highly coordinating solvents (e.g., diglyme, tetraglyme, and ethylene carbonate). As pointed out by Martins et al.,505 the formation of [Li(NTf2)2]− is also influenced by water content. The splitting of the 745 cm−1 band depends on the Li+ molar fraction, and a pseudoisosbestic point is observed in the Raman spectra.501 Furthermore, Umebayashi et al.506 showed that in the series of alkali cations, from Li+ to Cs+, the coordination number grows from two to four. Monteiro et al.496 argued that the formation of [Li(NTf2)2]− is the main reason for the decrease of Li+ ionic mobility and conductivity in Li[NTf2]− [C4C1C1im][NTf2] mixtures. Duluard et al.492 studying [C4C1im][NTf2] mixtures with Li+ pointed out that the conformation of the butyl chain remains essentially unchanged upon dissolution of Li+. Umebayashi et al.503 and Lassègues et al.499 addressed the issue of [NTf2]− conformation around the Li+ ion, analyzing the fingerprint range of the Raman spectrum of the [NTf2]− anion with the aid of quantum chemistry calculations at the DFT/B3LYP level of theory. In both of these papers, the authors concluded that the cisoid conformation is favored in the 1:1 (Li+:[NTf2]−) complex, in contrast to the situation in the neat ionic liquid, in which the transoid conformation dominates. In the case of 1:2 complex, the authors498,503 suggest that the complex structures involves the two anions in cisoid or transoid, or a mix of cisoid−transoid conformations. Lassègues et al.499 claim that the mix of [NTf2]− conformers gives better agreement to the fingerprint region of the Raman spectra. The above-mentioned effects of Li+ dissolution on the Raman spectrum of [C4C1im][NTf2] is illustrated in Figure 45. This figure shows Raman spectra of pure [C4C1im][NTf2], solid Li[NTf2], and the mixture, in the range of 250−450 cm−1, which characterizes the [NTf2]− conformation and the 741

[C4C1im]Cl and ZnCl2-[C4C1im]Cl over a wide range of compositions at room temperature. Besides speciation of niobium ([NbCl6]− and Nb2Cl10) and zinc ([ZnCl4]2−, [Zn3Cl8]2−, and [Zn4Cl10]2−), these authors reported several changes in the Raman spectra of the mixtures in comparison with the neat ionic liquid spectrum, in particular, in the C−H modes of the imidazolium481 as it has also been found by Goujon et al.482 in water-ZnCl2 or water-MgCl2 mixtures with [C8C1im]Cl. Andriyko et al.483 studied TiF4-[C4C1C1im][BF4] using IR spectroscopy over a wide range of concentrations. The authors were able to observe spectral features, indicating the formation of [TiF6]2− and a heteronuclear complex between TiF4 and the anion, [TiBF8]−. They also found decreasing intensity of anion bands because of a side reaction leading to BF3 evolution and formation of [TiF6]2−. Arellano et al.484 studied Zn[NTf2]2-[C2C1im][NTf2] mixtures using IR spectroscopy with the support of quantum chemical calculations. These authors claim that a new anionic species, [Zn(NTf2)3]−, is formed upon the addition of a stoichiometric amount of the solute. Formation of [Zn(NTf 2 ) 3 ] − results in frequency shift of [NTf 2 ] − and [C2C1im]+ modes, in particular the ν(C(2)−H) mode. Curve fit of spectra showed significant changes, such as the number of components in the spectral range of 700−820 cm−1, between spectra of mixture and the neat ionic liquid, but the authors did not address this issue.484 In line with the zinc coordination compound, there are similar reports of stable compounds formed between ytterbium485 and europium486 with [NTf2]−. Liu et al.487 studied 0.2 mol L−1 solutions of Zn[CF3SO3]2 in [C1im][CF3SO3], [C2C1im][CF3SO3], and [C2C1C1im][CF3SO3] at 393 K, using Raman spectroscopy. The authors considered the δs(CF3) in order to infer about the coordination of Zn2+ ions and the changes on local environment upon addition of the solute. The authors found that addition of Zn2+ causes a higher wavenumber shoulder in the δs(CF3) Raman band due to those anions coordinating the Zn2+, while the lower wavenumber band is assigned to uncoordinated (“free”) anions.487 Methylation of the imidazolium ring (i.e., from [C1im]+ to [C2C1C1im]+), implies larger splitting of vibrational frequencies between free and bounded anions. The authors estimated the coordination number of anions around Zn2+ from the ratio between the band areas of free and bounded anions. In the case of ionic liquid based on [C1im]+, [C2C1im]+, and [C2C1C1im]+, the most favored species are [Zn(CF3SO3)3]−, [Zn(CF3SO3)4]2−, and [Zn(CF3SO3)5]3−, respectively.487 Oliveira et al.488 using Raman spectroscopy studied the dissolution of [NH4]+, with [CH3COO]−, Cl−, [SCN]−, and [C2SO3]− as counterions, and Na+, with [CH3COO]− and [SCN]− as counterions, in [C2C1im][CH3COO]. The authors showed that the [CH3COO]− mode at 910 cm−1 is a good probe of the local environment as the corresponding Raman band is sensitive to the dissolved species. Chimdi et al.489 studied Na[N(CN) 2]−[Pyr1,1 ][N(CN) 2] mixtures using Raman spectroscopy, whereas Carstens et al.490 studied Na[N(SO2F)2]−[Py1,4][N(SO2F)2] and Forsyth et al.491 studied Na[NTf2]−[Pyr1,2][NTf2] mixtures using Raman and ATR spectroscopies. Chimdi et al.489 and Carstens et al.490 reported only frequency and intensity changes of Raman bands related to anion modes, but Forsyth et al.491 observed also the occurrence of shoulders at 534, 576, 598 651, and 1150 cm−1 in IR bands, the latter being assigned to the twisting mode of the pyrrolidinium ring. AQ


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spectra of the liquid phase. Fortunately, the computational resources available today allow for quantum chemistry calculations of vibrational frequencies for clusters made of a few ion pairs. However, the approach of cluster calculation still relies on an optimized geometry of minimum energy. Thus, ab initio molecular dynamics simulations of ionic liquids open the perspective for vibrational frequency calculations with the proper account of liquid dynamics. On the experimental point of view, linear IR and Raman spectroscopies of ionic liquids, which were the focus of this review, are being recently extended for time-resolved vibrational spectroscopy. Structural fluctuations of the local environment experienced by a probe oscillator cause vibrational dephasing due to energy relaxation and loss of phase (pure dephasing). The traditional approach using linear IR and Raman spectroscopies by Fourier transforming the band shape in order to get time correlation functions of vibrational dephasing and reorientational dynamics74,83,85,92 has been applied for the CN stretching mode of ionic liquids containing cyano-anions since the corresponding Raman band is relatively free of overlapping bands.106 However, this methodology is not fully appropriate when processes of multiple time ranges simultaneously contribute with homogeneous and inhomogeneous mechanisms for the band shape. Thus, perspectives for vibrational spectroscopy of ionic liquids include ongoing studies by time-resolved techniques, such as time-resolved IR spectroscopy and coherent anti-Stokes Raman scattering (CARS),6−11 providing insights on the short-time molecular dynamics. For instance, time-resolved CARS measurements give dephasing times in the subpicosecond range for C−H stretching modes of 1-alkyl-3-methylimidazolium cations that can be related to flexibility of molecular structure and the strength of different sites for hydrogen bonding to the anion.10 Time-resolved IR spectroscopy has also been used to follow the time evolution of modes belonging to molecule probes dissolved in ionic liquids. This allows addressing the molecular dynamics at the different time scales of fluctuations of the hydrogen bond structure and reorientational motions and the relative contributions of homogeneous and inhomogeneous broadening to the band shape.517,518 Thus, time-resolved vibrational spectroscopy is expected to provide experimental data related to short-time molecular dynamics allowing for direct comparison to results of computer simulations of ionic liquids. Nevertheless, there is plenty of room for applications of linear IR and Raman spectroscopies in many areas related to the ones discussed in this review. Besides the application on ionic liquid solutions discussed in this review, vibrational spectroscopy is a complementary technique within the broad field of research on hybrid materials. Vibrational spectroscopy has been used in combination with other techniques to characterize ionic liquid interactions with nanotube and graphene,519−524 clays,525−529 polymers,530,492,531−539 carbohydrates,540−544 etc. Furthermore, surface-enhanced Raman scattering (SERS) studies have been reported for ionic liquids with different metals as substrates.545−549 Within the context of studies on phase transitions, vibrational spectra of ionic liquids have been reported as a function of temperature, at atmospheric pressure, or as a function of pressure, at room temperature. Simultaneous variation of temperature and pressure aiming more complete mapping of the phase diagrams most probably will be the subject of further vibrational spectroscopy studies of ionic liquids.

Figure 45. Comparison between Raman spectra of pure [C4C1im][NTf2] (red line), solid Li[NTf2] (black line), and the mixture Li[NTf2]−[C4C1im][NTf2]) at molar fraction of Li[NTf2] equal to 0.37 (blue line). Spectra have been normalized by the most intense band at each spectral window.

cm−1 band, whose frequency shift and split characterizes coordinated and uncoordinated [NTf2]−. Despite identifying more than one possible arrangement for the [Li(NTf2)2]− complex,499,503 no more than two components were assumed under the band at ∼745 cm−1. In contrast, Pitawala et al.495,507 and Watkins et al.,508 the latter studying Mg[NTf2]2, accounted for three or more components while fitting this band with the support of previously reported quantum chemistry calculations.500,499,501−503 Pitawala et al.507 considered three bands in the fit of the 741 cm−1 Raman band in Li[NTf2]−[Pyr1,4][NTf2] mixtures and reached the same coordination number as previous works for high molar fractions of Li+. However, for low concentrations of Li+ (equal or below 0.05), the authors obtained a coordination number as high as four. Change in coordination number might have consequences in transport coefficients, glass transition, and melting temperatures of the mixtures.114,495−497,507 It is worth stressing that only part of the literature concerning ionic liquid solutions was reviewed in this section proper to the broadness of the theme and the large number of examples in which vibrational spectroscopy served only as a complementary tool to better understand some process (e.g., cellulose and carbohydrates dissolution) or for the characterization of new materials. It is also worth mentioning solvate ionic liquids,509−516 which are systems composed from weakly coordinating anions such as [NTf2]−, small cations (e.g., Li+) and a chelating molecular solvent such as tetraglyme,512 not discussed in this review. Such systems are of increasing interest as their properties might extend the range of applications of ionic liquids.

8. CONCLUDING REMARKS In this review, we discussed the application of vibrational spectroscopy for getting information on structure and intermolecular interactions in ionic liquids and the related experimental and theoretical issues. Quantum chemistry calculations of vibrational frequencies beyond the harmonic approximation may be needed for proper assignment and for including important anharmonicity effects on vibrational spectra of ionic liquids (e.g., Fermi resonance). Strong ionic interactions may imply that an ab initio calculation of vibrational frequencies for an isolated ion eventually only estimates the frequencies when compared with the actual AR


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ASSOCIATED CONTENT

ionic liquids with particular focus on changes observed along the glass transition and crystallization taking place under low temperature or high pressure.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemrev.6b00461. IR and Raman spectra recorded in this work are available as TXT files. Each file is identified by the number of the figure, the ionic liquid name, and whether it corresponds to Raman or IR spectrum. The files give Raman and IR spectra covering the same spectral window as shown in the corresponding figure of the paper. The file DAC_Crystallization.wmv is a movie showing the real time crystallization process of [C2C1im][NTf2] inside the diamond anvil cell at 0.7 GPa and room temperature (ZIP)

ACKNOWLEDGMENTS The authors acknowledge the Brazilian agencies CNPq and FAPESP (Grant nos. 2015/07516-8, 2015/05803-0, and 2012/ 13119-3) for fellowships and financial support. REFERENCES (1) Papatheodorou, G. N.; Yannopoulos, S. N. Light Scattering from Molten Salts: Structure and Dynamics. In Molten Salts: From Fundamentals to Applications; Gaune-Escard, M., Ed. Springer: Dordrecht, The Netherlands, 2002; pp 47−106. (2) Papatheodorou, G. N.; Kalampounias, A. G.; Yannopoulos, S. N. Raman Spectroscopy of High Temperature Melts. In Molten Salts and Ionic Liquids. Never the Twain?, Gaune-Escard, M., Seddon, K. R., Eds.; John Wiley & Sons: Hoboken, NJ, 2010; pp 301−340. (3) Berg, R. W. Raman Spectroscopy and Ab-initio Model Calculations on Ionic Liquids. Monatsh. Chem. 2007, 138, 1045−1075. (4) Saha, S.; Hiroi, T.; Iwata, K.; Hamaguchi, H. Raman spectroscopy and the heterogeneous liquid structure In Ionic Liquids Completely UnCOILed: Critical Expert Overviews; Plechkova, N., Seddon, K. R., Eds.; Wiley: Hoboken, NJ, 2015; p 165. (5) Penalber-Johnstone, C.; Baldelli, S. Vibrational spectroscopy of ionic liquids surface. In Ionic Liquids Completely UnCOILed: Critical Expert Overviews; Plechkova, N., Seddon, K. R., Eds.; Wiley: Hoboken, NJ, 2015; p 145. (6) Dahl, K.; Sando, G. M.; Fox, D. M.; Sutto, T. E.; Owrutsky, J. C. Vibrational Spectroscopy and Dynamics of Small Anions in Ionic Liquid Solutions. J. Chem. Phys. 2005, 123, 084504. (7) Shigeto, S.; Hamaguchi, H. Evidence for Mesoscopic Local Structures in Ionic Liquids: CARS Signal Spatial Distribution of C(n)mim PF6 (n = 4,6,8). Chem. Phys. Lett. 2006, 427, 329−332. (8) Iwata, K.; Okajima, H.; Saha, S.; Hamaguchi, H. Local Structure Formation in Alkyl-Imidazolium-Based Ionic Liquids as Revealed by Linear and Nonlinear Raman Spectroscopy. Acc. Chem. Res. 2007, 40, 1174−1181. (9) Namboodiri, M.; Kazemi, M. M.; Zeb Khan, T.; Materny, A.; Kiefer, J. Ultrafast Vibrational Dynamics and Energy Transfer in Imidazolium Ionic Liquids. J. Am. Chem. Soc. 2014, 136, 6136−6141. (10) Chatzipapadopoulos, S.; Zentel, T.; Ludwig, R.; Luetgens, M.; Lochbrunner, S.; Kühn, O. Vibrational Dephasing in Ionic Liquids as a Signature of Hydrogen Bonding. ChemPhysChem 2015, 16, 2519− 2523. (11) Kiefer, J.; Namboodiri, M.; Kazemi, M. M.; Materny, A. TimeResolved Femtosecond CARS of the Ionic Liquid 1-Ethyl-3methylimidazolium Ethylsulfate. J. Raman Spectrosc. 2015, 46, 722− 726. (12) Stuchebryukov, S. D.; Rudoy, V. M. Attenuated total reflection spectra under conditions of weak absorption: Physical nature. Opt. Commun. 1997, 140, 36−40. (13) Bertie, J. E.; Michaelian, K. H. Comparison of Infrared And Raman Wave Numbers of Neat Molecular Liquids: Which is the Correct Infrared Wave Number to Use? J. Chem. Phys. 1998, 109, 6764−6771. (14) Hancer, M.; Sperline, R. P.; Miller, J. D. Anomalous Dispersion Effects in the IR-ATR Spectroscopy of Water. Appl. Spectrosc. 2000, 54, 138−143. (15) Buffeteau, T.; Grondin, J.; Lassègues, J.-C. Infrared Spectroscopy of Ionic Liquids: Quantitative Aspects and Determination of Optical Constants. Appl. Spectrosc. 2010, 64, 112−119. (16) Mirabella, F. M. Principles, Theory and Practice of Internal Reflection Spectroscopy. In Handbook of Vibrational Spectroscopy, Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, U.K., 2002; Vol. 2, pp 1091−1102. (17) Fitzpatrick, J.; Reffner, J. A. Macro and Micro Internal Reflection Accessories. In Handbook of Vibrational Spectroscopy; Chalmers, J. M.,

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Vitor H. Paschoal: 0000-0002-0935-3772 Luiz F. O. Faria: 0000-0002-0711-4604 Mauro C. C. Ribeiro: 0000-0002-4301-5021 Notes

The authors declare no competing financial interest. Biographies Vitor Hugo Paschoal received his degree in chemistry from the State University of Londrina (Brazil) in 2013 starting his Ph.D. studies under the supervision of Prof. Mauro C. C. Ribeiro in early 2014. His research interests are molecular dynamics simulations and spectroscopy of high-frequency collective dynamics of liquids and their glassy phases (especially ionic liquids and solutions). Luiz Felipe de Oliveira Faria received his degree in chemistry from Federal University of Juiz de Fora in 2010 and his Ph.D. degree from University of São Paulo (Brazil) in 2015. He completed his Ph.D. under the supervision of Prof. Dr. Mauro C. C. Ribeiro on the structure and phase transitions of ionic liquids in different conditions of temperature and pressure. He is currently a postdoc in Prof. Mauro C. C. Ribeiro group, and his research has been focused on the structure and phase transitions of ionic liquids under high pressure using Raman spectroscopy and X-ray scattering techniques. Mauro Carlos Costa Ribeiro is an Associate Professor at the Chemistry Institute of the Universidade de São Paulo, IQ-USP. Mauro obtained his Bachelor degree in Chemistry in 1989 from Universidade Santa ́ (Santos-SP). He obtained his Master’s degree in 1992 and his Cecilia Ph.D. degree in 1995, both from Universidade de São Paulo under supervision of Prof. Paulo S. Santos. His Master’s studies concerned calculations of resonant Raman spectra and his Ph.D. thesis on Raman spectroscopy and molecular dynamics of liquids. He started teaching at IQ-USP in 1996. Mauro spent one and a half years (1996−1998) as a postdoc in the group of Prof. Paul A. Madden at Oxford University, U.K., working with molecular dynamics of high-temperature molten salts. After returning to São Paulo, he started working with vibrational spectroscopy and molecular dynamics simulation of room-temperature ionic liquids. He also spent shorter periods working in the groups of Prof. Giancarlo Ruocco in 2007 (Universitá di Roma La Sapienza) and ́ A. H. Pádua in 2013 (Université Blaise Pascal, ClermontProf. Agilio Ferrand, France). His research concerns structure and dynamics of AS


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(36) Bassett, W. A. Diamond Anvil Cell, 50th Birthday. High Pressure Res. 2009, 29, CP5−163. (37) Polsky, C. H.; Valkenburg, E. V. The Diamond Anvil Cell. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, UK, 2002; Vol. 2, pp 1352− 1360. (38) Forman, R. A.; Block, S.; Barnett, J. D.; Piermarini, G. J. Pressure Measurement Made by Utilization of Ruby Sharp-Line Luminescence. Science 1972, 176 (4032), 284−285. (39) Syassen, K. Ruby Under Pressure. High Pressure Res. 2008, 28, 75−126. (40) Faria, L. F. O.; Nobrega, M. M.; Temperini, M. L. A.; Ribeiro, M. C. C. Ionic Liquids Based on the Bis(Trifluoromethylsulfonyl)Imide Anion for High-Pressure Raman Spectroscopy Measurements. J. Raman Spectrosc. 2013, 44, 481−484. (41) Wilson, E. B., Jr.; Decius, J. C.; Cross, P. C. Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra; McGraw-Hill: New York, 1955. (42) Herzberg, G. Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; D. van Nostrand Co.: Princeton, NJ, 1945; Vol. 2. (43) Baddiel, C. B.; Janz, G. J. Molten Thiocynates: Raman Spectra and Structure. Trans. Faraday Soc. 1964, 60 (503P), 2009−2012. (44) Izgorodina, E. I.; Bernard, U. L.; MacFarlane, D. R. Ion-Pair Binding Energies of Ionic Liquids: Can DFT Compete with Ab InitioBased Methods? J. Phys. Chem. A 2009, 113, 7064−7072. (45) Rigby, J.; Izgorodina, E. I. New SCS- and SOS-MP2 Coefficients Fitted to Semi-Coulombic Systems. J. Chem. Theory Comput. 2014, 10, 3111−3122. (46) Garcia, G.; Atilhan, M.; Aparicio, S. Assessment of DFT Methods for Studying Acid Gas Capture by Ionic Liquids. Phys. Chem. Chem. Phys. 2015, 17, 26875−26891. (47) Berg, R. W.; Deetlefs, M.; Seddon, K. R.; Shim, I.; Thompson, J. M. Raman and ab initio Studies of Simple and Binary 1-Alkyl-3methylimidazolium Ionic Liquids. J. Phys. Chem. B 2005, 109, 19018− 19025. (48) Katsyuba, S. A.; Zvereva, E. E.; Vidis, A.; Dyson, P. J. Application of Density Functional Theory and Vibrational Spectroscopy Toward the Rational Design of Ionic Liquids. J. Phys. Chem. A 2007, 111, 352−370. (49) Umebayashi, Y.; Fujimori, T.; Sukizaki, T.; Asada, M.; Fujii, K.; Kanzaki, R.; Ishiguro, S. Evidence of Conformational Equilibrium of 1Ethyl-3-methylimidazolium in its Ionic Liquid Salts: Raman Spectroscopic Study and Quantum Chemical Calculations. J. Phys. Chem. A 2005, 109, 8976−8982. (50) Jamróz, M. H. Vibrational Energy Distribution Analysis (VEDA): Scopes and Limitations. Spectrochim. Acta, Part A 2013, 114, 220−230. (51) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. (52) Talaty, E. R.; Raja, S.; Storhaug, V. J.; Dölle, A.; Carper, W. R. Raman and Infrared Spectra and ab Initio Calculations of C2−4MIM Imidazolium Hexafluorophosphate Ionic Liquids. J. Phys. Chem. B 2004, 108, 13177−13184. (53) Heimer, N. E.; Del Sesto, R. E.; Meng, Z. Z.; Wilkes, J. S.; Carper, W. R. Vibrational Spectra of Imidazolium Tetrafluoroborate Ionic Liquids. J. Mol. Liq. 2006, 124, 84−95. (54) Katsyuba, S. A.; Dyson, P. J.; Vandyukova, E. E.; Chernova, A. V.; Vidis, A. Molecular Structure, Vibrational Spectra, and Hydrogen Bonding of the Ionic Liquid 1-Ethyl-3-methyl-1H-imidazolium Tetrafluoroborate. Helv. Chim. Acta 2004, 87, 2556−2565. (55) Darling, B. T.; Dennison, D. M. The Water Vapor Molecule. Phys. Rev. 1940, 57, 128−139. (56) Barone, V. Anharmonic Vibrational Properties by a Fully Automated Second-Order Perturbative Approach. J. Chem. Phys. 2005, 122, 014108.

Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, UK, 2002; pp 1103−1116. (18) Bertie, J. E. Optical Constants. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, UK, 2002; Vol. 1, pp 88−100. (19) Bertie, J. E.; Eysel, H. H. Infrared Intensities of Liquids 0.1. Determination of Infrared Optical and Dielectric-Constants by FT-IR Using the Circle ATR Cell. Appl. Spectrosc. 1985, 39, 392−401. (20) Burba, C. M.; Frech, R. Existence of Optical Phonons in the Room Temperature Ionic Liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. J. Chem. Phys. 2011, 134, 134503. (21) Burba, C. M.; Janzen, J.; Butson, E. D.; Coltrain, G. L. Using FT-IR Spectroscopy to Measure Charge Organization in Ionic Liquids. J. Phys. Chem. B 2013, 117, 8814−8820. (22) Burba, C. M.; Janzen, J.; Butson, E. D.; Coltrain, G. L. Correction to ″Using FT-IR Spectroscopy to Measure Charge Organization in Ionic Liquids″. J. Phys. Chem. B 2016, 120, 3591− 3592. (23) Decius, J. C. Dipolar Coupling and Molecular Vibration in Crystals. I. General Theory. J. Chem. Phys. 1968, 49, 1387−1392. (24) Frech, R.; Decius, J. C. Dipolar Coupling and Molecular Vibrations in Crystals. IV. Frequency Shifts and Dipole Moment Derivatives. J. Chem. Phys. 1971, 54, 2374−2379. (25) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Characterization and Comparison of Hydrophilic and Hydrophobic Room Temperature Ionic Liquids Incorporating the Imidazolium Cation. Green Chem. 2001, 3, 156− 164. (26) Jacquemin, J.; Husson, P.; Padua, A. A. H.; Majer, V. Density and Viscosity of Several Pure and Water-Saturated Ionic Liquids. Green Chem. 2006, 8, 172−180. (27) Andanson, J. M.; Meng, X.; Traikia, M.; Husson, P. Quantification of the Impact of Water as an Impurity on Standard Physico-Chemical Properties of Ionic Liquids. J. Chem. Thermodyn. 2016, 94, 169−176. (28) Fadeeva, T. A.; Husson, P.; DeVine, J. A.; Gomes, M. F. C.; Greenbaum, S. G.; Castner, E. W., Jr. Interactions Between Water and 1-Butyl-1-methylpyrrolidinium Ionic Liquids. J. Chem. Phys. 2015, 143, 064503. (29) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular States of Water in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192−5200. (30) Jeon, Y.; Sung, J.; Kim, D.; Seo, C.; Cheong, H.; Ouchi, Y.; Ozawa, R.; Hamaguchi, H.-o. Structural Change of 1-Butyl-3methylimidazolium Tetrafluoroborate + Water Mixtures Studied by Infrared Vibrational Spectroscopy. J. Phys. Chem. B 2008, 112, 923− 928. (31) Bertie, J. E.; Lan, Z. D. Infrared Intensities of Liquids 0.20. The Intensity of the OH Stretching Band of Liquid Water Revisited, and the Best Current Values of the Optical Constants of H2O(1) at 25 Degrees C between 15,000 and 1 cm(−1). Appl. Spectrosc. 1996, 50, 1047−1057. (32) Gurau, G.; Rodríguez, H.; Kelley, S. P.; Janiczek, P.; Kalb, R. S.; Rogers, R. D. Demonstration of Chemisorption of Carbon Dioxide in 1,3-Dialkylimidazolium Acetate Ionic Liquids. Angew. Chem., Int. Ed. 2011, 50, 12024−12026. (33) Thomas, M.; Brehm, M.; Holloczki, O.; Kelemen, Z.; Nyulaszi, L.; Pasinszki, T.; Kirchner, B. Simulating the Vibrational Spectra of Ionic Liquid Systems: 1-Ethyl-3-methylimidazolium acetate and its Mixtures. J. Chem. Phys. 2014, 141, 024510. (34) Marekha, B. A.; Bria, M.; Moreau, M.; De Waele, I.; Miannay, F.-A.; Smortsova, Y.; Takamuku, T.; Kalugin, O. N.; Kiselev, M.; Idrissi, A. Intermolecular Interactions in Mixtures of 1-N-Butyl-3Methylimidazolium Acetate and Water: Insights from IR, Raman, NMR Spectroscopy and Quantum Chemistry Calculations. J. Mol. Liq. 2015, 210, 227−237. (35) Bertie, J. E.; Zhang, S. L.; Keefe, C. D. Measurement and use of Absolute Infrared-Absorption Intensities of Neat Liquids. Vib. Spectrosc. 1995, 8, 215−229. AT


Chemical Reviews

Review

(57) Barone, V.; Biczysko, M.; Bloino, J. Fully Anharmonic IR and Raman Spectra of Medium-Size Molecular Systems: Accuracy and Interpretation. Phys. Chem. Chem. Phys. 2014, 16, 1759−1787. (58) Grondin, J.; Lassègues, J.-C.; Cavagnat, D.; Buffeteau, T.; Johansson, P.; Holomb, R. Revisited Vibrational Assignments of Imidazolium-Based Ionic Liquids. J. Raman Spectrosc. 2011, 42, 733− 743. (59) Lassègues, J.-C.; Grondin, J.; Cavagnat, D.; Johansson, P. New Interpretation of the CH Stretching Vibrations in Imidazolium-Based Ionic Liquids. J. Phys. Chem. A 2009, 113, 6419−6421. (60) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Claredon: Oxford, 1987. (61) Buchner, M.; Ladanyi, B. M.; Stratt, R. M. The Short-Time Dynamics of Molecular Liquids. Instantaneous-Normal-Mode Theory. J. Chem. Phys. 1992, 97, 8522−8535. (62) Stratt, R. M. The Instantaneous Normal-Modes of Liquids. Acc. Chem. Res. 1995, 28, 201−207. (63) Keyes, T. Unstable Modes in Supercooled and Normal Liquids: Density-Of-States, Energy Barriers, and Self-Diffusion. J. Chem. Phys. 1994, 101, 5081−5092. (64) Ribeiro, M. C. C.; Madden, P. A. Instantaneous Normal Mode Prediction for Cation and Anion Diffusion in Ionic Melts. J. Chem. Phys. 1997, 106 (20), 8616−8619. (65) Ribeiro, M. C. C.; Madden, P. A. Unstable Modes in Ionic Melts. J. Chem. Phys. 1998, 108, 3256−3263. (66) Ribeiro, M. C. C.; Wilson, M.; Madden, P. A. On the Observation of Propagating Sound Modes at High Momentum Transfer in Viscous Liquids and Glasses. J. Chem. Phys. 1998, 108, 9027−9038. (67) Ribeiro, M. C. C.; Wilson, M.; Madden, P. A. The Nature of the ″Vibrational Modes″ of the Network-Forming Liquid ZnCl2. J. Chem. Phys. 1998, 109, 9859−9869. (68) Ribeiro, M. C. C.; Wilson, M.; Madden, P. A. Raman Scattering in the Network Liquid ZnCl2 Relationship to the Vibrational Density of States. J. Chem. Phys. 1999, 110, 4803−4811. (69) Wilson, M.; Ribeiro, M. C. C. The Vibrational Motion of ’Polymeric’ BeCl2. Mol. Phys. 1999, 96, 867−876. (70) Castiglione, M. J.; Ribeiro, M. C. C.; Wilson, M.; Madden, P. A. Al3+ Coordination in Cryolitic Melts: A Computer Simulation Study. Z. Naturforsch., A: Phys. Sci. 1999, 54 (10-11), 605−610. (71) Hansen, J.-P.; McDonald, I. R. Theory of Simple Liquids, 3rd ed.; Academic Press: Amsterdam, 2006. (72) Kubo, R. Statistical-Mechanical Theory of Irreversible Processes. I. General Theory and Simple Applications to Magnetic and Conduction Problems. J. Phys. Soc. Jpn. 1957, 12, 570−586. (73) Harp, G. D.; Berne, B. J. Time-Correlation Functions, Memory Functions, and Molecular Dynamics. Phys. Rev. A: At., Mol., Opt. Phys. 1970, 2, 975−996. (74) McQuarrie, D. A. Statistical Mechanics; University Science Books: Sausalito, California, 2000. (75) Canongia Lopes, J. N.; Padua, A. A. H. CL&P: A Generic and Systematic Force Field for Ionic Liquids Modeling. Theor. Chem. Acc. 2012, 131, 1129. (76) Anderson, P. W. A Mathematical Model for the Narrowing of Spectral Lines by Exchange or Motion. J. Phys. Soc. Jpn. 1954, 9, 316− 339. (77) Wang, C. H. Spectroscopy of Condensed Media; Academic Press: Amsterdam, 1985. (78) Thomas, M.; Brehm, M.; Fligg, R.; Voehringer, P.; Kirchner, B. Computing Vibrational Spectra from ab initio Molecular Dynamics. Phys. Chem. Chem. Phys. 2013, 15, 6608−6622. (79) Benson, A. M.; Drickamer, H. G. Stretching Vibrations in Condensed Systems: Especially Bonds Containing Hydrogen. J. Chem. Phys. 1957, 27, 1164−1174. (80) Buckingham, A. D. Solvent Effects in Vibrational Spectroscopy. Trans. Faraday Soc. 1960, 56, 753−760. (81) Schweizer, K. S.; Chandler, D. Vibrational Dephasing and Frequency-Shifts of Polyatomic-Molecules in Solution. J. Chem. Phys. 1982, 76, 2296−2314.

(82) Wendler, K.; Brehm, M.; Malberg, F.; Kirchner, B.; Delle Site, L. Short Time Dynamics of Ionic Liquids in AIMD-Based Power Spectra. J. Chem. Theory Comput. 2012, 8, 1570−1579. (83) Gordon, R. G. Molecular Motion in Infrared and Raman Spectra. J. Chem. Phys. 1965, 43, 1307−1312. (84) Shimizu, H. Time-Correlation Function of Molecular Random Motion and Shape of Spectral Bands. J. Chem. Phys. 1965, 43, 2453− 2465. (85) Rothschild, W. G. Dynamics of Molecular Liquids; John Wiley & Sons: New York, 1984. (86) Madden, P. A. Simulation of Properties of Spectroscopic Interest. In Molecular Dynamics Simulation of Statistical-Mechanical (Proceedings of the International School of Physics); Hoover, W. G., Ciccoti, G., Eds. North-Holland: Amsterdam, 1986; pp 371−400. (87) Brehm, M.; Kirchner, B. TRAVIS - A Free Analyzer and Visualizer for Monte Carlo and Molecular Dynamics Trajectories. J. Chem. Inf. Model. 2011, 51, 2007−2023. (88) Sarangi, S. S.; Reddy, S. K.; Balasubramanian, S. Low Frequency Vibrational Modes of Room Temperature Ionic Liquids. J. Phys. Chem. B 2011, 115, 1874−1880. (89) Mondal, A.; Balasubramanian, S. Vibrational Signatures of Cation-Anion Hydrogen Bonding in Ionic Liquids: A Periodic Density Functional Theory and Molecular Dynamics Study. J. Phys. Chem. B 2015, 119, 1994−2002. (90) Mathias, G.; Baer, M. D. Generalized Normal Coordinates for the Vibrational Analysis of Molecular Dynamics Simulations. J. Chem. Theory Comput. 2011, 7, 2028−2039. (91) Mathias, G.; Ivanov, S. D.; Witt, A.; Baer, M. D.; Marx, D. Infrared Spectroscopy of Fluxional Molecules from (ab Initio) Molecular Dynamics: Resolving Large-Amplitude Motion, Multiple Conformations, and Permutational Symmetries. J. Chem. Theory Comput. 2012, 8, 224−234. (92) Kirillov, S. A. Interactions and Picosecond Dynamics in Molten Salts: A Review with Comparison to Molecular Liquids. J. Mol. Liq. 1998, 76, 35−95. (93) Janz, G. J.; James, D. W. Raman Spectra and Ionic Interactions in Molten Nitrates. J. Chem. Phys. 1961, 35, 739−744. (94) Chabanel, M.; Bencheikh, A.; Puchalska, D. Vibrational Study of Ionic Association in Aprotic Solvents. Part 12. Isothiocyanate M(NCS) Complexes of Non-Transition-Metal Ions. J. Chem. Soc., Dalton Trans. 1989, No. 11, 2193−2197. (95) Nyquist, R. A.; Putzig, C. L.; Leugers, M. A. The Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salt; Academic Press: San Diego, 1997; Vol. 1. (96) Takahashi, S.; Curtiss, L. A.; Gosztola, D.; Koura, N.; Saboungi, M. L. Molecular-Orbital Calculations and Raman Measurements for 1ethyl-3-methylimidazolium Chloroaluminates. Inorg. Chem. 1995, 34, 2990−2993. (97) Janz, G. J.; Kozlowski, T. R.; Wait, S. C. Raman Spectrum and Vibrational Assignment for Molten Thallous Nitrate. J. Chem. Phys. 1963, 39, 1809−1812. (98) Wait, S. C.; Ward, A. T.; Janz, G. J. AnionCation Interactions in Molten Inorganic Nitrates: Vibrational Analysis. J. Chem. Phys. 1966, 45, 133−137. (99) Holomb, R.; Martinelli, A.; Albinsson, I.; Lassègues, J. C.; Johansson, P.; Jacobsson, P. Ionic Liquid Structure: the Conformational Isomerism in 1-Butyl-3-methyl-imidazolium Tetrafluoroborate ([bmim][BF4]). J. Raman Spectrosc. 2008, 39, 793−805. (100) Madden, P. A.; O’Sullivan, K.; Board, J. A.; Fowler, P. W. Light Scattering by Alkali Halide Melts: A Computer Simulation Study. J. Chem. Phys. 1991, 94, 918−927. (101) Madden, P. A.; O’Sullivan, K. F. The Raman Spectra of Alkali Halide Melts: A Theoretical Description. J. Chem. Phys. 1991, 95, 1980−1990. (102) Hegemann, B.; Jonas, J. Separation of Temperature and Density Effects on Collision-Induced Rayleigh and Raman LineShapes of Liquid Carbon-Disulfide. J. Chem. Phys. 1985, 82, 2845− 2855. AU


Chemical Reviews

Review

perature Ionic Liquids: Vibrational and Theoretical Studies. Vib. Spectrosc. 2014, 75, 107−117. (121) Vitucci, F. M.; Trequattrini, F.; Palumbo, O.; Brubach, J. B.; Roy, P.; Paolone, A. Infrared Spectra of Bis(trifluoromethanesulfonyl)imide Based Ionic Liquids: Experiments and DFT Simulations. Vib. Spectrosc. 2014, 74, 81−87. (122) Vitucci, F. M.; Trequattrini, F.; Palumbo, O.; Brubach, J. B.; Roy, P.; Navarra, M. A.; Panero, S.; Paolone, A. Stabilization of Different Conformers of Bis(trifluoromethanesulfonyl)imide Anion in Ammonium-Based Ionic Liquids at Low Temperatures. J. Phys. Chem. A 2014, 118, 8758−8764. (123) Faria, L. F. O.; Matos, J. R.; Ribeiro, M. C. C. Thermal Analysis and Raman Spectra of Different Phases of the Ionic Liquid Butyltrimethylammonium Bis(trifluoromethylsulfonyl)imide. J. Phys. Chem. B 2012, 116, 9238−9245. (124) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y.; Ishiguro, S. I. Conformational Equilibrium of Bis(Trifluoromethanesulfonyl) Imide Anion of a Room-Temperature Ionic Liquid: Raman Spectroscopic Study and DFT Calculations. J. Phys. Chem. B 2006, 110, 8179−8183. (125) Lassègues, J. C.; Grondin, J.; Holomb, R.; Johansson, P. Raman and ab initio Study of the Conformational Isomerism in the 1-Ethyl-3methylimidazolium Bis(trifluoromethanesulfonyl)imide Ionic Liquid. J. Raman Spectrosc. 2007, 38, 551−558. (126) Lopes, J. N. C.; Shimizu, K.; Padua, A. A. H.; Umebayashi, Y.; Fukuda, S.; Fujii, K.; Ishiguro, S.-i. A Tale of Two Ions: The Conformational Landscapes of Bis(trifluoromethanesulfonyl)amide and N,N-Dialkylpyrrolidinium. J. Phys. Chem. B 2008, 112, 1465− 1472. (127) Umebayashi, Y.; Mitsugi, T.; Fujii, K.; Seki, S.; Chiba, K.; Yamamoto, H.; Lopes, J. N. C.; Padua, A. A. H.; Takeuchi, M.; Kanzaki, R.; et al. Raman Spectroscopic Study, DFT Calculations and MD Simulations on the Conformational Isomerism of N-Alkyl-Nmethylpyrrolidinium Bis-(trifluoromethanesulfonyl) Amide Ionic Liquids. J. Phys. Chem. B 2009, 113, 4338−4346. (128) Lima, T. A.; Paschoal, V. H.; Faria, L. F. O.; Ribeiro, M. C. C.; Ferreira, F. F.; Costa, F. N.; Giles, C. Comparing Two Tetraalkylammonium Ionic Liquids. II. Phase Transitions. J. Chem. Phys. 2016, 144, 224505. (129) Fujii, K.; Seki, S.; Fukuda, S.; Kanzaki, R.; Takamuku, T.; Umebayashi, Y.; Ishiguro, S.-i. Anion Conformation of Low-Viscosity Room-Temperature Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(fluorosulfonyl)imide. J. Phys. Chem. B 2007, 111, 12829−12833. (130) Fujii, K.; Seki, S.; Fukuda, S.; Takamuku, T.; Kohara, S.; Kameda, Y.; Umebayashi, Y.; Ishiguro, S.-i. Liquid Structure and Conformation of a Low-Viscosity Ionic Liquid, N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide Studied by High-Energy X-ray Scattering. J. Mol. Liq. 2008, 143, 64−69. (131) Matsumoto, K.; Oka, T.; Nohira, T.; Hagiwara, R. Polymorphism of Alkali Bis(fluorosulfonyl)amides (M N(SO2F)(2), M = Na, K, and Cs). Inorg. Chem. 2013, 52, 568−576. (132) Shimizu, Y.; Fujii, K.; Imanari, M.; Nishikawa, K. Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid Exhibiting Scanning Rate Dependence. J. Phys. Chem. B 2015, 119, 12552−12560. (133) Giffin, G. A.; Laszczynski, N.; Jeong, S.; Jeremias, S.; Passerini, S. Conformations and Vibrational Assignments of the (Fluorosulfonyl) (trifluoromethanesulfonyl)imide Anion in Ionic Liquids. J. Phys. Chem. C 2013, 117, 24206−24212. (134) Manning, J.; Frech, R. The Structure of Associated Ionic Species in Poly(Propylene oxide) Alkali-Metal trifluoromethanesulfonate Complexes. Polymer 1992, 33, 3487−3494. (135) Dissanayake, M.; Frech, R. Infrared Spectroscopic Study of the Phases and Phase-Transitions in Poly(Ethylene Oxide) and Poly(Ethylene Oxide)-Lithium Trifluoromethanesulfonate Complexes. Macromolecules 1995, 28, 5312−5319. (136) Rhodes, C. P.; Frech, R. Cation-Anion and Cation-Polymer Interactions in (PEO)(n)NaCF3SO3 (n = 1−80). Solid State Ionics 1999, 121, 91−99.

