The Anion Dependence of the Interaction Strength between Ions in

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The Anion Dependence of the Interaction Strength between Ions in Imidazolium-Based Ionic Liquids Probed by Far Infrared Spectroscopy Koichi Fumino, Kai Wittler, and Ralf Ludwig J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp306173t • Publication Date (Web): 19 Jul 2012 Downloaded from http://pubs.acs.org on July 28, 2012

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The anion dependence of the interaction strength between ions in imidazolium-based ionic liquids probed by far infrared spectroscopy Koichi Fumino,a Kai Wittler,a and Ralf Ludwig*abc Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059 Rostock, Germany. b Leibniz-Institut für Katalyse an der Universität Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany. c Faculty of Interdisciplinary Research, Department „Science and Technology of Life, Light and Matter“, University of Rostock, Rostock, Germany. a

KEYWORDS Ionic liquids, far infrared spectroscopy, density functional theory calculation, hydrogen bonding, cellulose. Supporting Information Placeholder ABSTRACT: We have shown that far infrared (FIR) spectroscopy is a sensitive probe for detecting the anioncation interaction strength in imidazolium-based ionic liquids (ILs). Supported by density functional theory (DFT) calculations of frequencies and energies for large IL aggregates, it is observed that frequency shifts stem mainly from varying interaction strength rather than from different reduced masses of the anions. The strongest interaction is observed for the IL containing the acetate anion which is well known for its effective dissolution power for cellulose.

Ionic liquids present a new liquid material with potential application in science and technology.1-7 ILs are formed solely by ions and reveal ubiquitous properties. Structure, dynamics and thermodynamics of these Coulomb systems are strongly related to the type and strength of interaction between cations and anions. There are indirect probes for the anion-cation interaction such as the NMR proton chemical shifts or the IR CH stretch frequencies of the ring protons in imidazoliumbased ILs.8-21 For the direct measure of this interaction strength several experiments provide access to the frequency range of interest. Spectral contributions indicating the cation-anion interactions are expected to lie in the far infrared frequency range from 50 to 200 cm-1 or 1.5 to 6 THz. The palette of suitable methods includes optical heterodyne-detected Raman-induced Kerr-effect spectroscopy,22-31 terahertz (THz) spectroscopy,31-36 dielectric relaxation spectroscopy,31,37,38 low-energy neutron scattering,39 far infrared (FIR) spectroscopy as well as Raman spectroscopy.40-45 However, the interpretation of the recorded low frequency spectra requires theoretical modeling to assign intra- and intermolecular vibrational modes and to deduce the origins of spectral absorptions. This has been usually done by using ab initio or DFT cal-

culations on ion-pairs or ion-pair aggregates of ionic liquids as well as classical or quantum chemical molecular dynamics simulations.46-56 However, there are still some difficulties to identify the anion-cation interaction precisely. Firstly, this frequency range can be strongly populated. Beside the pure anion-cation interaction, other contributions can stem from weak intramolecular vibrational modes of the anions and cations or from unspecific librational motions or lattice modes. Secondly, even if the vibrational band of the anion-cation interaction can be clearly assigned, its position in the spectral range is determined by two parameters. Following the equation of the simple harmonic oscillator ν~=(1/2πc)(k/µ)1/2, the frequency (wavenumber) can be obtained from the square root of the ratio between the force constant k and the reduced mass µ. Thus frequency shifts in the spectrum can be referred either to changing force constants or to different reduced masses or even both. It is also possible that both effects cancel each other and no frequency shift is observed. Recently, we tried to overcome these difficulties by using ILs comprising the same anion (e.g. NTf2–) and varying imidazolium cations with different interaction sites and interaction strength.41 In this way we could show that imidazolium-based ILs having the same reduced mass lead to different shifts in the far infrared frequency range indicating different interaction strength between anion and cation. The observed frequency shifts to higher wavenumbers could be related to increasing interaction strength between anion and cation supported by DFT calculated frequencies for larger clusters. However, the frequency shifts are ranged from weak to moderate only. Additionally, the frequency shifts could be observed only for the most intense vibrational modes, not for distinct vibrational bands. Stronger frequency shifts can be expected by using the same imidazolium cation but varying anions owing significantly different interaction potential.