(103) Madden, P. A.; Tildesley, D. J. Interaction-Induced Contributions to Rayleigh and Allowed Raman Bands - A Simulation Study of CS2. Mol. Phys. 1985, 55, 969−998. (104) Ikawa, S.; Whalley, E. Polarized and Depolarized Raman Spectra of Liquid Carbon Disulfide at 0−10 kbar. 3. InteractionInduced.nu.2 and.nu.3 Scattering and the Fluctuation of the Local Field. J. Phys. Chem. 1990, 94, 7834−7839. (105) Hipps, K. W.; Aplin, A. T. The Tricyanomethanide Ion: an Infrared, Raman, and Tunneling Spectroscopy Study Including Isotopic-Substitution. J. Phys. Chem. 1985, 89, 5459−5464. (106) Penna, T. C.; Faria, L. F. O.; Ribeiro, M. C. C. Raman Band Shape Analysis of Cyanate-Anion Ionic Liquids. J. Mol. Liq. 2015, 209, 676−682. (107) Sprague, J. W.; Ritchey, W. M.; Grasselli, J. G. The Synthesis and Infrared and Nuclear Magnetic Resonance Spectra of Ammonium Dicyanamide. J. Phys. Chem. 1964, 68, 431−433. (108) Reckeweg, O.; DiSalvo, F. J.; Schulz, A.; Blaschkowski, B.; Jagiella, S.; Schleid, T. Synthesis, Crystal Structure, and Vibrational Spectra of the Anhydrous Lithium Dicyanamide Li[N(CN)2]. Z. Anorg. Allg. Chem. 2014, 640, 851−855. (109) Kiefer, J.; Noack, K.; Penna, T. C.; Ribeiro, M. C. C.; Weber, H.; Kirchner, B. Vibrational Signatures of Cyano Groups in Imidazolium Ionic Liquids. Vib. Spectrosc. 2016, DOI: 10.1016/ j.vibspec.2016.05.004. (110) Bernhardt, E.; Henkel, G.; Willner, H. Die Tetracyanoborate M[B(CN)4], M = [Bu4N]+, Ag+, K+. Z. Anorg. Allg. Chem. 2000, 626, 560−568. (111) Rey, I.; Johansson, P.; Lindgren, J.; Lassègues, J. C.; Grondin, J.; Servant, L. Spectroscopic and Theoretical Study of (CF3SO2)2N(TFSI-) and (CF3SO2)2NH (HTFSI). J. Phys. Chem. A 1998, 102, 3249−3258. (112) Herstedt, M.; Smirnov, M.; Johansson, P.; Chami, M.; Grondin, J.; Servant, L.; Lassègues, J. C. Spectroscopic Characterization of the Conformational States of the Bis(trifluoromethanesulfonyl)imide Anion (TFSI-). J. Raman Spectrosc. 2005, 36, 762−770. (113) Rey, I.; Lassègues, J. C.; Grondin, J.; Servant, L. Infrared and Raman Study of the PEO-LiTFSI Polymer Electrolyte. Electrochim. Acta 1998, 43, 1505−1510. (114) Castriota, M.; Caruso, T.; Agostino, R. G.; Cazzanelli, E.; Henderson, W. A.; Passerini, S. Raman Investigation of the Ionic Liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide and its Mixture with LiN(SO2CF3) (2). J. Phys. Chem. A 2005, 109, 92−96. (115) Kiefer, J.; Fries, J.; Leipertz, A. Experimental Vibrational Study of Imidazolium-Based Ionic Liquids: Raman and Infrared Spectra of 1ethyl-3-methylimidazolium bis(Trifluoromethylsulfonyl)imide and 1ethyl-3-methylimidazolium ethylsulfate. Appl. Spectrosc. 2007, 61, 1306−1311. (116) Martinelli, A.; Matic, A.; Johansson, P.; Jacobsson, P.; Borjesson, L.; Fernicola, A.; Panero, S.; Scrosati, B.; Ohno, H. Conformational Evolution of TFSI- in Protic and Aprotic Ionic Liquids. J. Raman Spectrosc. 2011, 42, 522−528. (117) Mao, J. X.; Nulwala, H. B.; Luebke, D. R.; Damodaran, K. Spectroscopic and Computational Analysis of the Molecular Interactions in the Ionic Liquid Ion Pair BMP(+) TFSI(−). J. Mol. Liq. 2012, 175, 141−147. (118) Moschovi, A. M.; Ntais, S.; Dracopoulos, V.; Nikolakis, V. Vibrational Spectroscopic Study of the Protic Ionic Liquid 1-H-3methylimidazolium Bis(Trifluoromethanesulfonyl)imide. Vib. Spectrosc. 2012, 63, 350−359. (119) Dhumal, N. R.; Noack, K.; Kiefer, J.; Kim, H. J. Molecular Structure and Interactions in the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(Trifluoromethylsulfonyl)imide. J. Phys. Chem. A 2014, 118, 2547−2557. (120) Shukla, M.; Noothalapati, H.; Shigeto, S.; Saha, S. Importance of Weak Interactions and Conformational Equilibrium in N-Butyl-NMethylpiperidinium Bis(trifluromethanesulfonyl)imide Room TemAV


Chemical Reviews

Review

(158) Phamthi, M.; Colomban, P.; Novak, A.; Blinc, R. Vibrational Spectra of and Phase Transitions in Cesium Hydrogen Sulfate. J. Raman Spectrosc. 1987, 18, 185−194. (159) Colomban, P.; Phamthi, M.; Novak, A. Influence of Thermal and Mechanical Treatment and of Water on Structural PhaseTransitions in CsHSO4. Solid State Ionics 1987, 24, 193−203. (160) Varma, V.; Rangavittal, N.; Rao, C. N. R. A Study of Superionic CsHSO4 and Cs1-XLixHSO4 by Vibrational Spectroscopy and X-Ray Diffraction. J. Solid State Chem. 1993, 106, 164−173. (161) Rogers, S. E.; Ubbelohde, A. R. Melting and Crystal Structure. III. Low-Melting Acid Sulfates. Trans. Faraday Soc. 1950, 46, 1051− 1061. (162) Cabaço, M. I.; Besnard, M.; Danten, Y.; Coutinho, J. A. P. Carbon Dioxide in 1-Butyl-3-methylimidazolium Acetate. I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations. J. Phys. Chem. A 2012, 116, 1605−1620. (163) Ito, K.; Bernstein, H. J. The Vibrational Spectra of the Formate, Acetate, and Oxalate Ions. Can. J. Chem. 1956, 34, 170−178. (164) Alcock, N. W.; Tracy, V. M.; Waddington, T. C. Acetates and Acetato-Complexes. Part 2. Spectroscopic Studies. J. Chem. Soc., Dalton Trans. 1976, No. 21, 2243−2246. (165) Deacon, G. B.; Phillips, R. J. Relationships Between the Carbon-Oxygen Stretching Frequencies of Carboxylato Complexes and the Type of Carboxylate Coordination. Coord. Chem. Rev. 1980, 33, 227−250. (166) Nara, M.; Torii, H.; Tasumi, M. Correlation between the Vibrational Frequencies of the Carboxylate Group and the Types of its Coordination to a Metal Ion: An ab initio Molecular Orbital Study. J. Phys. Chem. 1996, 100, 19812−19817. (167) Holloczki, O.; Firaha, D. S.; Friedrich, J.; Brehm, M.; Cybik, R.; Wild, M.; Stark, A.; Kirchner, B. Carbene Formation in Ionic Liquids: Spontaneous, Induced, or Prohibited? J. Phys. Chem. B 2013, 117, 5898−5907. (168) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B, 6th ed.; John Wiley & Sons: Hoboken, NJ, 2009. (169) Rudolph, W. W.; Irmer, G. Raman Spectroscopic Studies and DFT Calculations on NaCH3CO2 and NaCD3CO2 Solutions in Water and Heavy Water. RSC Adv. 2015, 5, 21897−21908. (170) Tanzi, L.; Benassi, P.; Nardone, M.; Ramondo, F. Vibrations of Bioionic Liquids by Ab Initio Molecular Dynamics and Vibrational Spectroscopy. J. Phys. Chem. A 2014, 118, 12229−12240. (171) Quiles, F.; Burneau, A. Infrared and Raman Spectra of Alkaline-Earth and Copper(II) Acetates in Aqueous Solutions. Vib. Spectrosc. 1998, 16, 105−117. (172) Bowron, D. T.; D’Agostino, C.; Gladden, L. F.; Hardacre, C.; Holbrey, J. D.; Lagunas, M. C.; McGregor, J.; Mantle, M. D.; Mullan, C. L.; Youngs, T. G. A. Structure and Dynamics of 1-Ethyl-3methylimidazolium Acetate via Molecular Dynamics and Neutron Diffraction. J. Phys. Chem. B 2010, 114, 7760−7768. (173) Cabaço, M. I.; Besnard, M.; Danten, Y.; Coutinho, J. A. P. Solubility of CO2 in 1-Butyl-3-methyl-imidazolium-trifluoro Acetate Ionic Liquid Studied by Raman Spectroscopy and DFT Investigations. J. Phys. Chem. B 2011, 115, 3538−3550. (174) Robinson, R. E.; Taylor, R. C. Raman Spectrum and Vibrational Assignments for the Trifluoroacetate Ion. Spectrochim. Acta 1962, 18, 1093−1095. (175) Shukla, M.; Srivastava, N.; Saha, S. Theoretical and Spectroscopic Studies of 1-butyl-3-methylimidazolium iodide Room Temperature Ionic Liquid: Its Differences with chloride and bromide Derivatives. J. Mol. Struct. 2010, 975, 349−356. (176) Jeon, Y.; Sung, J.; Seo, C.; Lim, H.; Cheong, H.; Kang, M.; Moon, B.; Ouchi, Y.; Kim, D. Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy. J. Phys. Chem. B 2008, 112, 4735−4740. (177) Dhumal, N. R. Molecular Interactions in 1,3-dimethylimidazolium-bis(trifluromethanesulfonyl)imide Ionic Liquid. Chem. Phys. 2007, 342, 245−252.

(137) Alloin, F.; Hirankumar, G.; Pagnier, T. TemperatureDependent Raman Spectroscopy of Lithium Triflate-PEO Complexes: Phase Equilibrium and Component Interactions. J. Phys. Chem. B 2009, 113, 16465−16471. (138) Huang, W. W.; Frech, R.; Wheeler, R. A. Molecular Structures and Normal Vibrations of Trifluoromethane Sulfonate (CF3SO3-) and Its Lithium Ion Pairs and Aggregates. J. Phys. Chem. 1994, 98, 100− 110. (139) Gejji, S. P.; Hermansson, K.; Tegenfeldt, J.; Lindgren, J. Geometry and Vibrational Frequencies of the Lithium Triflate IonPair: An Ab-Initio Study. J. Phys. Chem. 1993, 97, 11402−11407. (140) Frech, R.; Huang, W. W. Anion-Solvent and Anion-Cation Interactions in Lithium and Tetrabutylammonium Trifluoromethanesulfonate Solutions. J. Solution Chem. 1994, 23, 469−481. (141) Alia, J. M.; Edwards, H. G. M. Ion Solvation and Ion Association in Lithium Trifluoromethanesulfonate Solutions in Three Aprotic Solvents. An FT-Raman Spectroscopic Study. Vib. Spectrosc. 2000, 24, 185−200. (142) Mikhailov, G. P. Calculation of Vibrational Spectra for Coordinated Trifluoromethanesulfonate Ion in Dipolar Aprotic Solvents. J. Appl. Spectrosc. 2015, 82, 165−168. (143) Varetti, E. L.; Fernandez, E. L.; Altabef, A. B. The Polarized Infrared and Raman-Spectra of Crystalline ammonium trifluoromethanesulfonate. Spectrochim. Acta, Part A 1991, 47, 1767−1774. (144) Schwenzer, B.; Kerisit, S. N.; Vijayakumar, M. Anion Pairs in Room Temperature Ionic Liquids Predicted by Molecular Dynamics Simulation, Verified by Spectroscopic Characterization. RSC Adv. 2014, 4, 5457−5464. (145) Akai, N.; Kawai, A.; Shibuya, K. Ion-Pair Structure of Vaporized Ionic Liquid Studied by Matrix-Isolation FTIR Spectroscopy with DFT Calculations: A Case of 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate. J. Phys. Chem. A 2010, 114, 12662−12666. (146) Mao, J. X.; Damodaran, K. Spectroscopic and Computational Analysis of the Molecular Interactions in the Ionic Liquid Emim (+) FAP (−). Ionics 2015, 21, 1605−1613. (147) Voroshylova, I. V.; Teixeira, F.; Costa, R.; Pereira, C. M.; Cordeiro, M. N. D. S. Interactions in the Ionic Liquid [EMIM][FAP]: a Coupled Experimental and Computational Analysis. Phys. Chem. Chem. Phys. 2016, 18, 2617−2628. (148) Dhumal, N. R.; Kim, H. J.; Kiefer, J. Electronic Structure and Normal Vibrations of the 1-Ethyl-3-methylimidazolium Ethyl Sulfate Ion Pair. J. Phys. Chem. A 2011, 115, 3551−3558. (149) Koda, S.; Nomura, H. Aqueous-Solutions of Sodium Methylsulfate by Raman-Scattering, NMR, Ultrasound, and DensityMeasurements. J. Solution Chem. 1985, 14, 355−366. (150) Walrafen, G. E.; Young, T. F.; Irish, D. E. Raman Spectral Studies of Molten Potassium Bisulfate and Vibrational Frequencies of S2O7 Groups. J. Chem. Phys. 1962, 37, 662−670. (151) Fehrmann, R.; Hansen, N. H.; Bjerrum, N. J. RamanSpectroscopic and Spectrophotometric Study of the System K2S2O7KHSO4 in the Temperature-Range 200−450 °C. Inorg. Chem. 1983, 22, 4009−4014. (152) Knudsen, C. B.; Kalampounias, A. G.; Fehrmann, R.; Boghosian, S. Thermal Dissociation of Molten KHSO4: Temperature Dependence of Raman Spectra and Thermodynamics. J. Phys. Chem. B 2008, 112, 11996−12000. (153) Irish, D. E.; Chen, H. Equilibria and Proton Transfer in Bisulfate-Sulfate System. J. Phys. Chem. 1970, 74, 3796−3801. (154) Turner, D. J. Raman Spectral Study of Bisulphate Ion Hydration. J. Chem. Soc., Faraday Trans. 2 1972, 68, 643−648. (155) Kiefer, J.; Pye, C. C. Structure of the Room-Temperature Ionic Liquid 1-Hexyl-3-methylimidazolium Hydrogen Sulfate: Conformational Isomerism. J. Phys. Chem. A 2010, 114, 6713−6720. (156) Ribeiro, M. C. C. High Viscosity of Imidazolium Ionic Liquids with the Hydrogen Sulfate Anion: A Raman Spectroscopy Study. J. Phys. Chem. B 2012, 116, 7281−7290. (157) Goypiron, A.; Devillepin, J.; Novak, A. Raman and Infrared Study of KHSO4 Crystal. J. Raman Spectrosc. 1980, 9, 297−303. AW


Chemical Reviews

Review

(178) Wulf, A.; Fumino, K.; Ludwig, R. Comment on ″New Interpretation of the CH Stretching Vibrations in Imidazolium-Based Ionic Liquids″. J. Phys. Chem. A 2010, 114, 685−686. (179) Lassègues, J.-C.; Grondin, J.; Cavagnat, D.; Johansson, P. Reply to the ″Comment on ’New Interpretation of the CH Stretching Vibrations in Imidazolium-Based Ionic Liquids’″. J. Phys. Chem. A 2010, 114, 687−688. (180) Hunt, P. A.; Ashworth, C. R.; Matthews, R. P. Hydrogen Bonding in Ionic Liquids. Chem. Soc. Rev. 2015, 44, 1257−1288. (181) Hayashi, S.; Ozawa, R.; Hamaguchi, H. Raman Spectra, Crystal Polymorphism, and Structure of a Prototype Ionic-Iiquid [bmim]Cl. Chem. Lett. 2003, 32, 498−499. (182) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Johnson, S.; Seddon, K. R.; Rogers, R. D. Crystal Polymorphism in 1-Butyl-3Methylimidazolium Halides: Supporting Ionic Liquid Formation by Inhibition of Crystallization. Chem. Commun. 2003, No. 14, 1636− 1637. (183) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Crystal Structure of 1-butyl-3-methylimidazolium chloride. A Clue to the Elucidation of the Ionic Liquid Structure. Chem. Lett. 2003, 32, 740− 741. (184) Ozawa, R.; Hayashi, S.; Saha, S.; Kobayashi, A.; Hamaguchi, H. Rotational isomerism and Structure of the 1-butyl-3-methylimidazolium Cation in the Ionic Liquid State. Chem. Lett. 2003, 32, 948−949. (185) Hamaguchi, H.; Ozawa, R. Structure of Ionic Liquids and Ionic Liquid Compounds: Are Ionic Liquids Genuine Liquids in the Conventional Sense? In Advances in Chemical Physics, Rice, S. A., Ed.; John Wiley & Sons, Inc.: Hoboken, 2005; Vol. 131, pp 85−104. (186) Endo, T.; Hoshino, S.; Shimizu, Y.; Fujii, K.; Nishikawa, K. Comprehensive Conformational and Rotational Analyses of the Butyl Group in Cyclic Cation: DFT Calculations for Imidazolium, Pyridinium, Pyrrolidinium, and Piperidinium. J. Phys. Chem. B 2016, 120, 10336−10349. (187) Tsuzuki, S.; Arai, A. A.; Nishikawa, K. Conformational Analysis of 1-Butyl-3-methylimidazolium by CCSD(T) level ab initio Calculations: Effects of Neighboring Anions. J. Phys. Chem. B 2008, 112, 7739−7747. (188) Umebayashi, Y.; Hamano, H.; Tsuzuki, S.; Canongia Lopes, J. N.; Padua, A. A. H.; Kameda, Y.; Kohara, S.; Yamaguchi, T.; Fujii, K.; Ishiguro, S.-i. Dependence of the Conformational Isomerism in 1-nButyl-3-methylimidazolium Ionic Liquids on the Nature of the Halide Anion. J. Phys. Chem. B 2010, 114, 11715−11724. (189) Katayanagi, H.; Hayashi, S.; Hamaguchi, H. O.; Nishikawa, K. Structure of an Ionic Liquid, 1-n-butyl-3-methylimidazolium iodide, Studied by Wide-Angle X-ray Scattering and Raman Spectroscopy. Chem. Phys. Lett. 2004, 392, 460−464. (190) Singh, D. K.; Cha, S.; Nam, D.; Cheong, H.; Joo, S.-W.; Kim, D. Raman Spectroscopic Study on Alkyl Chain Conformation in 1Butyl-3-methylimidazolium Ionic Liquids and their Aqueous Mixtures. ChemPhysChem 2016, 17, 3040−3046. (191) Endo, T.; Nishikawa, K. Isomer Populations in Liquids for 1Isopropyl-3-methylimidazolium Bromide and its Iodide and Their Conformational Changes Accompanying the Crystallizing and Melting Processes. J. Phys. Chem. A 2008, 112, 7543−7550. (192) Hunt, P. A. Why Does a Reduction in Hydrogen Bonding Lead to an Increase in Viscosity for the 1-Butyl-2,3-dimethyl-imidazoliumBased Ionic Liquids? J. Phys. Chem. B 2007, 111, 4844−4853. (193) Fumino, K.; Wulf, A.; Ludwig, R. Strong, Localized, and Directional Hydrogen Bonds Fluidize Ionic Liquids. Angew. Chem., Int. Ed. 2008, 47, 8731−8734. (194) Izgorodina, E. I.; Maganti, R.; Armel, V.; Dean, P. M.; Pringle, J. M.; Seddon, K. R.; MacFarlane, D. R. Understanding the Effect of the C2 Proton in Promoting Low Viscosities and High Conductivities in Imidazolium-Based Ionic Liquids: Part I. Weakly Coordinating Anions. J. Phys. Chem. B 2011, 115, 14688−14697. (195) Chen, Z. J.; Lee, J.-M. Free Volume Model for the Unexpected Effect of C2-Methylation on the Properties of Imidazolium Ionic Liquids. J. Phys. Chem. B 2014, 118, 2712−2718.

(196) Noack, K.; Schulz, P. S.; Paape, N.; Kiefer, J.; Wasserscheid, P.; Leipertz, A. The Role of the C2 Position in Interionic Interactions of Imidazolium Based Ionic Liquids: a Vibrational and NMR Spectroscopic Study. Phys. Chem. Chem. Phys. 2010, 12, 14153−14161. (197) Hunt, P. A.; Kirchner, B.; Welton, T. Characterising the Electronic Structure of Ionic Liquids: An Examination of the 1-butyl-3methylimidazolium chloride Ion Pair. Chem. - Eur. J. 2006, 12, 6762− 6775. (198) Endo, T.; Kato, T.; Nishikawa, K. Effects of Methylation at the 2 Position of the Cation Ring on Phase Behaviors and Conformational Structures of Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2010, 114, 9201−9208. (199) Moschovi, A. M.; Dracopoulos, V.; Nikolakis, V. Inter- and Intramolecular Interactions in Imidazolium Protic Ionic Liquids. J. Phys. Chem. B 2014, 118, 8673−8683. (200) Garaga, M. N.; Nayeri, M.; Martinelli, A. Effect of the Alkyl Chain Length in 1-Alkyl-3-Methylimidazolium Ionic Liquids on InterMolecular Interactions and Rotational Dynamics: A Combined Vibrational and NMR Spectroscopic Study. J. Mol. Liq. 2015, 210, 169−177. (201) Moschovi, A. M.; Dracopoulos, V. Structure of Protic (HC(n)ImNTf(2), n = 0−12) and Aprotic (C(1)C(n)ImNTf(2), n = 1−12) Imidazolium Ionic Liquids: A Vibrational Spectroscopic Study. J. Mol. Liq. 2015, 210, 189−199. (202) Xuan, X.; Guo, M.; Pei, Y.; Zheng, Y. Theoretical Study on Cation-Anion Interaction and Vibrational Spectra of 1-Allyl-3methylimidazolium-Based Ionic Liquids. Spectrochim. Acta, Part A 2011, 78, 1492−1499. (203) Xu, H.; Liao, J.; Li, Q.; You, L.; Kang, S.; Chen, H.; He, B.; Tang, Q. Synthesis and Properties of 1-allyl-3-methyl-imidazolium bicarbonate Room Temperature Ionic Liquid. Mater. Res. Innovations 2014, 18, 457−460. (204) Shirota, H.; Matsuzaki, H.; Ramati, S.; Wishart, J. F. Effects of Aromaticity in Cations and Their Functional Groups on the LowFrequency Spectra and Physical Properties of Ionic Liquids. J. Phys. Chem. B 2015, 119, 9173−9187. (205) Xue, L.; Tamas, G.; Matthews, R. P.; Stone, A. J.; Hunt, P. A.; Quitevis, E. L.; Lynden-Bell, R. M. An OHD-RIKES and Simulation Study Comparing a Benzylmethylimidazolium Ionic Liquid with an Equimolar Mixture of Dimethylimidazolium and Benzene. Phys. Chem. Chem. Phys. 2015, 17, 9973−9983. (206) Knorr, A.; Ludwig, R. Cation-cation clusters in ionic liquids: Cooperative hydrogen bonding overcomes like-charge repulsion. Sci. Rep. 2015, 5, 17505. (207) Knorr, A.; Stange, P.; Fumino, K.; Weinhold, F.; Ludwig, R. Spectroscopic Evidence for Clusters of Like-Charged Ions in Ionic Liquids Stabilized by Cooperative Hydrogen Bonding. ChemPhysChem 2016, 17, 458−462. (208) Katsyuba, S. A.; Vener, M. V.; Zvereva, E. E.; Fei, Z.; Scopelliti, R.; Laurenczy, G.; Yan, N.; Paunescu, E.; Dyson, P. J. How Strong Is Hydrogen Bonding in Ionic Liquids? Combined X-ray Crystallographic, Infrared/Raman Spectroscopic, and Density Functional Theory Study. J. Phys. Chem. B 2013, 117, 9094−9105. (209) Knorr, A.; Fumino, K.; Bonsa, A. M.; Ludwig, R. Spectroscopic Evidence of ’Jumping and Pecking’ of Cholinium and H-bond Enhanced Cation-Cation Interaction in Ionic Liquids. Phys. Chem. Chem. Phys. 2015, 17, 30978−30982. (210) Weinhold, F.; Klein, R. A. Anti-Electrostatic Hydrogen Bonds. Angew. Chem., Int. Ed. 2014, 53, 11214−11217. (211) Gale, R. J.; Gilbert, B.; Osteryoung, R. A. Raman-Spectra of Molten Aluminum Chloride: 1-Butylpyridinium Chloride Systems at Ambient Temperatures. Inorg. Chem. 1978, 17, 2728−2729. (212) Gale, R. J.; Osteryoung, R. A. Infrared Spectral Investigations of Room-Temperature Aluminum Chloride-1-Butylpyridinium Chloride Melts. Inorg. Chem. 1980, 19, 2240−2242. (213) Tait, S.; Osteryoung, R. A. Infrared Study of AmbientTemperature Chloroaluminates as a Function of Melt Acidity. Inorg. Chem. 1984, 23, 4352−4360. AX


Chemical Reviews

Review

(214) Zhao, H.; Malhotra, S. V.; Luo, R. G. Preparation and characterization of three room-temperature ionic liquids. Phys. Chem. Liq. 2003, 41, 487−492. (215) Zheng, Y.; Zhuo, Z.; He, Y.; Mo, Q. Structure and Spectral Studies of N-Ethyl Pyridinium Bromide Ionic Liquids: DFT and ab initio Study. Asian J. Chem. 2013, 25, 3233−3236. (216) Sashina, E. S.; Kashirskii, D. A.; Artamonova, T. V.; Myznikov, L. V. 1-Alkyl-3-Methylpyridinium Halide Structural Features Studied by Vibrational Spectroscopic Methods. Fibre Chem. 2015, 47, 171− 176. (217) Rocher, N. M.; Izgorodina, E. I.; Ruether, T.; Forsyth, M.; MacFarlane, D. R.; Rodopoulos, T.; Horne, M. D.; Bond, A. M. Aluminium Speciation in 1-Butyl-1-Methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide/AlCl3Mixtures. Chem. - Eur. J. 2009, 15, 3435−3447. (218) Fujimori, T.; Fujii, K.; Kanzaki, R.; Chiba, K.; Yamamoto, H.; Umebayashi, Y.; Ishiguro, S.-i. Conformational Structure of Room Temperature Ionic Liquid N-Butyl-N-Methyl-pyrrolidinium Bis(trifluoromethanesulfonyl)imide: Raman Spectroscopic Study and DFT Calculations. J. Mol. Liq. 2007, 131−132, 216−224. (219) Trequattrini, F.; Palumbo, O.; Gatto, S.; Appetecchi, G. B.; Paolone, A. A Computational and Experimental Study of the Conformers of Pyrrolidinium Ionic Liquid Cations Containing an Ethoxy Group in the Alkyl Side Chain. Adv. Chem. 2016, 2016, 1−9. (220) Menzies, A. C.; Mills, H. R. Raman Effect and Temperature I Ammonium Chloride, Bromide, Iodide. Proc. R. Soc. London, Ser. A 1935, 148 (A864), 407−422. (221) Holmes, F. T. Raman Spectrum of Crystalline Ammonium Chloride. J. Chem. Phys. 1936, 4, 88−90. (222) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Evolving Structure-Property Relationships and Expanding Applications. Chem. Rev. 2015, 115, 11379−11448. (223) Palumbo, O.; Vitucci, F. M.; Trequattrini, F.; Paolone, A. A Study of the Conformers of the N,N-Diethyl-N-Methyl-N-Propylammonium Ion by Means of Infrared Spectroscopy and DFT Calculations. Vib. Spectrosc. 2015, 80, 11−16. (224) Domańska, U.; Bogel-L̷ ukasik, R. Physicochemical Properties and Solubility of Alkyl-(2-hydroxyethyl)-dimethylammonium Bromide. J. Phys. Chem. B 2005, 109, 12124−12132. (225) Aparicio, S.; Atilhan, M.; Khraisheh, M.; Alcalde, R. Study on Hydroxylammonium-Based Ionic Liquids. I. Characterization. J. Phys. Chem. B 2011, 115, 12473−12486. (226) Arkas, M.; Tsiourvas, D.; Paleos, C. M.; Skoulios, A. Smectic Mesophases from Dihydroxy Derivatives of Quaternary Alkylammonium Salts. Chem. - Eur. J. 1999, 5, 3202−3207. (227) Campetella, M.; Bodo, E.; Caminiti, R.; Martino, A.; D’Apuzzo, F.; Lupi, S.; Gontrani, L. Interaction and Dynamics of Ionic Liquids Based on Choline and Amino Acid Anions. J. Chem. Phys. 2015, 142, 234502. (228) Harmon, K. M.; Avci, G. F.; Thiel, A. C. Hydrogen-Bonding Part 21. Infrared Spectral Study of the High-Temperature Phases of Choline Bromide and Choline Iodide. J. Mol. Struct. 1986, 145, 83−91. (229) Perkins, S. L.; Painter, P.; Colina, C. M. Molecular Dynamic Simulations and Vibrational Analysis of an Ionic Liquid Analogue. J. Phys. Chem. B 2013, 117, 10250−10260. (230) Bodo, E.; Postorino, P.; Mangialardo, S.; Piacente, G.; Ramondo, F.; Bosi, F.; Ballirano, P.; Caminiti, R. Structure of the Molten Salt Methyl Ammonium Nitrate Explored by Experiments and Theory. J. Phys. Chem. B 2011, 115, 13149−13161. (231) Bodo, E.; Mangialardo, S.; Ramondo, F.; Ceccacci, F.; Postorino, P. Unravelling the Structure of Protic Ionic Liquids with Theoretical and Experimental Methods: Ethyl-, Propyl- and Butylammonium Nitrate Explored by Raman Spectroscopy and DFT Calculations. J. Phys. Chem. B 2012, 116, 13878−13888. (232) Henderson, W. A.; Fylstra, P.; De Long, H. C.; Trulove, P. C.; Parsons, S. Crystal Structure of the Ionic Liquid EtNH3NO3-Insights into the Thermal Phase Behavior of Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 16041−16046.

(233) Hagemann, H.; Bill, H. Raman Spectroscopic Study of EtNH3X (X = Cl,Br) and Several Deuterated Analogs. J. Chem. Phys. 1984, 80, 111−118. (234) Prasad, P. S. R.; Bist, H. D. Raman Investigation of OrderDisorder Transition in Normal-Propylammonium Chloride. Solid State Commun. 1990, 74, 885−888. (235) Faria, L. F. O.; Penna, T. C.; Ribeiro, M. C. C. Raman Spectroscopic Study of Temperature and Pressure Effects on the Ionic Liquid Propylammonium Nitrate. J. Phys. Chem. B 2013, 117, 10905− 10912. (236) Bodo, E.; Sferrazza, A.; Caminiti, R.; Mangialardo, S.; Postorino, P. A prototypical Ionic Liquid Explored by ab initio Molecular Dynamics and Raman Spectroscopy. J. Chem. Phys. 2013, 139, 144309. (237) Luo, J.; Conrad, O.; Vankelecom, I. F. J. Physicochemical Properties of Phosphonium-Based and Ammonium-Based Protic Ionic Liquids. J. Mater. Chem. 2012, 22, 20574−20579. (238) Berg, R. W.; Riisager, A.; Van Buu, O. N.; Fehrmann, R.; Harris, P.; Tomaszowska, A. A.; Seddon, K. R. Crystal Structure, Vibrational Spectroscopy and ab Initio Density Functional Theory Calculations on the Ionic Liquid forming 1,1,3,3-Tetramethylguanidinium bis{(trifluoromethyl)sulfonyl}amide. J. Phys. Chem. B 2009, 113, 8878−8886. (239) Horikawa, M.; Akai, N.; Kawai, A.; Shibuya, K. Vaporization of Protic Ionic Liquids Studied by Matrix-Isolation Fourier Transform Infrared Spectroscopy. J. Phys. Chem. A 2014, 118, 3280−3287. (240) Berg, R. W.; Riisager, A.; Fehrmann, R. Formation of an IonPair Molecule with a Single NH+···Cl− Hydrogen Bond: Raman Spectra of 1,1,3,3-Tetramethylguanidinium Chloride in the Solid State, in Solution, and in the Vapor Phase. J. Phys. Chem. A 2008, 112, 8585−8592. (241) Berg, R. W.; Riisager, A.; Van Buu, O. N.; Kristensen, S. B.; Fehrmann, R.; Harris, P.; Brunetti, A. C. X-ray Crystal Structure, Raman Spectroscopy, and Ab Initio Density Functional Theory Calculations on 1,1,3,3-Tetramethylguanidinium Bromide. J. Phys. Chem. A 2010, 114, 13175−13181. (242) Carper, W. R.; Langenwalter, K.; Nooruddin, N. S.; Kullman, M. J.; Gerhard, D.; Wasserscheid, P. Aggregation Models of Potential Cyclical Trimethylsulfonium Dicyanamide Ionic Liquid Clusters. J. Phys. Chem. B 2009, 113, 2031−2041. (243) Frenkel, M. L.; Kabo, G. J.; Marsh, K. N.; Roganov, G. N.; Wilhoit, R. C. Thermodynamics of Organic Compounds in the Gas Phase; Thermodynamics Research Center: Texas, 1994; Vol. 1. (244) Paulechka, Y. U.; Kabo, G. J.; Blokhin, A. V.; Vydrov, O. A.; Magee, J. W.; Frenkel, M. Thermodynamic Properties of 1-butyl-3methylimidazolium Hexafluorophosphate in the Ideal Gas State. J. Chem. Eng. Data 2003, 48, 457−462. (245) Blokhin, A. V.; Paulechka, Y. U.; Strechan, A. A.; Kabo, G. J. Physicochemical Properties, Structure, and Conformations of 1-Butyl3-methylimidazolium Bis(Trifluoromethanesulfonyl)imide C(4)mim NTf2 Ionic Liquid. J. Phys. Chem. B 2008, 112, 4357−4364. (246) Paulechka, Y. U.; Kabo, G. J.; Emel’yanenko, V. N. Structure, Conformations, Vibrations, and Ideal-Gas Properties of 1-Alkyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide Ionic Pairs and Constituent Ions. J. Phys. Chem. B 2008, 112, 15708−15717. (247) Leal, J. P.; Esperanca, J. M. S. S.; Minas da Piedade, M. E.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Seddon, K. R. The Nature of Ionic Liquids in the Gas Phase. J. Phys. Chem. A 2007, 111, 6176− 6182. (248) Armstrong, J. P.; Hurst, C.; Jones, R. G.; Licence, P.; Lovelock, K. R. J.; Satterley, C. J.; Villar-Garcia, I. J. Vapourisation of Ionic Liquids. Phys. Chem. Chem. Phys. 2007, 9, 982−990. (249) Dong, K.; Zhao, L.; Wang, Q.; Song, Y.; Zhang, S. Are Ionic Liquids Pairwise in Gas Phase? A Cluster Approach and in situ IR Study. Phys. Chem. Chem. Phys. 2013, 15, 6034−6040. (250) Obi, E. I.; Leavitt, C. M.; Raston, P. L.; Moradi, C. P.; Flynn, S. D.; Vaghjiani, G. L.; Boatz, J. A.; Chambreau, S. D.; Douberly, G. E. Helium Nanodroplet Isolation and Infrared Spectroscopy of the Isolated Ion-Pair 1-Ethyl-3-methylimidazolium bisAY


Chemical Reviews

Review

(trifluoromethylsulfonyl)imide. J. Phys. Chem. A 2013, 117, 9047− 9056. (251) Cooper, R.; Zolot, A. M.; Boatz, J. A.; Sporleder, D. P.; Stearns, J. A. IR and UV Spectroscopy of Vapor-Phase Jet-Cooled Ionic Liquid [emim]+ [Tf2N]−: Ion Pair Structure and Photodissociation Dynamics. J. Phys. Chem. A 2013, 117, 12419−12428. (252) Fournier, J. A.; Wolke, C. T.; Johnson, C. J.; McCoy, A. B.; Johnson, M. A. Comparison of the Local Binding Motifs in the Imidazolium-Based Ionic Liquids [EMIM][BF4] and [EMMIM][BF4] Through Cryogenic Ion Vibrational Predissociation Spectroscopy: Unraveling the Roles of Anharmonicity and Intermolecular Interactions. J. Chem. Phys. 2015, 142, No. 064306. (253) Hanke, K.; Kaufmann, M.; Schwaab, G.; Wolke, C. T.; Gorlova, O.; Johnson, M. A.; Kar, B. P.; Sander, W.; Sanchez-Garcia, E.; Havenith, M. Understanding the Ionic Liquid [NC4111][NTf2] from Individual Building Blocks: An IR-Spectroscopic Study. Phys. Chem. Chem. Phys. 2015, 17, 8518−8529. (254) Booth, R. S.; Annesley, C. J.; Young, J. W.; Vogelhuber, K. M.; Boatz, J. A.; Stearns, J. A. Identification of Multiple Conformers of the Ionic Liquid [emim][tf2n] in the Gas Phase Using IR/UV Action Spectroscopy. Phys. Chem. Chem. Phys. 2016, 18, 17037−17043. (255) Berg, R. W.; Canongia Lopes, J. N.; Ferreira, R.; Rebelo, L. P. N.; Seddon, K. R.; Tomaszowska, A. A. Raman Spectroscopic Study of the Vapor Phase of l-Methylimidazolium Ethanoate, a Protic Ionic Liquid. J. Phys. Chem. A 2010, 114, 10834−10841. (256) Materazzi, S.; Vecchio, S. Evolved Gas Analysis by Infrared Spectroscopy. Appl. Spectrosc. Rev. 2010, 45, 241−273. (257) Chambreau, S. D.; Schenk, A. C.; Sheppard, A. J.; Yandek, G. R.; Vaghjiani, G. L.; Maciejewski, J.; Koh, C. J.; Golan, A.; Leone, S. R. Thermal Decomposition Mechanisms of Alkylimidazolium Ionic Liquids with Cyano-Functionalized Anions. J. Phys. Chem. A 2014, 118, 11119−11132. (258) Liaw, H.-J.; Chen, C.-C.; Chen, Y.-C.; Chen, J.-R.; Huang, S.K.; Liu, S.-N. Relationship Between Flash Point of Ionic Liquids and Their Thermal Decomposition. Green Chem. 2012, 14, 2001−2008. (259) Palumbo, O.; Trequattrini, F.; Vitucci, F. M.; Navarra, M. A.; Panero, S.; Paolone, A. An Infrared Spectroscopy Study of the Conformational Evolution of the Bis(trifluoromethanesulfonyl) imide Ion in the Liquid and in the Glass State. Adv. Condens. Matter Phys. 2015, 2015, 176067. (260) Dommert, F.; Wendler, K.; Berger, R.; Delle Site, L.; Holm, C. Force Fields for Studying the Structure and Dynamics of Ionic Liquids: A Critical Review of Recent Developments. ChemPhysChem 2012, 13, 1625−1637. (261) Borodin, O. Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids. J. Phys. Chem. B 2009, 113, 11463−11478. (262) Canongia Lopes, J. N.; Padua, A. A. H. Using Spectroscopic Data on Imidazolium Cation Conformations to Test a Molecular Force Field for Ionic Liquids. J. Phys. Chem. B 2006, 110, 7485−7489. (263) Allerhand, A.; Schleyer, P. v. R. Solvent Effects in Infrared Spectroscopic Studies of Hydrogen Bonding. J. Am. Chem. Soc. 1963, 85, 371−380. (264) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman & Co.: San Francisco, 1960. (265) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, 1997. (266) Steiner, T. The Hydrogen Bond in the Solid State. Angew. Chem., Int. Ed. 2002, 41, 48−76. (267) Buckingham, A. D.; Del Bene, J. E.; McDowell, S. A. C. The Hydrogen Bond. Chem. Phys. Lett. 2008, 463, 1−10. (268) Pitawala, J.; Scheers, J.; Jacobsson, P.; Matic, A. Physical Properties, Ion-Ion Interactions, and Conformational States of Ionic Liquids with Alkyl-Phosphonate Anions. J. Phys. Chem. B 2013, 117, 8172−8179. (269) Dong, K.; Song, Y.; Liu, X.; Cheng, W.; Yao, X.; Zhang, S. Understanding Structures and Hydrogen Bonds of Ionic Liquids at the Electronic Level. J. Phys. Chem. B 2012, 116, 1007−1017.