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For that purpose, we recently used ILs including the same C2mim+ imidazolium cation but diverse anions NTf2–, EtSO4–, N(CN)2– and SCN–. The most intense bands shifted from 82 to 116 cm-1 in this order and could be related linearly to the DFT calculated binding energies.57 Although there has been some evidence that not the full masses of the anions and cations take part in the intermolecular vibrational motion and that the interaction is local in nature, the shifts to higher frequencies could be also explained by the decreasing reduced masses of the ILs instead of increasing force constants due to stronger interaction. Although it is obvious to use anions of similar mass, this approach is not easy to realize. Of course, voluminous and heavy anions only weakly interact, whereas small and light anions provide strong interaction potential due to their higher surface charge. Thus, both parameters the reduced mass as well as the force constant lead to shifts into the same direction, namely to lower or higher frequencies, respectively. For this purpose we used imidazolium-based ionic liquids with C2mim+ cations and anions which are close in molecular mass as shwon in Figure 1. Additionally to the ILs including anions, N(CN)2– and SCN–, which we remeasured for a broader frequency range down to 10 cm-1, we recorded the far infrared spectra of 1-ethyl-3methyl-imidazolium nitrate [C2mim][NO3] and 1-ethyl3-methyl-imidazolium acetate [C2mim][CH3COO].

30 and 600 cm-1 (0.9 and 18 THz) Further improvement could be achieved by using a high pressure mercury lamp and an silica beam splitter. This configuration allowed measurements down to 10 cm-1 (0.3 THz) and significantly better signal-to-noise ratios. Table 1. Masses of cations and anions, reduced masses and their square roots of ion-pairs in the ILs I-IV. The masses are all given in atomic mass units (AMUs). Additionally, the correction factors for the measured frequencies due to different reduced masses are given relative to IL II. IL

cation

anion

µ

(µ µ)1/2

correction factors

I

111,09

66.01

41.406

6.435

1.043

II

111,09

57.97

38.09 2

6.172

1

III

111,09

62,01

39.796

6.30 8

1.022

IV

111,09

59,01

38.539

6.20 8

1.006

The low-frequency FTIR spectra for [C2mim][N(CN)2] (I), [C2mim][SCN] (II), [C2mim][NO3] (III) and [C2mim][CH3COO] (IV) in the range between 20 and 300 cm-1 are shown in Figure 2 (see SI1). All the spectra were recorded at 298 K. Only III has been measured at 323 K due to its higher melting point. Overall it is observed that the spectra show similarities but also significant differences.

Fig.1 Structures of anions dicyanamide (N(CN)2–), thiocyanate (SCN–), nitrate (NO3–) and acetate (CH3COO–) in C2mim+ imidazolium-based ionic liquids.

The molecular weights of these four anions are nearly similar. Based on the calculated reduced masses of these ILs, the frequency should shift by only 4 cm-1 to higher values going from the heaviest [C2mim][N(CN)2] (167 g/mol) to lightest [C2mim][SCN] (159 g/mol) IL (see Table 1). Consequently, the other two ILs with similar masses should have frequencies in this range, if the frequency shifts are mainly determined by the reduced masses and not by the anion interaction strength expressed by the force constants. The FTIR measurements were performed with a Bruker Vertex 70 FTIR spectrometer equipped with an extension for measurements in the FIR region that consists a multilayer mylar beam splitter and a room temperature DLATGS detector with preamplifier. The accessible spectral region for this configuration lies between

Fig.2 Far infrared spectra of imidazolium-based ionic liquids I-IV. The maxima in the measured FIR spectra indicate the anion-cation interaction as supported by combined experimental and theoretical analysis.