(270) Kato, T.; Takenaka, T. Raman-Study of Rotational Motion and Vibrational Dephasing Dynamics of NO3- in Molten Nitrates. Mol. Phys. 1985, 54, 1393−1414. (271) Griffiths, P. R. Far-Infrared Spectroscopy. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, UK, 2002; Vol. 1, pp 229−239. (272) Griffiths, P. R.; Homes, C. C. Instrumentation for Far-Infrared Spectroscopy. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, UK, 2002; Vol. 1, pp 326−336. (273) Fumino, K.; Wulf, A.; Ludwig, R. The Cation-Anion Interaction in Ionic Liquids Probed by Far-Infrared Spectroscopy. Angew. Chem., Int. Ed. 2008, 47, 3830−3834. (274) Fumino, K.; Wulf, A.; Ludwig, R. The Potential Role of Hydrogen Bonding in Aprotic and Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2009, 11, 8790−8794. (275) Fumino, K.; Ludwig, R. Analyzing the Interaction Energies between Cation and Anion in Ionic Liquids: the Subtle Balance between Coulomb Forces and Hydrogen Bonding. J. Mol. Liq. 2014, 192, 94−102. (276) Fumino, K.; Wulf, A.; Ludwig, R. Hydrogen Bonding in Protic Ionic Liquids: Reminiscent of Water. Angew. Chem., Int. Ed. 2009, 48, 3184−3186. (277) Fumino, K.; Reichert, E.; Wittler, K.; Hempelmann, R.; Ludwig, R. Low-Frequency Vibrational Modes of Protic Molten Salts and Ionic Liquids: Detecting and Quantifying Hydrogen Bonds. Angew. Chem., Int. Ed. 2012, 51, 6236−6240. (278) Yamada, T.; Tominari, Y.; Tanaka, S.; Mizuno, M.; Fukunaga, K. Vibration Modes at Terahertz and Infrared Frequencies of Ionic Liquids Consisting of an Imidazolium Cation and a Halogen Anion. Materials 2014, 7, 7409−7422. (279) Wulf, A.; Fumino, K.; Ludwig, R. Spectroscopic Evidence for an Enhanced Anion-Cation Interaction from Hydrogen Bonding in Pure Imidazolium Ionic Liquids. Angew. Chem., Int. Ed. 2010, 49, 449− 453. (280) Fumino, K.; Wittler, K.; Ludwig, R. The Anion Dependence of the Interaction Strength between Ions in Imidazolium-Based Ionic Liquids Probed by Far-Infrared Spectroscopy. J. Phys. Chem. B 2012, 116, 9507−9511. (281) Fumino, K.; Fossog, V.; Wittler, K.; Hempelmann, R.; Ludwig, R. Dissecting Anion-Cation Interaction Energies in Protic Ionic Liquids. Angew. Chem., Int. Ed. 2013, 52, 2368−2372. (282) Fumino, K.; Wulf, A.; Verevkin, S. P.; Heintz, A.; Ludwig, R. Estimating Enthalpies of Vaporization of Imidazolium-Based Ionic Liquids from Far-Infrared Measurements. ChemPhysChem 2010, 11, 1623−1626. (283) Roth, C.; Peppel, T.; Fumino, K.; Köckerling, M.; Ludwig, R. The Importance of Hydrogen Bonds for the Structure of Ionic Liquids: Single-Crystal X-ray Diffraction and Transmission and Attenuated Total Reflection Spectroscopy in the Terahertz Region. Angew. Chem., Int. Ed. 2010, 49, 10221−10224. (284) Fumino, K.; Fossog, V.; Stange, P.; Paschek, D.; Hempelmann, R.; Ludwig, R. Controlling the Subtle Energy Balance in Protic Ionic Liquids: Dispersion Forces Compete with Hydrogen Bonds. Angew. Chem., Int. Ed. 2015, 54, 2792−2795. (285) Ludwig, R. The Effect of Dispersion Forces on the Interaction Energies and Far Infrared Spectra of Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2015, 17, 13790−13793. (286) Zaitsau, D. H.; Emel’yanenko, V. N.; Stange, P.; Schick, C.; Verevkin, S. P.; Ludwig, R. Dispersion and Hydrogen Bonding Rule: Why the Vaporization Enthalpies of Aprotic Ionic Liquids Are Signifcantly Lager than those of Protic Ionic Liquids. Angew. Chem., Int. Ed. 2016, 55, 11682−11686. (287) Buffeteau, T.; Grondin, J.; Danten, Y.; Lassègues, J.-C. Imidazolium-Based Ionic Liquids: Quantitative Aspects in the FarInfrared Region. J. Phys. Chem. B 2010, 114, 7587−7592. (288) Wulf, A.; Fumino, K.; Ludwig, R.; Taday, P. F. Combined THz, FIR and Raman Spectroscopy Studies of Imidazolium-Based Ionic AZ


Chemical Reviews

Review

Liquids Covering the Frequency Range 2−300 cm(−1). ChemPhysChem 2010, 11, 349−353. (289) Winterling, G. Very-Low-Frequency Raman-Scattering in Vitreous Silica. Physi. Rev. B 1975, 12, 2432−2440. (290) Gochiyaev, V. Z.; Malinovsky, V. K.; Novikov, V. N.; Sokolov, A. P. Structure of the Rayleigh Line Wing in Highly Viscous Liquids. Philos. Mag. B 1991, 63, 777−787. (291) Kojima, S. Low-Frequency Raman Investigation of the LiquidGlass Transition in Glycerol. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 2924−2927. (292) Sokolov, A. P.; Kisliuk, A.; Quitmann, D.; Kudlik, A.; Rossler, E. The Dynamics of Strong and Fragile Glass Formers: Vibrational and Relaxation Contributions. J. Non-Cryst. Solids 1994, 172−174, 138− 153. (293) Alba-Simionesco, C.; Krakoviack, V.; Krauzman, M.; Migliardo, P.; Romain, F. Low-Wavenumber Raman Scattering of Molecular Glass-Forming Liquids. J. Raman Spectrosc. 1996, 27, 715−721. (294) Papatheodorou, G. N.; Kalogrianitis, S. G.; Mihopoulos, T. G.; Pavlatou, E. A. Isotropic and Anisotropic Raman Scattering from Molten LiCl-CsCl Mixtures: Composition and Temperature Effects. J. Chem. Phys. 1996, 105, 2660−2667. (295) Dracopoulos, V.; Papatheodorou, G. N. Isotropic and Anisotropic Raman Scattering from Molten Alkali-Metal Fluorides. Phys. Chem. Chem. Phys. 2000, 2, 2021−2025. (296) Kirillov, S. A.; Pavlatou, E. A.; Papatheodorou, G. N. Instantaneous Collision Complexes in Molten Alkali Halides: Picosecond Dynamics from Low-Frequency Raman Data. J. Chem. Phys. 2002, 116, 9341−9351. (297) Aliotta, F.; Maisano, G.; Migliardo, P.; Vasi, C.; Wanderlingh, F.; Smith, G. P.; Triolo, R. Vibrational Dynamics of Glassy and Molten ZnCl2. J. Chem. Phys. 1981, 75, 613−618. (298) Yannopoulos, S. N.; Papatheodorou, G. N. Critical Experimental Facts Pertaining to Models and Associated Universalities for Low-Frequency Raman Scattering in Inorganic Glass Formers. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 3728−3734. (299) Kirillov, S. A.; Yannopoulos, S. N. Charge-Current Contribution to Low-Frequency Raman Scattering from GlassForming Ionic Liquids. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 11391−11399. (300) Kalampounias, A. G.; Papatheodorou, G. N.; Yannopoulos, S. N. Light Scattering from Glass-Forming Molten Salts. Z. Naturforsch., A: Phys. Sci. 2002, 57, 65−70. (301) Ribeiro, M. C. C.; de Oliveira, L. F. C.; Goncalves, N. S. Boson Peak in the Room-Temperature Molten Salt Tetra(N-Butyl)ammonium Croconate. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 104303. (302) Ribeiro, M. C. C. Correlation Between Quasielastic Raman Scattering and Configurational Entropy in an Ionic Liquid. J. Phys. Chem. B 2007, 111, 5008−5015. (303) Shuker, R.; Gammon, R. W. Raman-Scattering Selection-Rule Breaking and Density of States in Amorphous Materials. Phys. Rev. Lett. 1970, 25, 222−225. (304) Shuker, R.; Gammon, R. W. Low-Frequency Vibrational Light Scattering in Viscous Liquids. J. Chem. Phys. 1971, 55, 4784−4788. (305) Surovtsev, N. V.; Sokolov, A. P. Frequency Behavior of Raman Coupling Coefficient in Glasses. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 054205−054211. (306) Yannopoulos, S. N.; Andrikopoulos, K. S.; Ruocco, G. On the Analysis of the Vibrational Boson Peak and Low-Energy Excitations in Glasses. J. Non-Cryst. Solids 2006, 352, 4541−4551. (307) Buchenau, U.; Zorn, R. A Relation Between Fast and Slow Motions in Glassy and Liquid Selenium. Europhys. Lett. 1992, 18, 523−528. (308) Krüger, M.; Soltwisch, M.; Petscherizin, I.; Quitmann, D. Light-Scattering from Disorder and Glass-Transition-Dynamics in GeSBr2. J. Chem. Phys. 1992, 96, 7352−7363. (309) Kojima, S.; Novikov, V. N. Correlation of Temperature Dependence of Quasielastic-Light-Scattering Intensity and Alpha-

Relaxation Time. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 222−227. (310) Yannopoulos, S. N.; Papatheodorou, G. N.; Fytas, G. LowEnergy Excitations in Noncrystalline Arsenic Trioxide. J. Chem. Phys. 1997, 107, 1341−1349. (311) Ribeiro, M. C. C. Low-frequency Raman Spectra and Fragility of Imidazolium Ionic Liquids. J. Chem. Phys. 2010, 133, 024503. (312) Ribeiro, M. C. C. Intermolecular Vibrations and Fast Relaxations in Supercooled Ionic Liquids. J. Chem. Phys. 2011, 134, 244507. (313) Kinoshita, S.; Kai, Y.; Yamaguchi, M.; Yagi, T. Direct Comparison Between Ultrafast Optical Kerr-Effect and HighResolution Light Scattering Spectroscopy. Phys. Rev. Lett. 1995, 75, 148−151. (314) Yannopoulos, S. N. The Frequency-Dependent Depolarization Ratio of the Low-Frequency Raman Scattering of Two Inorganic Systems in Their Glassy, Supercooled, and Molten States. J. Chem. Phys. 2000, 113, 5868−5872. (315) Hyun, B. R.; Dzyuba, S. V.; Bartsch, R. A.; Quitevis, E. L. Intermolecular Dynamics of Room-Temperature Ionic Liquids: Femtosecond Optical Kerr Effect Measurements on 1-Alkyl-3methylimidazolium Bis((trifluoromethyl)sulfonyl)imides. J. Phys. Chem. A 2002, 106, 7579−7585. (316) Giraud, G.; Gordon, C. M.; Dunkin, I. R.; Wynne, K. The Effects of Anion and Cation Substitution on the Ultrafast Solvent Dynamics of Ionic Liquids: A Time-Resolved Optical Kerr-Effect Spectroscopic Study. J. Chem. Phys. 2003, 119, 464−477. (317) Rajesh Rajian, J.; Li, S. F.; Bartsch, R. A.; Quitevis, E. L. Temperature-Dependence of the Low-Frequency Spectrum of 1pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide Studied by Optical Kerr Effect Spectroscopy. Chem. Phys. Lett. 2004, 393, 372−377. (318) Shirota, H.; Funston, A. M.; Wishart, J. F.; Castner, E. W. Ultrafast Dynamics of Pyrrolidinium Cation Ionic Liquids. J. Chem. Phys. 2005, 122, 184512. (319) Xiao, D.; Rajian, J. R.; Cady, A.; Li, S.; Bartsch, R. A.; Quitevis, E. L. Nanostructural Organization and Anion Effects on the Temperature Dependence of the Optical Kerr Effect Spectra of Ionic Liquids. J. Phys. Chem. B 2007, 111, 4669−4677. (320) Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R.; Xiao, D.; Hines, L. G., Jr.; Bartsch, R. A.; Quitevis, E. L.; Pleckhova, N.; Seddon, K. R. Morphology and Intermolecular Dynamics of 1-Alkyl-3methylimidazolium Bis{(trifluoromethane)sulfonyl}amide Ionic Liquids: Structural and Dynamic Evidence of Nanoscale Segregation. J. Phys.: Condens. Matter 2009, 21, 424121. (321) Zhong, Q.; Fourkas, J. T. Optical Kerr Effect Spectroscopy of Simple Liquids. J. Phys. Chem. B 2008, 112, 15529−15539. (322) Shirota, H. Comparison of Low-Frequency Spectra between Aromatic and Nonaromatic Cation Based Ionic Liquids Using Femtosecond Raman-Induced Kerr Effect Spectroscopy. ChemPhysChem 2012, 13, 1638−1648. (323) Sonnleitner, T.; Turton, D. A.; Hefter, G.; Ortner, A.; Waselikowski, S.; Walther, M.; Wynne, K.; Buchner, R. UltraBroadband Dielectric and Optical Kerr-Effect Study of the Ionic Liquids Ethyl and Propylammonium Nitrate. J. Phys. Chem. B 2015, 119, 8826−8841. (324) Krüger, M.; Bründermann, E.; Funkner, S.; Weingärtner, H.; Havenith, M. Communications: Polarity Fluctuations of the Protic Ionic Liquid Ethylammonium Nitrate in the Terahertz Regime. J. Chem. Phys. 2010, 132, 101101. (325) Clarke, J. H. R. Raman Spectra of Lattice Vibrations in Liquid and Solid Monovalent Metal Nitrates. Chem. Phys. Lett. 1969, 4, 39− 42. (326) Kirillov, S. A.; Nielsen, O. F. Boson Peak in the LowFrequency Raman Spectra of an Ordinary Liquid. J. Mol. Struct. 2000, 526, 317−321. (327) Surovtsev, N. V.; Pugachev, A. M.; Nenashev, B. G.; Malinovsky, V. K. Low-frequency Raman Scattering in As2S3 Glass BA


Chemical Reviews

Review

Former around the Liquid-glass Transition. J. Phys.: Condens. Matter 2003, 15, 7651−7662. (328) Kalampounias, A. G.; Yannopoulos, S. N.; Papatheodorou, G. N. A Low-Frequency Raman Study of Glassy, Supercooled and Molten Silica and the Preservation of the Boson Peak in the Equilibrium Liquid State. J. Non-Cryst. Solids 2006, 352, 4619−4624. (329) Kalampounias, A. G. Low-energy Excitations in a Low-Viscous Glass-Forming Liquid. Bull. Mater. Sci. 2013, 36, 607−611. (330) Li, J.; Wang, I.; Fruchey, K.; Fayer, M. D. Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis. J. Phys. Chem. A 2006, 110, 10384−10391. (331) Balucani, U.; Zoppi, M. Dynamics of the Liquid State; Clarendon Press: Oxford, 1994. (332) Sette, F.; Krisch, M. H.; Masciovecchio, C.; Ruocco, G.; Monaco, G. Dynamics of Glasses and Glass-Forming Liquids Studied by Inelastic X-ray Scattering. Science 1998, 280, 1550−1555. (333) Ruocco, G.; Sette, F. High-Frequency Vibrational Dynamics in Glasses. J. Phys.: Condens. Matter 2001, 13, 9141−9164. (334) Monaco, G. High-Resolution Inelastic X-Ray Scattering to Study the High-Frequency Atomic Dynamics of Disordered Systems. C. R. Phys. 2008, 9, 608−623. (335) Shintani, H.; Tanaka, H. Universal Link between the Boson Peak and Transverse Phonons in Glass. Nat. Mater. 2008, 7, 870−877. (336) Chumakov, A. I.; Monaco, G.; Monaco, A.; Crichton, W. A.; Bosak, A.; Rüffer, R.; Meyer, A.; Kargl, F.; Comez, L.; Fioretto, D.; et al. Equivalence of the Boson Peak in Glasses to the Transverse Acoustic van Hove Singularity in Crystals. Phys. Rev. Lett. 2011, 106, 225501. (337) Kumar, P.; Wikfeldt, K. T.; Schlesinger, D.; Pettersson, L. G. M.; Stanley, H. E. The Boson Peak in Supercooled Water. Sci. Rep. 2013, 3, 1980. (338) Chumakov, A. I.; Monaco, G. Understanding the Atomic Dynamics and Thermodynamics of Glasses: Status and Outlook. J. Non-Cryst. Solids 2015, 407, 126−132. (339) Buchenau, U.; Wischnewski, A.; Richter, D.; Frick, B. Is the Fast Process at the Glass Transition Mainly Due to Long Wavelength Excitations? Phys. Rev. Lett. 1996, 77, 4035−4038. (340) Sokolov, A. P. Vibrations at the boson peak: Random- and Coherent-Phase Contributions. J. Phys.: Condens. Matter 1999, 11, A213−A218. (341) Urahata, S. M.; Ribeiro, M. C. C. Single Particle Dynamics in Ionic Liquids of 1-alkyl-3-methylimidazolium Cations. J. Chem. Phys. 2005, 122, 024511. (342) Cavalcante, A. d. O.; Ribeiro, M. C. C.; Skaf, M. S. Polarizability Effects on the Structure and Dynamics of Ionic Liquids. J. Chem. Phys. 2014, 140, 114108. (343) Sunda, A. P.; Mondal, A.; Balasubramanian, S. Atomistic Simulations of Ammonium-Based Protic Ionic Liquids: Steric Effects on Structure, Low-Frequency Vibrational Modes and Electrical Conductivity. Phys. Chem. Chem. Phys. 2015, 17, 4625−4633. (344) Hu, Z.; Huang, X.; Annapureddy, H. V. R.; Margulis, C. J. Molecular Dynamics Study of the Temperature-Dependent Optical Kerr Effect Spectra and Intermolecular Dynamics of Room Temperature Ionic Liquid 1-Methoxyethylpyridinium Dicyanoamide. J. Phys. Chem. B 2008, 112, 7837−7849. (345) Shirota, H.; Castner, E. W. Physical Properties and Intermolecular Dynamics of an Ionic Liquid Compared with Its Isoelectronic Neutral Binary Solution. J. Phys. Chem. A 2005, 109, 9388−9392. (346) Elola, M. D.; Ladanyi, B. M. Polarizability Response in Polar Solvents: Molecular-Dynamics Simulations of Acetonitrile And Chloroform. J. Chem. Phys. 2005, 122, 224506. (347) McGreevy, R. L. Experimental Studies of the Structure and Dynamics of Molten Alkali and Alkaline-Earth Halides. Solid State Phys. 1987, 40, 247−325. (348) Wilkes, J. S.; Zaworotko, M. J. Air and Water Stable 1-Ethyl-3methylimidazolium Based Ionic Liquids. J. Chem. Soc., Chem. Commun. 1992, 13, 965−967.

(349) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. Ionic Liquid Crystals: Hexafluorophosphate Salts. J. Mater. Chem. 1998, 8, 2627−2636. (350) Holbrey, J. D.; Seddon, K. R. The Phase Behaviour of 1-Alkyl3-methylimidazolium Tetrafluoroborates; Ionic Liquids and Ionic Liquid Crystals. J. Chem. Soc., Dalton Trans. 1999, No. 13, 2133−2140. (351) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases. J. Phys. Chem. B 1999, 103, 4164−4170. (352) Dupont, J. On the Solid, Liquid and Solution Structural Organization of Imidazolium Ionic Liquids. J. Braz. Chem. Soc. 2004, 15, 341−350. (353) Goossens, K.; Lava, K.; Nockemann, P.; Van Hecke, K.; Van Meervelt, L.; Driesen, K.; Görller-Walrand, C.; Binnemans, K.; Cardinaels, T. Pyrrolidinium Ionic Liquid Crystals. Chem. - Eur. J. 2009, 15, 656−674. (354) Binnemans, K. Ionic Liquid Crystals. Chem. Rev. 2005, 105, 4148−4204. (355) De Roche, J.; Gordon, C. M.; Imrie, C. T.; Ingram, M. D.; Kennedy, A. R.; Lo Celso, F.; Triolo, A. Application of Complementary Experimental Techniques to Characterization of the Phase Behavior of [C16mim][PF6] and [C14mim][PF6]. Chem. Mater. 2003, 15, 3089−3097. (356) Goossens, K.; Lava, K.; Bielawski, C. W.; Binnemans, K. Ionic Liquid Crystals: Versatile Materials. Chem. Rev. 2016, 116, 4643− 4807. (357) Endo, T.; Kato, T.; Tozaki, K.-i.; Nishikawa, K. Phase Behaviors of Room Temperature Ionic Liquid Linked with Cation Conformational Changes: 1-Butyl-3-methylimidazolium Hexafluorophosphate. J. Phys. Chem. B 2010, 114, 407−411. (358) Endo, T.; Murata, H.; Imanari, M.; Mizushima, N.; Seki, H.; Nishikawa, K. NMR Study of Cation Dynamics in Three Crystalline States of 1-Butyl-3-methylimidazolium Hexafluorophosphate Exhibiting Crystal Polymorphism. J. Phys. Chem. B 2012, 116, 3780−3788. (359) Endo, T.; Nishikawa, K. Thermal Phase Behavior of 1-Butyl-3Methylimidazolium Hexafluorophosphate: Simultaneous Measurements of the Melting of Two Polymorphic Crystals by Raman Spectroscopy and Calorimetry. Chem. Phys. Lett. 2013, 584, 79−82. (360) Herstedt, M.; Henderson, W. A.; Smirnov, M.; Ducasse, L.; Servant, L.; Talaga, D.; Lassègues, J. C. Conformational Isomerism and Phase Transitions in Tetraethylammonium bis(trifluoromethanesulfonyl)imide Et4NTFSI. J. Mol. Struct. 2006, 783, 145−156. (361) Bakker, A.; Gejji, S.; Lindgren, J.; Hermansson, K.; Probst, M. M. Contact Ion Pair Formation and Ether Oxygen Coordination in the Polymer Electrolytes M[N(CF3SO2)2]2PEOn for M = Mg, Ca, Sr and Ba. Polymer 1995, 36, 4371−4378. (362) Henderson, W. A.; Herstedt, M.; Young, V. G.; Passerini, S.; De Long, H. C.; Trulove, P. C. New Disordering Mode for TFSIanions: The Nonequilibrium, Plastic Crystalline Structure of Et4NTFSI. Inorg. Chem. 2006, 45, 1412−1414. (363) Martinelli, A. Conformational Changes and Phase Behaviour in the Protic Ionic Liquid 1-Ethylimidazolium Bis(trifluoromethylsulfonyl)imide in the Bulk and Nano-Confined State. Eur. J. Inorg. Chem. 2015, 2015, 1300−1308. (364) Capitani, F.; Gatto, S.; Postorino, P.; Palumbo, O.; Trequattrini, F.; Deutsch, M.; Brubach, J. B.; Roy, P.; Paolone, A. The Complex Dance of the Two Conformers of Bis(trifluoromethanesulfonyl)imide as a Function of Pressure and Temperature. J. Phys. Chem. B 2016, 120, 1312. (365) Saouane, S.; Norman, S. E.; Hardacre, C.; Fabbiani, F. P. A. Pinning Down the Solid-State Polymorphism of the Ionic Liquid [bmim][PF6]. Chem. Sci. 2013, 4, 1270−1280. (366) Xu, W.; Cooper, E. I.; Angell, C. A. Ionic liquids: Ion Mobilities, Glass Temperatures, and Fragilities. J. Phys. Chem. B 2003, 107, 6170−6178. (367) Mudring, A.-V. Solidification of Ionic Liquids: Theory and Techniques. Aust. J. Chem. 2010, 63, 544−564. BB


Chemical Reviews

Review

(368) Nishikawa, K.; Wang, S. L.; Katayanagi, H.; Hayashi, S.; Hamaguchi, H. O.; Koga, Y.; Tozaki, K. I. Melting and Freezing Behaviors of Prototype Ionic Liquids, 1-Butyl-3-methylimidazolium Bromide and Its Chloride, Studied by Using a Nano-Watt Differential Scanning Calorimeter. J. Phys. Chem. B 2007, 111, 4894−4900. (369) Nishikawa, K.; Wang, S.; Tozaki, K.-i. Rhythmic Melting and Crystallizing of Ionic Liquid 1-butyl-3-methylimidazolium bromide. Chem. Phys. Lett. 2008, 458, 88−91. (370) Nishikawa, K.; Tozaki, K.-i. Intermittent Crystallization of an Ionic Liquid: 1-isopropyl-3-methylimidazolium bromide. Chem. Phys. Lett. 2008, 463, 369−372. (371) Nishikawa, K.; Wang, S.; Endo, T.; Tozaki, K.-i. Melting and Crystallization Behaviors of an Ionic Liquid, 1-Isopropyl-3-methylimidazolium Bromide, Studied by Using Nanowatt-Stabilized Differential Scanning Calorimetry. Bull. Chem. Soc. Jpn. 2009, 82, 806−812. (372) Okajima, H.; Hamaguchi, H. Unusually Long trans/gauche Conformational Equilibration Time during the Melting Process of BmimC1, a Prototype Ionic Liquid. Chem. Lett. 2011, 40, 1308−1309. (373) Imanari, M.; Fujii, K.; Endo, T.; Seki, H.; Tozaki, K.-i.; Nishikawa, K. Ultraslow Dynamics at Crystallization of a RoomTemperature Ionic Liquid, 1-Butyl-3-methylimidazolium Bromide. J. Phys. Chem. B 2012, 116, 3991−3997. (374) Chang, H.-C.; Chang, C.-Y.; Su, J.-C.; Chu, W.-C.; Jiang, J.-C.; Lin, S. H. Conformations of 1-butyl-3-methylimidazolium chloride Probed by High Pressure Raman Spectroscopy. Int. J. Mol. Sci. 2006, 7, 417−424. (375) Chang, H.-C.; Jiang, J.-C.; Su, J.-C.; Chang, C.-Y.; Lin, S. H. Evidence of Rotational Isomerism in 1-Butyl-3-methylimidazolium Halides: A Combined High-Pressure Infrared and Raman Spectroscopic Study. J. Phys. Chem. A 2007, 111, 9201−9206. (376) Su, L.; Li, M.; Zhu, X.; Wang, Z.; Chen, Z.; Li, F.; Zhou, Q.; Hong, S. In Situ Crystallization of Low-Melting Ionic Liquid BMIM PF(6) under High Pressure up to 2 GPa. J. Phys. Chem. B 2010, 114, 5061−5065. (377) Russina, O.; Fazio, B.; Schmidt, C.; Triolo, A. Structural Organization and Phase Behaviour of 1-Butyl-3-methylimidazolium Hexafluorophosphate: An High Pressure Raman Spectroscopy Study. Phys. Chem. Chem. Phys. 2011, 13, 12067−12074. (378) Takekiyo, T.; Imai, Y.; Hatano, N.; Abe, H.; Yoshimura, Y. Conformational Preferences of Two Imidazolium-Based Ionic Liquids at High Pressures. Chem. Phys. Lett. 2011, 511, 241−246. (379) Su, L.; Zhu, X.; Wang, Z.; Cheng, X.; Wang, Y.; Yuan, C.; Chen, Z.; Ma, C.; Li, F.; Zhou, Q.; et al. In Situ Observation of Multiple Phase Transitions in Low-Melting Ionic Liquid BMIM BF4 under High Pressure up to 30 GPa. J. Phys. Chem. B 2012, 116, 2216− 2222. (380) Yoshimura, Y.; Abe, H.; Takekiyo, T.; Shigemi, M.; Hamaya, N.; Wada, R.; Kato, M. Superpressing of a Room Temperature Ionic Liquid, 1-Ethyl-3-methylimidazolium Tetrafluoroborate. J. Phys. Chem. B 2013, 117, 12296−12302. (381) Abe, H.; Takekiyo, T.; Hatano, N.; Shigemi, M.; Hamaya, N.; Yoshimura, Y. Pressure-Induced Frustration−Frustration Process in 1Butyl-3-methylimidazolium Hexafluorophosphate, a Room-Temperature Ionic Liquid. J. Phys. Chem. B 2014, 118, 1138−1145. (382) Li, J.; Su, L.; Zhu, X.; Li, H.; Cheng, X.; Li, L. DecompressionInduced Disorder to Order Phase Transition in Low-Melting Ionic Liquid [OMIM][PF6]. Chin. Sci. Bull. 2014, 59, 2980−2986. (383) Li, H.; Su, L.; Zhu, X.; Cheng, X.; Yang, K.; Yang, G. In Situ Crystallization of Ionic Liquid [Emim][PF6] from Methanol Solution under High Pressure. J. Phys. Chem. B 2014, 118, 8684−8690. (384) Ren, Y.; Li, H.; Zhu, X.; Chen, L.; Su, L.; Yang, K.; Yang, G.; Wang, H. Pressure-induced Amorphization of Ionic Liquid [HMIM][PF6]. Chem. Phys. Lett. 2015, 629, 8−12. (385) Wu, J.; Zhu, X.; Li, H.; Su, L.; Yang, K.; Cheng, X.; Yang, G.; Liu, J. Combined Raman Scattering and X-ray Diffraction Study of Phase Transition of the Ionic Liquid [BMIM][TFSI] Under High Pressure. J. Solution Chem. 2015, 44, 2106−2116. (386) Yoshimura, Y.; Shigemi, M.; Takaku, M.; Yamamura, M.; Takekiyo, T.; Abe, H.; Hamaya, N.; Wakabayashi, D.; Nishida, K.;

Funamori, N.; et al. Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature. J. Phys. Chem. B 2015, 119, 8146−8153. (387) Faria, L. F. O.; Ribeiro, M. C. C. Phase Transitions of TriflateBased Ionic Liquids under High Pressure. J. Phys. Chem. B 2015, 119, 14315−14322. (388) Li, H.; Wang, Z.; Chen, L.; Wu, J.; Huang, H.; Yang, K.; Wang, Y.; Su, L.; Yang, G. Kinetic Effect on Pressure-Induced Phase Transitions of Room Temperature Ionic Liquid, 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate. J. Phys. Chem. B 2015, 119, 14245−14251. (389) Yoshimura, Y.; Takekiyo, T.; Abe, H.; Hamaya, N. HighPressure Phase Behavior of the Room Temperature Ionic Liquid 1ethyl-3-methylimidazolium Nitrate. J. Mol. Liq. 2015, 206, 89−94. (390) Zhu, X.; Yuan, C.; Li, H.; Zhu, P.; Su, L.; Yang, K.; Wu, J.; Yang, G.; Liu, J. Successive Disorder to Disorder Phase Transitions in Ionic Liquid [HMIM][BF4] Under High Pressure. J. Mol. Struct. 2016, 1106, 70−75. (391) Imai, Y.; Abe, H.; Goto, T.; Takekiyo, T.; Yoshimura, Y. Pressure-Induced Raman Spectral Changes of N,N,diethyl-N-methylN-(2-methoxyethyl) Ammonium Tetrafluoroborate. High Pressure Res. 2009, 29, 536−541. (392) Capitani, F.; Fasolato, C.; Mangialardo, S.; Signorelli, S.; Gontrani, L.; Postorino, P. Heterogeneity of Propyl-ammonium Nitrate Solid Phases Obtained Under High Pressure. J. Phys. Chem. Solids 2015, 84, 13−16. (393) Yoshimura, Y.; Takekiyo, T.; Imai, Y.; Abe, H. PressureInduced Spectral Changes of Room-Temperature Ionic Liquid, N,NDiethyl-N-methyl-N-(2-methoxyethyl)ammonium Bis(trifluoromethylsulfonyl)imide, DEME TFSI. J. Phys. Chem. C 2012, 116, 2097−2101. (394) Capitani, F.; Trequattrini, F.; Palumbo, O.; Paolone, A.; Postorino, P. Phase Transitions of PYR14-TFSI as a Function of Pressure and Temperature: the Competition between Smaller Volume and Lower Energy Conformer. J. Phys. Chem. B 2016, 120, 2921− 2928. (395) Piermari, G. J.; Block, S.; Barnett, S. Hydrostatic Limits in Liquids and Solids to 100 kbar. J. Appl. Phys. 1973, 44, 5377−5382. (396) Ribeiro, M. C. C.; Pádua, A. A. H.; Gomes, M. F. C. Glass Transition of Ionic Liquids Under High Pressure. J. Chem. Phys. 2014, 140, 244514. (397) Yoshimura, Y.; Abe, H.; Imai, Y.; Takekiyo, T.; Hamaya, N. Decompression-Induced Crystal Polymorphism in a Room-Temperature Ionic Liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Tetrafluoroborate. J. Phys. Chem. B 2013, 117, 3264−3269. (398) García-Saiz, A.; de Pedro, I.; Blanco, J. A.; González, J.; Fernández, J. R. Pressure Effects on Emim[FeCl4], a Magnetic Ionic Liquid with Three-Dimensional Magnetic Ordering. J. Phys. Chem. B 2013, 117, 3198−3206. (399) Chen, H.-K.; Srivastava, N.; Saha, S.; Shigeto, S. Complementing Crystallography with Ultralow-Frequency Raman Spectroscopy: Structural Insights into Nitrile-Functionalized Ionic Liquids. ChemPhysChem 2016, 17, 93−97. (400) Penna, T. C.; Faria, L. F. O.; Matos, J. R.; Ribeiro, M. C. C. Pressure and Temperature Effects on Intermolecular Vibrational Dynamics of Ionic Liquids. J. Chem. Phys. 2013, 138, 104503. (401) Abe, H.; Takekiyo, T.; Aono, M.; Kishimura, H.; Yoshimura, Y.; Hamaya, N. Polymorphs in Room-Temperature Ionic Liquids: Hierarchical Structure, Confined Water and Pressure-Induced Frustration. J. Mol. Liq. 2015, 210, 200−214. (402) Rogers, R. D.; Seddon, K. R. Ionic liquids–solvents of the future? Science 2003, 302 (5646), 792−793. (403) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37, 123−150. (404) Chatel, G.; Pereira, J. F. B.; Debbeti, V.; Wang, H.; Rogers, R. D. Mixing Ionic Liquids − “Simple Mixtures” or “Double Salts”? Green Chem. 2014, 16, 2051. BC


Chemical Reviews

Review

(405) Kiefer, J.; Molina, M. M.; Noack, K. The Peculiar Nature of Molecular Interactions between an Imidazolium Ionic Liquid and Acetone. ChemPhysChem 2012, 13, 1213−1220. (406) Zhang, Q.-G.; Wang, N.-N.; Yu, Z.-W. The Hydrogen Bonding Interactions between the Ionic Liquid 1-Ethyl-3-Methylimidazolium Ethyl Sulfate and Water. J. Phys. Chem. B 2010, 114, 4747−4754. (407) Koch, H.; Noack, K.; Will, S. Raman Excess Spectroscopy vs Principal Component Analysis: Probing the Intermolecular Interactions Between Chiral Molecules and Imidazolium-Based Ionic Liquids. Phys. Chem. Chem. Phys. 2016, 18, 28370−28375. (408) Brereton, R. G. Applied Chemometrics for Scientists; John Wiley & Sons: Wiltshire, 2007. (409) Fumino, K.; Peppel, T.; Geppert-Rybczyńska, M.; Zaitsau, D. H.; Lehmann, J. K.; Verevkin, S. P.; Köckerling, M.; Ludwig, R. The Influence of Hydrogen Bonding on the Physical Properties of Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 14064−14075. (410) Fumino, K.; Bonsa, A. M.; Golub, B.; Paschek, D.; Ludwig, R. Non-Ideal Mixing Behaviour of Hydrogen Bonding in Mixtures of Protic Ionic Liquids. ChemPhysChem 2015, 16, 299−304. (411) Xiao, D.; Rajian, J. R.; Li, S.; Bartsch, R. A.; Quitevis, E. L. Additivity in the Optical Kerr Effect Spectra of Binary Ionic Liquid Mixtures: Implications for Nanostructural Organization. J. Phys. Chem. B 2006, 110, 16174−16178. (412) Quitevis, E. L.; Bardak, F.; Xiao, D.; Hines, L. G.; Son, P.; Bartsch, R. A.; Yang, P.; Voth, G. A. OKE Spectroscopy and Molecular Dynamics Simulations of Nonpolar and Polar Molecules in Ionic Liquids. In Ionic Liquids: Science and Applications; Visser, A. E., Bridges, N. J., Rogers, R. D., Eds.; American Chemical Society: Washington, DC, 2012; Vol. 1117, pp 271−287. (413) Fumino, K.; Reimann, S.; Ludwig, R. Probing Molecular Interaction in Ionic Liquids by Low Frequency Spectroscopy: Coulomb Energy, Hydrogen Bonding and Dispersion Forces. Phys. Chem. Chem. Phys. 2014, 16, 21903−21929. (414) Cha, S.; Kim, D. Anion Exchange in Ionic Liquid Mixtures. Phys. Chem. Chem. Phys. 2015, 17, 29786−29792. (415) Aparicio, S.; Atilhan, M. Mixed Ionic Liquids: The Case of Pyridinium-Based Fluids. J. Phys. Chem. B 2012, 116, 2526−2537. (416) Miran, M. S.; Yasuda, T.; Susan, M. A. B. H.; Dokko, K.; Watanabe, M. Binary Protic Ionic Liquid Mixtures as a Proton Conductor: High Fuel Cell Reaction Activity and Facile Proton Transport. J. Phys. Chem. C 2014, 118, 27631−27639. (417) Navia, P.; Troncoso, J.; Romaní, L. Excess Magnitudes for Ionic Liquid Binary Mixtures with a Common Ion. J. Chem. Eng. Data 2007, 52, 1369−1374. (418) Navia, P.; Troncoso, J.; Romaní, L. Viscosities for Ionic Liquid Binary Mixtures with a Common Ion. J. Solution Chem. 2008, 37, 677− 688. (419) Clough, M. T.; Crick, C. R.; Gräsvik, J.; Hunt, P. A.; Niedermeyer, H.; Welton, T.; Whitaker, O. P. A Physicochemical Investigation of Ionic Liquid Mixtures. Chem. Sci. 2015, 6, 1101−1114. (420) Annat, G.; Forsyth, M.; MacFarlane, D. R. Ionic Liquid MixturesVariations in Physical Properties and Their Origins in Molecular Structure. J. Phys. Chem. B 2012, 116, 8251−8258. (421) Katayanagi, H.; Nishikawa, K.; Shimozaki, H.; Miki, K.; Westh, P.; Koga, Y. Mixing Schemes in Ionic Liquid-H2O Systems: A Thermodynamic Study. J. Phys. Chem. B 2004, 108, 19451−19457. (422) Abe, H.; Fukushima, R.; Onji, M.; Hirayama, K.; Kishimura, H.; Yoshimura, Y.; Ozawa, S. Two-Length Scale Description of Hydrophobic Room-Temperature Ionic Liquid-Alcohol Systems. J. Mol. Liq. 2016, 215, 417−422. (423) Noack, K.; Leipertz, A.; Kiefer, J. Molecular Interactions and Macroscopic Effects in Binary Mixtures of an Imidazolium Ionic Liquid with Water, Methanol, and Ethanol. J. Mol. Struct. 2012, 1018, 45−53. (424) Yoshimura, Y.; Kimura, H.; Okamoto, C.; Miyashita, T.; Imai, Y.; Abe, H. Glass Transition Behaviour of Ionic Liquid, 1-Butyl-3methylimidazolium Tetrafluoroborate−H2O Mixed Solutions. J. Chem. Thermodyn. 2011, 43, 410−412.