The vibrational band at about 250 cm-1 is present in all spectra and can be assigned to the out-of-plane bending



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mode of the CH3-(N) methyl group in the imidazolium cation (C2mim+). The specific vibrational band at 170 cm1 in the spectrum of [C2mim][N(CN)2] can be attributed to the bending mode of the N(CN)2–, and thus is not observed in the other spectra. The out-of-plane bending mode of the CH3CH2-(N) ethyl group in the imidazolium cation (C2mim+) is suggested to be at 125 cm-1 but is not explicitly observed in the measured spectra because of its low intensity. With the exception of the above given contributions no further intramolecular vibrational modes are present in the low frequency range. The frequencies below 150 cm-1 can be exclusively assigned to the stretching modes +C-H…A– decribing the anion-cation interaction.57

Fig. 3 Recorded FIR (bottom) and calculated DFT (top) low frequency spectra between 20 and 600 cm-1 of [C2mim][CH3COO]. The calculated spectrum of a ion-pair tetramer for this IL was obtained by using half-widths of about 20 cm-1. The frequencies were not corrected for the harmonic approximation. All spectral features are reproduced by the DFT frequencies.

The measured FIR spectra are nicley reproduced by the DFT (B3LYP/6-31+G*) calculated frequencies obtained for IL clusters including four ion-pairs (SI2-7).58 Here, we would like to focus in detail on the spectra for the IL [C2mim][CH3COO]. In Figure 3 the measured and calculated spectra are shown between 20 and 600 cm-1. All the spectral features are reproduced in the DFT calculated spectrum, the intramolecular frequencies above as well as the intermolecular frequencies below 150 cm-1. For the IL ion-pair monomers we also calculated the energies and frequencies by applying Grimme’s DFT-D3 method for calculating non-covalent interactions.59,60. In general, the energies per ion increase from 6-8 kJmol-1 reflected in inconsistent frequency shifts of about 2-3 cm-1 to higher or lower wavenumbers (see SI5). Thus possible frequency shifts due to dispersion forces are smaller than those observed due to variation of the anion interaction strength. For the IL 1-etyl-3-methylimidazolium thiocyanate which is also considered in this study (system II) it has been shown by Pensado et al. that the vibrational density of states (VDOS) from a Molecular Dynamics (MD) trajectory as the power spectrum (Fourier Trans-

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form) from the time autocorrelation function of the atomic velocities (VACF) do not change significantly if dispersion forces are taken into account in agreement with our observation.61 In Figure 2 it is shown that the FIR spectra of ILs I-III are very similar in the expected anion-cation stretching region between 100 and 150 cm-1. The maxima only shift from 111 cm-1 for I to 116 cm-1 for II and 119 cm-1 for III, respectively. Such small frequency shifts (5 and 3 cm-1) could be explained essentially by the different reduced masses. Above, we predicted a frequency shift to higher wavenumbers in the order of 4 cm-1 due to the lower reduced mass of II compared to that of I (Table 1, SI2). Also the calculated binding energies for the IL tetramers for I-III are relatively close. Fortunately, there is one exception among the measured and calculated ILs. For [C2mim][CH3COO] (IV) the maximum in the FIR spctrum describing the +C-H…A– stretching modes occurs at 136 cm-1 and is significantly shifted to higher wavenumbers although the reduced mass is only the second lowest of all ILs under investigation. Such a strong increase in frequency (> 22 cm-1) can be only explained by increasing anion-cation interaction strength. This finding is supported by the DFT calculations which give up to 35 kJmol-1 higher energies per ion for [C2mim][CH3COO] compared to the other ILs (see Table 2). In principle we should be able to relate the FIR frequencies to measured enthalpies of vaporization because both properties include information about the interaction strength between anion and cation. However, the known ∆vapH values are 157.2±1.1 kJmol-1 for I, 151±2 and 165±1 kJmol-1 for II and 163.7±5.3 kJmol-1 for III, respectively. 62-64 The values are relatively close and ambiguous. Unfortunately, for the most interesting IL (IV) under investigation, no enthalpy of vaporization is reported so far.

Fig. 4 Average interaction energies Ebin per ion for ion-pair clusters n=1-4 of the ILs 1-IV plotted versus the measured frequencies +C…A–. For [C2mim][NO3] the reduced mass corrected frequencies (III versus II) are indicated by bars.


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Table 2. DFT calculated energies per ion of ionpair monomers, dimers, trimers and tetramers of the ILs I-IV. The energies are given in kJmol-1. IL

monomer

dimer

trimer

tetramer

I

-159.5

-186.3

-191.2

-198.9

II

-170.4

-193.0

-199.6

-207.4

III

-180.7

-204.0

-215.0

-220.4

IV

-205.4

-221.5

-227.3

-232.5

If we plot the calculated binding energies per ion versus the measured +C-H…A– stretching modes, we obtain a nearly linear dependency. That is true for all the calculated species taken into account. Whereas the average energies for all ILs are increasing from the monomer to the tetramers due to cooperative effects, the slope for the given series of ILs is the same. Because the similar reduced masses can have only minor effects on the spectra, the shifts to higher wavenumbers can only stem from increasing force constants.41,57 The strongest deviation from the linear dependence is found for III. However, if the small reduced mass effect from II to III is taken into account, slightly higher frequencies towards linear behavior is observed (Fig 4). In Figure 5 it is shown that the shortest distances (given in Å) between cation and anion are found for interactions via the C(2)-H position of the imidazolium cation. In conclusion, the frequency shifts to higher wavenumbers can be clearly referred to increasing force constants indicating stronger anion-cation interaction. Unfortunately, the present set of ILs does not allow deriving to which extent the overall interaction energy between anion and cation is determined by Coulomb forces, hydrogen bonding or dispersion forces. Howver this was not the aim of the present work. Herein we wanted to demonstrate that the variation of interaction strength is properly reflected in the far infrared frequency shifts if reduced mass effects can be avoided.

Fig. 5 The calculated tetramer of IV indicates that the shortest distances (given in Å) between cation and anion are

found for interactions via the C(2)-H position of the imidazolium cation.

The observed interaction power of CH3COO– is well known.65-68 The acetate anion is even able to overcome the strong hydrogen bond network present in cellulose.69 Imidazolium-based ILs including other anions such as those used in this work (I-III) are not able to compete with the cellulose-cellulose interaction. Recently, we could show that the dissolution power of ILs largely depends on the ability of the anion to disrupt the H-bond network in a well-defined model compound for cellulose.70 Additionally it could be shown that the frequency shifts in the OH streching region of the model compounds provide guidance for developing effective IL cellulose solvents. Strong redshifted OH stretches of the model compounds correspond to high wavenumber shifts in the far infrared spectra, both indicating stronger solute-solvent or anion-cation interaction, in accordance with our result obtained for the FIR spectra of [C2mim][CH3COO] in this work.

ACKNOWLEDGMENT This reserach was funded from the DFG priority programme 1191 “Ionic Liquids” and the DFG Sonderforschungsbereich SFB 652. The authors also thank the University of Rostock for continued support. R. L. gratefully acknowledges the support of the Leibniz-Institut for Catalysis.

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ASSOCIATED CONTENT “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * Universität Rostock, Institut für Chemie, Abteilung für Physikalische Chemie, Dr.-Lorenz-Weg 1, 18059 Rostock, Germany. Fax: 49 381 498 6524; Tel: 49 381 498 6517; E-mail: [email protected] b Faculty of Interdisciplinary Research, Department „Science and Technology of Life, Light and Matter“, University of Rostock, Rostock, Germany. c Leibniz-Institut für Katalyse an der Universität Rostock, AlbertEinstein-Strasse 29a, 18059 Rostock, Germany.



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SYNOPSIS TOC The strongest frequency shift to higher wavenumbers in the far infrared spectra is observed for the acetate containing of imidazolium-based ionic liquid. The tremendous interaction potential of this anion is reflected in its dissolution power for cellulose.


The Anion Dependence of the Interaction Strength between Ions in

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