(425) Pal, A.; Saini, M.; Kumar, B. Volumetric, Ultrasonic and Spectroscopic (FT-IR) Studies for the Binary Mixtures of Imidazolium Based ILs with 1,2-propanediol. Fluid Phase Equilib. 2016, 411, 66−73. (426) Roth, C.; Appelhagen, A.; Jobst, N.; Ludwig, R. Microheterogeneities in Ionic-Liquid-Methanol Solutions Studied by FTIR Spectroscopy, DFT Calculations and Molecular Dynamics Simulations. ChemPhysChem 2012, 13, 1708−1717. (427) Singh, T.; Rao, K. S.; Kumar, A. Polarity Behaviour and Specific Interactions of Imidazolium-Based Ionic Liquids in Ethylene Glycol. ChemPhysChem 2011, 12, 836−845. (428) Köddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. The Association of Water in Ionic Liquids: A Reliable Measure of Polarity. Angew. Chem., Int. Ed. 2006, 45, 3697−3702. (429) Wülf, A.; Köddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. Water Vibrational Bands as a Polarity Indicator in Ionic Liquids. Z. Phys. Chem. 2006, 220, 1361−1376. (430) Zhang, Q.-G.; Wang, N.-N.; Wang, S.-L.; Yu, Z.-W. Hydrogen Bonding Behaviors of Binary Systems Containing the Ionic Liquid 1Butyl-3-methylimidazolium Trifluoroacetate and Water/Methanol. J. Phys. Chem. B 2011, 115, 11127−11136. (431) Joshi, R.; Pasilis, S. P. The Effect of Tributylphosphate and Tributyl Phosphine Oxide On Hydrogen Bonding Interactions between Water and the 1-Ethyl-3-Methylimidazolium Cation in 1Ethyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Imide. J. Mol. Liq. 2015, 209, 381−386. (432) Chang, H. C.; Jiang, J. C.; Tsai, W. C.; Chen, G. C.; Lin, S. H. Hydrogen Bond Stabilization in 1,3-Dimethylimidazolium Methyl Sulfate and 1-Butyl-3-Methylimidazolium Hexafluorophosphate Probed by High Pressure: The Role of Charge-Enhanced C−H···O Interactions in the Room-Temperature Ionic Liquid. J. Phys. Chem. B 2006, 110, 3302−3307. (433) Andanson, J.-M.; Beier, M. J.; Baiker, A. Binary Ionic Liquids with a Common Cation: Insight into Nanoscopic Mixing by Infrared Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 2959−2964. (434) Tran, C. D.; De Paoli Lacerda, S. H.; Oliveira, D. Absorption of Water by Room-Temperature Ionic Liquids: Effect of Anions on Concentration and State of Water. Appl. Spectrosc. 2003, 57, 152−157. (435) Dominguez-Vidal, A.; Kaun, N.; Ayora-Canada, M. J.; Lendl, B. Probing Intermolecular Interactions in Water/Ionic Liquid Mixtures by Far-Infrared Spectroscopy. J. Phys. Chem. B 2007, 111, 4446−4452. (436) Marekha, B. A.; Koverga, V. A.; Moreau, M.; Kiselev, M.; Takamuku, T.; Kalugin, O. N.; Idrissi, A. Intermolecular Interactions, Ion Solvation, and Association in Mixtures of 1-N-Butyl-3-Methylimidazolium Hexafluorophosphate and Gamma-Butyrolactone: Insights from Raman Spectroscopy. J. Raman Spectrosc. 2015, 46, 339− 352. (437) Cha, S.; Ao, M.; Sung, W.; Moon, B.; Ahlström, B.; Johansson, P.; Ouchi, Y.; Kim, D. Structures of Ionic Liquid-Water Mixtures Investigated by IR and NMR Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 9591−9601. (438) Yaghini, N.; Pitawala, J.; Matic, A.; Martinelli, A. Effect of Water on the Local Structure and Phase Behavior of ImidazoliumBased Protic Ionic Liquids. J. Phys. Chem. B 2015, 119, 1611−1622. (439) Singh, T.; Kumar, A. Cation−Anion−Water Interactions in Aqueous Mixtures of Imidazolium Based Ionic Liquids. Vib. Spectrosc. 2011, 55, 119−125. (440) Saha, S.; Hamaguchi, H. Effect of Water on the Molecular Structure and Arrangement of Nitrile-Functionalized Ionic Liquids. J. Phys. Chem. B 2006, 110, 2777−2781. (441) Bodo, E.; Mangialardo, S.; Capitani, F.; Gontrani, L.; Leonelli, F.; Postorino, P. Interaction of a Long Alkyl Chain Protic Ionic Liquid and Water. J. Chem. Phys. 2014, 140, 204503. (442) Danten, Y.; Cabaço, M. I.; Besnard, M. Interaction of Water Highly Diluted in 1-Alkyl-3-methylimidazolium Ionic Liquids with the PF6(−) and BF4(−) Anions. J. Phys. Chem. A 2009, 113, 2873−2889. (443) Danten, Y.; Cabaço, M. I.; Besnard, M. Interaction of Water Diluted in 1-butyl-3-methylimidazolium Ionic Liquids by Vibrational Spectroscopy Modeling. J. Mol. Liq. 2010, 153, 57−66. BD


Chemical Reviews

Review

(444) Dahi, A.; Fatyeyeva, K.; Chappey, C.; Langevin, D.; Rogalsky, S. P.; Tarasyuk, O. P.; Marais, S. Water Sorption Properties of RoomTemperature Ionic Liquids over the Whole Range of Water Activity and Molecular States of Water in These Media. RSC Adv. 2015, 5, 76927−76938. (445) Romanos, G. E.; Zubeir, L. F.; Likodimos, V.; Falaras, P.; Kroon, M. C.; Iliev, B.; Adamova, G.; Schubert, T. J. S. Enhanced CO2 Capture in Binary Mixtures of 1-Alkyl-3-methylimidazolium Tricyanomethanide Ionic Liquids with Water. J. Phys. Chem. B 2013, 117, 12234−12251. (446) Yoshimura, Y.; Goto, T.; Abe, H.; Imai, Y. Existence of NearlyFree Hydrogen Bonds in an Ionic Liquid, N,N-Diethyl-N-methyl-N(2-methoxyethyl) Ammonium Tetrafluoroborate-Water at 77 K. J. Phys. Chem. B 2009, 113, 8091−8095. (447) Fumino, K.; Stange, P.; Fossog, V.; Hempelmann, R.; Ludwig, R. Equilibrium of Contact and Solvent-Separated Ion Pairs in Mixtures of Protic Ionic Liquids and Molecular Solvents Controlled by Polarity. Angew. Chem., Int. Ed. 2013, 52, 12439−12442. (448) Fumino, K.; Fossog, V.; Stange, P.; Wittler, K.; Polet, W.; Hempelmann, R.; Ludwig, R. Ion Pairing in Protic Ionic Liquids Probed by Far-Infrared Spectroscopy: Effects of Solvent Polarity and Temperature. ChemPhysChem 2014, 15, 2604−2609. (449) Jiang, J.-C.; Lin, K.-H.; Li, S.-C.; Shih, P.-M.; Hung, K.-C.; Lin, S. H.; Chang, H.-C. Association Structures of Ionic Liquid/DMSO Mixtures Studied by High-Pressure Infrared Spectroscopy. J. Chem. Phys. 2011, 134, 044506. (450) Rodrigues, F.; Santos, P. S. Effect of the Chain Length in the Structure of Imidazolic Ionic Liquids and Dimethylformamide Solutions Probed by Raman Spectroscopy. Vib. Spectrosc. 2010, 54, 123−126. (451) Wang, N.-N.; Zhang, Q.-G.; Wu, F.-G.; Li, Q.-Z.; Yu, Z.-W. Hydrogen Bonding Interactions Between a Representative PyridiniumBased Ionic Liquid [BuPy][BF4] and Water/Dimethyl Sulfoxide. J. Phys. Chem. B 2010, 114, 8689−8700. (452) Pal, A.; Kumar, B.; Singh Kang, T. Effect of Structural Alteration of Ionic Liquid on their Bulk and Molecular Level Interactions with Ethylene Glycol. Fluid Phase Equilib. 2013, 358, 241−249. (453) Shirota, H.; Kakinuma, S.; Itoyama, Y.; Umecky, T.; Tak am u ku , T . Effec ts of T e tr aflu or ob or ate and Bis(trifluoromethylsulfonyl)amide Anions on the Microscopic Structures of 1-Methyl-3-octylimidazolium-Based Ionic Liquids and Benzene Mixtures: A Multiple Approach by ATR-IR, NMR, and Femtosecond Raman-Induced Kerr Effect. J. Phys. Chem. B 2016, 120, 513−526. (454) Shimomura, T.; Takamuku, T.; Yamaguchi, T. Clusters of Imidazolium-Based Ionic Liquid in Benzene Solutions. J. Phys. Chem. B 2011, 115, 8518−8527. (455) Xue, L.; Tamas, G.; Quitevis, E. L. Comparative OHD-RIKES Study of the Low-Frequency (0−250 cm − 1) Vibrational Dynamics of Dibenzyl- and Monobenzyl-Substituted Imidazolium Ionic Liquids and Benzene/Dimethylimidazolium Mixtures. ACS Sustainable Chem. Eng. 2016, 4, 514−524. (456) Lynden-Bell, R. M.; Xue, L.; Tamas, G.; Quitevis, E. L. Local Structure and Intermolecular Dynamics of an Equimolar Benzene and 1,3-dimethylimidazolium bis[(trifluoromethane)sulfonyl]amide Mixture: Molecular Dynamics Simulations and OKE Spectroscopic Measurements. J. Chem. Phys. 2014, 141, 044506. (457) Shirota, H. Intermolecular/Interionic Vibrations of 1-Methyl3-n-octylimidazolium Tetrafluoroborate Ionic Liquid and Benzene Mixtures. J. Phys. Chem. B 2013, 117, 7985−7995. (458) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH Verlag: Weinheim, Germany, 2003. (459) Reichardt, C. Pyridinium N-Phenoxide Betaine Dyes and their Application to the Determination of Solvent Polarities Part 29 Polarity of Ionic Liquids Determined Empirically by Means of Solvatochromic Pyridinium N-Phenolate Betaine Dyes. Green Chem. 2005, 7, 339−351.

(460) Tao, G.-h.; Zou, M.; Wang, X.-h.; Chen, Z.-y.; Evans, D. G.; Kou, Y. Comparison of Polarities of Room-Temperature Ionic Liquids Using FT-IR Spectroscopic Probes. Aust. J. Chem. 2005, 58, 327. (461) Garcia, H. C.; de Oliveira, L. F. C.; Nicolau, B. G.; Ribeiro, M. C. C. Raman Spectra of Acetonitrile in Imidazolium Ionic Liquids. J. Raman Spectrosc. 2010, 41, 1720−1724. (462) Shiflett, M. B.; Kasprzak, D. J.; Junk, C. P.; Yokozeki, A. Phase Behavior of {carbon dioxide+[bmim][Ac]} Mixtures. J. Chem. Thermodyn. 2008, 40, 25−31. (463) Hollóczki, O.; Kelemen, Z.; Könczöl, L.; Szieberth, D.; Nyulászi, L.; Stark, A.; Kirchner, B. Significant Cation Effects in Carbon Dioxide-Ionic Liquid Systems. ChemPhysChem 2013, 14, 315− 320. (464) Zeng, S.; Gao, H.; Zhang, X.; Dong, H.; Zhang, X.; Zhang, S. Efficient and Reversible Capture of SO2 by Pyridinium-based Ionic Liquids. Chem. Eng. J. 2014, 251, 248−256. (465) Shang, Y.; Li, H.; Zhang, S.; Xu, H.; Wang, Z.; Zhang, L.; Zhang, J. Guanidinium-based Ionic Liquids for Sulfur Dioxide Sorption. Chem. Eng. J. 2011, 175, 324−329. (466) Huang, J.; Riisager, A.; Wasserscheid, P.; Fehrmann, R. Reversible Physical Absorption of SO2 by Ionic Liquids. Chem. Commun. 2006, 39, 4027−4029. (467) Yang, D.; Hou, M.; Ning, H.; Ma, J.; Kang, X.; Zhang, J.; Han, B. Reversible Capture of SO2 through Functionalized Ionic Liquids. ChemSusChem 2013, 6, 1191−1195. (468) Ando, R. A.; Siqueira, L. J. A.; Bazito, F. C.; Torresi, R. M.; Santos, P. S. The Sulfur Dioxide−1-Butyl-3-Methylimidazolium Bromide Interaction: Drastic Changes in Structural and Physical Properties. J. Phys. Chem. B 2007, 111, 8717−8719. (469) Siqueira, L. J. A.; Ando, R. m. A.; Bazito, F. F. C.; Torresi, R. M.; Santos, P. S.; Ribeiro, M. C. C. Shielding of Ionic Interactions by Sulfur Dioxide in an Ionic Liquid. J. Phys. Chem. B 2008, 112, 6430− 6435. (470) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Combining Ionic Liquids and Supercritical Fluids: In Situ ATR-IR Study of CO2 Dissolved in Two Ionic Liquids at High Pressures. Chem. Commun. 2000, No. 20, 2047−2048. (471) Andanson, J.-M.; Jutz, F.; Baiker, A. Supercritical CO2/Ionic Liquid Systems: What Can We Extract from Infrared and Raman Spectra? J. Phys. Chem. B 2009, 113, 10249−10254. (472) Seki, T.; Grunwaldt, J.-D.; Baiker, A. In Situ Attenuated Total Reflection Infrared Spectroscopy of Imidazolium-Based RoomTemperature Ionic Liquids under “Supercritical” CO2. J. Phys. Chem. B 2009, 113, 114−122. (473) Besnard, M.; Cabaço, M. I.; Vaca Chávez, F.; Pinaud, N.; Sebastião, P. J.; Coutinho, J. A. P.; Mascetti, J.; Danten, Y. CO2 in 1Butyl-3-methylimidazolium Acetate. 2. NMR Investigation of Chemical Reactions. J. Phys. Chem. A 2012, 116, 4890−4901. (474) Blath, J.; Deubler, N.; Hirth, T.; Schiestel, T. Chemisorption of Carbon Dioxide in Imidazolium Based Ionic Liquids with Carboxylic Anions. Chem. Eng. J. 2012, 181−182, 152−158. (475) Makino, T. In situ Raman Study of Dissolved Carbon-Dioxide Induced Changes of Imidazolium-Based Ionic Liquids. J. Phys. Conf. Ser. 2010, 215, 012068. (476) Gurkan, B. E.; de la Fuente, J. C.; Mindrup, E. M.; Ficke, L. E.; Goodrich, B. F.; Price, E. A.; Schneider, W. F.; Brennecke, J. F. Equimolar CO2 Absorption by Anion-Functionalized Ionic Liquids. J. Am. Chem. Soc. 2010, 132, 2116−2117. (477) Lee, T. B.; Oh, S.; Gohndrone, T. R.; Morales-Collazo, O.; Seo, S.; Brennecke, J. F.; Schneider, W. F. CO2 Chemistry of Phenolate-Based Ionic Liquids. J. Phys. Chem. B 2016, 120, 1509− 1517. (478) Giammanco, C. H.; Kramer, P. L.; Yamada, S. A.; Nishida, J.; Tamimi, A.; Fayer, M. D. Coupling of Carbon Dioxide Stretch and Bend Vibrations Reveals Thermal Population Dynamics in an Ionic Liquid. J. Phys. Chem. B 2016, 120, 549−556. (479) Rodopoulos, T.; Smith, L.; Horne, M. D.; Rüther, T. Speciation of Aluminium in Mixtures of the Ionic Liquids [C3mpip]BE


Chemical Reviews

Review

[Ntf2] and [C4mpyr][Ntf2] with AlCl3: An Electrochemical and NMR Spectroscopy Study. Chem. - Eur. J. 2010, 16, 3815−3826. (480) Babushkina, O. B. Phase Behaviour and FTIR Spectra of Ionic Liquids: The Mixtures of 1-Butyl-1-methylpyrrolidinium Chloride and TaCl5. Z. Naturforsch., A: Phys. Sci. 2008, 63, 66−72. (481) Alves, M. B.; Santos, V. O., Jr.; Soares, V. C. D.; Suarez, P. A. Z.; Rubim, J. C. Raman Spectroscopy of Ionic Liquids Derived from 1n-Butyl-3-methylimidazolium Chloride and Niobium Chloride or Zinc Chloride Mixtures. J. Raman Spectrosc. 2008, 39, 1388−1395. (482) Goujon, N.; Byrne, N.; Walsh, T. R.; Forsyth, M. The Influence of Water and Metal Salt on the Transport and Structural Properties of 1-Octyl-3-methylimidazolium Chloride. Aust. J. Chem. 2015, 68, 420. (483) Andriyko, Y.; Andriiko, A.; Babushkina, O. B.; Nauer, G. E. Electrochemistry of TiF4 in 1-butyl-2,3-dimethylimidazolium tetrafluoroborate. Electrochim. Acta 2010, 55, 1081−1089. (484) Arellano, I. H.; Huang, J.; Pendleton, P. Computational Insights into the Molecular Interaction and Ion-Pair Structures of a Novel Zinc-Functionalized Ionic Liquid, [Emim][Zn(TFSI)3]. Spectrochim. Acta, Part A 2016, 153, 6−15. (485) Mudring, A.-V.; Babai, A.; Arenz, S.; Giernoth, R. The “Noncoordinating” Anion Tf2N− Coordinates to Yb2+: A Structurally Characterized Tf2N− Complex from the Ionic Liquid [mppyr][Tf2N]. Angew. Chem., Int. Ed. 2005, 44, 5485−5488. (486) Tang, S.; Babai, A.; Mudring, A.-V. Europium-Based Ionic Liquids as Luminescent Soft Materials. Angew. Chem., Int. Ed. 2008, 47, 7631−7634. (487) Liu, Z.; El Abedin, S. Z.; Endres, F. Electrochemical and Spectroscopic Study of Zn(II) Coordination and Zn Electrodeposition in Three Ionic Liquids with the Trifluoromethylsulfonate Anion, Different Imidazolium Ions and Their Mixtures with Water. Phys. Chem. Chem. Phys. 2015, 17, 15945−15952. (488) Oliveira, F. S.; Cabrita, E. J.; Todorovic, S.; Bernardes, C. E. S.; Canongia Lopes, J. N.; Hodgson, J. L.; MacFarlane, D. R.; Rebelo, L. P. N.; Marrucho, I. M. Mixtures of the 1-Ethyl-3-methylimidazolium Acetate Ionic Liquid with Different Inorganic Salts: Insights into their Interactions. Phys. Chem. Chem. Phys. 2016, 18, 2756−2766. (489) Chimdi, T.; Gunzelmann, D.; Vongsvivut, J.; Forsyth, M. A study of phase behavior and conductivity of mixtures of the organic ionic plastic crystal N-methyl-N-methyl-pyrrolidinium dicyanamide with sodium dicyanamide. Solid State Ionics 2015, 272, 74−83. (490) Carstens, T.; Lahiri, A.; Borisenko, N.; Endres, F. [Py1,4]FSINaFSI-Based Ionic Liquid Electrolyte for Sodium Batteries: Na+ Solvation and Interfacial Nanostructure on Au(111). J. Phys. Chem. C 2016, 120, 14736−14741. (491) Forsyth, M.; Chimdi, T.; Seeber, A.; Gunzelmann, D.; Howlett, P. C. Structure and Dynamics in an Organic Ionic Plastic Crystal, Nethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide, Mixed with a Sodium Salt. J. Mater. Chem. A 2014, 2, 3993−4003. (492) Duluard, S.; Grondin, J.; Bruneel, J. L.; Campet, G.; Delville, M. H.; Lassègues, J. C. Lithium Solvation in a PMMA Membrane Plasticized by a Lithium-Conducting Ionic Liquid Based on 1-Butyl-3Methylimidazolium Bis(Trifluoromethanesulfonyl)Imide. J. Raman Spectrosc. 2008, 39, 1189−1194. (493) Nicolau, B. G.; Sturlaugson, A.; Fruchey, K.; Ribeiro, M. C. C.; Fayer, M. D. Room Temperature Ionic Liquid-Lithium Salt Mixtures: Optical Kerr Effect Dynamical Measurements. J. Phys. Chem. B 2010, 114, 8350−8356. (494) Li, Z.; Smith, G. D.; Bedrov, D. Li+ Solvation and Transport Properties in Ionic Liquid/Lithium Salt Mixtures: A Molecular Dynamics Simulation Study. J. Phys. Chem. B 2012, 116, 12801− 12809. (495) Pitawala, J.; Martinelli, A.; Johansson, P.; Jacobsson, P.; Matic, A. Coordination and Interactions in a Li-Salt Doped Ionic Liquid. J. Non-Cryst. Solids 2015, 407, 318−323. (496) Monteiro, M. J.; Bazito, F. F. C.; Siqueira, L. J. a.; Ribeiro, M. C. C.; Torresi, R. M. Transport Coefficients, Raman Spectroscopy, and Computer Simulation of Lithium Salt Solutions in an Ionic Liquid. J. Phys. Chem. B 2008, 112, 2102−2109.

(497) Le, M. L. P.; Alloin, F.; Strobel, P.; Leprêtre, J.-C.; Pérez del Valle, C.; Judeinstein, P. Structure-Properties Relationships of Lithium Electrolytes Based on Ionic Liquid. J. Phys. Chem. B 2010, 114, 894− 903. (498) Lassègues, J.-C.; Grondin, J.; Talaga, D. Lithium Solvation in Bis(Trifluoromethanesulfonyl)imide-Based Ionic Liquids. Phys. Chem. Chem. Phys. 2006, 8, 5629−5632. (499) Lassègues, J. C.; Grondin, J.; Aupetit, C.; Johansson, P. Spectroscopic Identification of the Lithium Ion Transporting Species in LiTFSI-Doped Ionic Liquids. J. Phys. Chem. A 2009, 113, 305−314. (500) Duluard, S.; Grondin, J.; Bruneel, J.-L.; Pianet, I.; Grélard, A.; Campet, G.; Delville, M.-H.; Lassègues, J.-C. Lithium Solvation and Diffusion in the 1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide Ionic Liquid. J. Raman Spectrosc. 2008, 39, 627−632. (501) Umebayashi, Y.; Mitsugi, T.; Fukuda, S.; Fujimori, T.; Fujii, K.; Kanzaki, R.; Takeuchi, M.; Ishiguro, S.-I. Lithium Ion Solvation in Room-Temperature Ionic Liquids Involving Bis(trifluoromethanesulfonyl) Imide Anion Studied by Raman Spectroscopy and DFT Calculations. J. Phys. Chem. B 2007, 111, 13028− 13032. (502) Umebayashi, Y.; Mori, S.; Fujii, K.; Tsuzuki, S.; Seki, S.; Hayamizu, K.; Ishiguro, S.-i. Raman Spectroscopic Studies and Ab Initio Calculations on Conformational Isomerism of 1-Butyl-3methylimidazolium Bis-(trifluoromethanesulfonyl)amide Solvated to a Lithium Ion in Ionic Liquids: Effects of the Second Solvation Sphere of the Lithium Ion. J. Phys. Chem. B 2010, 114, 6513−6521. (503) Umebayashi, Y.; Hamano, H.; Seki, S.; Minofar, B.; Fujii, K.; Hayamizu, K.; Tsuzuki, S.; Kameda, Y.; Kohara, S.; Watanabe, M. Liquid Structure of and Li+ Ion Solvation in Bis(trifluoromethanesulfonyl)amide Based Ionic Liquids Composed of 1-Ethyl-3-methylimidazolium and N-Methyl-N-propylpyrrolidinium Cations. J. Phys. Chem. B 2011, 115, 12179−12191. (504) Hardwick, L. J.; Holzapfel, M.; Wokaun, A.; Novák, P. Raman Study of Lithium Coordination in EMI-TFSI Additive Systems as Lithium-Ion Battery Ionic Liquid Electrolytes. J. Raman Spectrosc. 2007, 38, 110−112. (505) Martins, V. L.; Nicolau, B. G.; Urahata, S. M.; Ribeiro, M. C. C.; Torresi, R. M. Influence of the Water Content on the Structure and Physicochemical Properties of an Ionic Liquid and Its Li + Mixture. J. Phys. Chem. B 2013, 117, 8782−8792. (506) Umebayashi, Y.; Yamaguchi, T.; Fukuda, S.; Mitsugi, T.; Takeuchi, M.; Fujii, K.; Ishiguro, S.-i. Raman Spectroscopic Study on Alkaline Metal Ion Solvation in 1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquid. Anal. Sci. 2008, 24, 1297−1304. (507) Pitawala, J.; Kim, J.-K.; Jacobsson, P.; Koch, V.; Croce, F.; Matic, A. Phase Behaviour, Transport Properties, and Interactions in Li-Salt Doped Ionic Liquids. Faraday Discuss. 2012, 154, 71−80. (508) Watkins, T.; Buttry, D. A. Determination of Mg2+ Speciation in a TFSI−-Based Ionic Liquid With and Without Chelating Ethers Using Raman Spectroscopy. J. Phys. Chem. B 2015, 119, 7003−7014. (509) Deepa, M.; Agnihotry, S. A.; Gupta, D.; Chandra, R. IonPairing Effects and Ion−Solvent−Polymer Interactions in LiN(CF3SO2)2−PC−PMMA Electrolytes: A FTIR Study. Electrochim. Acta 2004, 49, 373−383. (510) Seo, D. M.; Boyle, P. D.; Sommer, R. D.; Daubert, J. S.; Borodin, O.; Henderson, W. A. Solvate Structures and Spectroscopic Characterization of LiTFSI Electrolytes. J. Phys. Chem. B 2014, 118, 13601−13608. (511) Mandai, T.; Tsuzuki, S.; Ueno, K.; Dokko, K.; Watanabe, M. Pentaglyme-K Salt Binary Mixtures: Phase Behavior, Solvate Structures, and Physicochemical Properties. Phys. Chem. Chem. Phys. 2015, 17, 2838−2849. (512) Mandai, T.; Yoshida, K.; Ueno, K.; Dokko, K.; Watanabe, M. Criteria for Solvate Ionic Liquids. Phys. Chem. Chem. Phys. 2014, 16, 8761−8772. (513) Zhang, C.; Ueno, K.; Yamazaki, A.; Yoshida, K.; Moon, H.; Mandai, T.; Umebayashi, Y.; Dokko, K.; Watanabe, M. Chelate Effects BF


Chemical Reviews

Review

in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids. I. Stability of Solvate Cations and Correlation with Electrolyte Properties. J. Phys. Chem. B 2014, 118, 5144−5153. (514) Aguilera, L.; Xiong, S.; Scheers, J.; Matic, A. A Structural Study of LiTFSI−tetraglyme Mixtures: From Diluted Solutions to Solvated Ionic Liquids. J. Mol. Liq. 2015, 210, 238−242. (515) Ueno, K.; Tatara, R.; Tsuzuki, S.; Saito, S.; Doi, H.; Yoshida, K.; Mandai, T.; Matsugami, M.; Umebayashi, Y.; Dokko, K.; et al. Li+ Solvation in Glyme-Li Salt Solvate Ionic Liquids. Phys. Chem. Chem. Phys. 2015, 17, 8248−8257. (516) Angell, C. A.; Ansari, Y.; Zhao, Z. Ionic Liquids: Past, Present and Future. Faraday Discuss. 2012, 154, 9−27. (517) Kramer, P. L.; Giammanco, C. H.; Fayer, M. D. Dynamics of Water, Methanol, and Ethanol in a Room Temperature Ionic Liquid. J. Chem. Phys. 2015, 142, 212408. (518) Wong, D. B.; Giammanco, C. H.; Fenn, E. E.; Fayer, M. D. Dynamics of Isolated Water Molecules in a Sea of Ions in a Room Temperature Ionic Liquid. J. Phys. Chem. B 2013, 117, 623−635. (519) Yu, B.; Liu, Z. L.; Zhou, F.; Liu, W. M.; Liang, Y. M. A Novel Lubricant Additive Based on Carbon Nanotubes for Ionic Liquids. Mater. Lett. 2008, 62, 2967−2969. (520) Wang, J. Y.; Chu, H. B.; Li, Y. Why Single-Walled Carbon Nanotubes Can Be Dispersed in Imidazolium-Based Ionic Liquids. ACS Nano 2008, 2, 2540−2546. (521) Tamailarasan, P.; Ramaprabhu, S. Carbon NanotubesGraphene-Solidlike Ionic Liquid Layer-Based Hybrid Electrode Material for High Performance Supercapacitor. J. Phys. Chem. C 2012, 116, 14179−14187. (522) Kleinschmidt, A. C.; Donato, R. K.; Perchacz, M.; Benes, H.; Stengl, V.; Amico, S. C.; Schrekker, H. S. ″Unrolling″ Multi-Walled Carbon Nanotubes with Ionic Liquids: Application as Fillers in EpoxyBased Nanocomposites. RSC Adv. 2014, 4, 43436−43443. (523) Attri, P.; Bhatia, R.; Arora, B.; Kumar, N.; Park, J. H.; Baik, K. Y.; Lee, G. J.; Kim, I. T.; Koo, J. H.; Choi, E. H. Molecular Interactions Between Carbon Nanotubes and Ammonium Ionic Liquids and their Catalysis Properties. Mater. Res. Bull. 2014, 58, 6−9. (524) Jiang, F. L.; Li, C.; Fu, H. Y.; Wang, C. Y.; Guo, X. J.; Jiang, Z.; Wu, G. Z.; Chen, S. M. Temperature-Induced Molecular Rearrangement of an Ionic Liquid Confined in Nanospaces: An in Situ X-ray Absorption Fine Structure Study. J. Phys. Chem. C 2015, 119, 22724− 22731. (525) Ding, Y. S.; Xiong, R. Y.; Wang, S. S.; Zhang, X. M. Aggregative Structure of the 1-Octadecyl-3-Methylimidazolium Cation in rhe Interlayer of Montmorillonite and Its Effect on Polypropylene Melt Intercalation. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 1252−1259. (526) Ha, J. U.; Xanthos, M. Sequential Modification of Cationic and Anionic Nanoclays with Ionic Liquids. Green Chem. Lett. Rev. 2011, 4, 103−107. (527) Ye, W.; He, S. L.; Ding, L. P.; Yao, Y. J.; Wan, L.; Miao, S. D.; Xu, J. Z. Supported Ionic-Liquid ″Semi-Heterogeneous Catalyst″: An Interfacial Chemical Study. J. Phys. Chem. C 2013, 117, 7026−7038. (528) Siqueira, L. J. A.; Constantino, V. R. L.; Camilo, F. F.; Torresi, R. M.; Temperini, M. L. A.; Ribeiro, M. C. C.; Izumi, C. M. S. Probing the Local Environment of Hybrid Materials Designed from Ionic Liquids and Synthetic Clay by Raman Spectroscopy. Spectrochim. Acta, Part A 2014, 122, 469−475. (529) Lv, G. C.; Li, Z. H.; Jiang, W. T.; Chang, P. H.; Liao, L. B. Interlayer Configuration of Ionic Liquids in a Ca-Montmorillonite as Evidenced by FTIR, TG-DTG, and XRD Analyses. Mater. Chem. Phys. 2015, 162, 417−424. (530) Martinelli, A.; Matic, A.; Jacobsson, P.; Börjesson, L.; Navarra, M. A.; Panero, S.; Scrosati, B. A Structural Study on Ionic-LiquidBased Polymer Electrolyte Membranes. J. Electrochem. Soc. 2007, 154, G183−G187. (531) Costa, L. T.; Siqueira, L. J. A.; Nicolau, B. G.; Ribeiro, M. C. C. Raman Spectra of Polymer Electrolytes Based on Poly(Ethylene Glycol)dimethylether, Lithium Perchlorate, and the Ionic Liquid 1Butyl-3-methylimidazolium Hexafluorophosphate. Vib. Spectrosc. 2010, 54, 155−158.

(532) Chaurasia, S. K.; Singh, R. K.; Chandra, S. Structural and Transport Studies on Polymeric Membranes of PEO Containing Ionic Liquid, EMIM-TY: Evidence of Complexation. Solid State Ionics 2011, 183, 32−39. (533) Bradley, L. C.; Gupta, M. Encapsulation of Ionic Liquids within Polymer Shells via Vapor Phase Deposition. Langmuir 2012, 28, 10276−10280. (534) Ramesh, S.; Lu, S.-C. Enhancement of Ionic Conductivity and Structural Properties by 1-butyl-3-methylimidazolium trifluoromethanesulfonate Ionic Liquid in poly(vinylidene fluoride-hexafluoropropylene)-Based Polymer Electrolytes. J. Appl. Polym. Sci. 2012, 126, E484−E492. (535) He, L. H.; Sun, J.; Wang, X. X.; Wang, C. D.; Song, R.; Hao, Y. M. Facile and Effective Promotion of Crystalline Phase in Poly(Vinylidene Fluoride) Via The Incorporation of Imidazolium Ionic Liquids. Polym. Int. 2013, 62, 638−646. (536) Shalu; Chaurasia, S. K.; Singh, R. K.; Chandra, S. Electrical, Mechanical, Structural, and Thermal Behaviors of Polymeric Gel Electrolyte Membranes of Poly(vinylidene fluoride-co-hexafluoropropylene) with the Ionic Liquid 1-Butyl-3-Methylimidazolium Tetrafluoroborate Plus Lithium Tetrafluoroborate. J. Appl. Polym. Sci. 2015, 132,10.1002/app.41456. (537) Yuan, C. S.; Zhu, X.; Su, L.; Yang, D. Y.; Wang, Y. Q.; Yang, K.; Cheng, X. R. Preparation and Characterization of a Novel Ionic Conducting Foam-Type Polymeric Gel Based on Polymer PVdF-HFP and Ionic Liquid EMIM TFSI. Colloid Polym. Sci. 2015, 293, 1945− 1952. (538) Vitucci, F. M.; Palumbo, O.; Trequattrini, F.; Brubach, J. B.; Roy, P.; Meschini, I.; Croce, F.; Paolone, A. Interaction of 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide with an Electrospun PVdF Membrane: Temperature Dependence of the Concentration of the Anion Conformers. J. Chem. Phys. 2015, 143, 094707. (539) Bahadur, I.; Momin, M. I. K.; Koorbanally, N. A.; Sattari, M.; Ebenso, E. E.; Katata-Seru, L. M.; Singh, S.; Ramjugernath, D. Interactions of polyvinylpyrrolidone with Imidazolium Based Ionic Liquids: Spectroscopic and Density Functional Theory studies. J. Mol. Liq. 2016, 213, 13−16. (540) Kiefer, J.; Obert, K.; Fries, J.; Bösmann, A.; Wasserscheid, P.; Leipertz, A. Determination of Glucose and Cellobiose Dissolved in the Ionic Liquid 1-Ethyl-3-Methylimidazolium Acetate Using Fourier Transform Infrared Spectroscopy. Appl. Spectrosc. 2009, 63, 1041− 1049. (541) Kunov-Kruse, A. J.; Riisager, A.; Saravanamurugan, S.; Berg, R. W.; Kristensen, S. B.; Fehrmann, R. Revisiting the Bronsted Acid Catalysed Hydrolysis Kinetics of Polymeric Carbohydrates in Ionic Liquids by in situ ATR-FTIR Spectroscopy. Green Chem. 2013, 15, 2843−2848. (542) Zhang, X.; Ma, J.; Ji, Z.; Yang, G. H.; Zhou, X.; Xu, F. Using Confocal Raman Microscopy to Real-Time Monitor Poplar Cell Wall Swelling and Dissolution During Ionic Liquid Pretreatment. Microsc. Res. Tech. 2014, 77, 609−618. (543) Papanyan, Z.; Roth, C.; Wittler, K.; Reimann, S.; Ludwig, R. The Dissolution of Polyols in Salt Soltuions and Ionic Liquids at Molecular Level: Ions, Counter Ions and Hofmeister Effects. ChemPhysChem 2013, 14, 3667−3671. (544) Papanyan, Z.; Roth, C.; Paschek, D.; Ludwig, R. Understanding the Dissolution of Polyols by Ionic Liquids Using the Example of a Well-Defined Model Compound. ChemPhysChem 2011, 12, 2400− 2404. (545) Brandão, C. R. R.; Costa, L. A. F.; Breyer, H. S.; Rubim, J. C. Surface-enhanced Raman Scattering (SERS) of a Copper Electrode in 1-N-Butyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid. Electrochem. Commun. 2009, 11, 1846−1848. (546) Rubim, J. C.; Trindade, F. A.; Gelesky, M. A.; Aroca, R. F.; Dupont, J. Surface-Enhanced Vibrational Spectroscopy of Tetrafluoroborate 1-N-Butyl-3-Methylimidazolium (BMIBF4) Ionic Liquid on Silver Surface. J. Phys. Chem. C 2008, 112, 19670−19675. BG


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(547) Santos, V. O., Jr.; Leite, I. R.; Brolo, A. G.; Rubim, J. C. The Electrochemical Reduction of CO2 on a Copper Electrode in 1-NButyl-3-Methylimidazolium Tetrafluoroborate (BMI.BF4) Monitored by Surface-Enhanced Raman Scattering (SERS). J. Raman Spectrosc. 2016, 47, 674−680. (548) Santos, V. O., Jr.; Alves, M. B.; Carvalho, M. S.; Suarez, P. A. Z.; Rubim, J. C. Surface-Enhanced Raman Scattering at the Silver Electrode/Ionic Liquid (BMIPF6) Interface. J. Phys. Chem. B 2006, 110, 20379−20385. (549) Harroun, S. G.; Abraham, T. J.; Prudhoe, C.; Zhang, Y.; Scammells, P. J.; Brosseau, C. L.; Pye, C. C.; Singer, R. D. Electrochemical Surface-Enhanced Raman Spectroscopy (E-SERS) of Novel Biodegradable Ionic Liquids. Phys. Chem. Chem. Phys. 2013, 15, 19205−19212.

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