Terahertz and Infrared Spectroscopy of Room-Temperature

Article pubs.acs.org/JPCB

Terahertz and Infrared Spectroscopy of Room-Temperature Imidazolium-Based Ionic Liquids Toshiki Yamada,*,† Yukihiro Tominari,† Shukichi Tanaka,† and Maya Mizuno‡ †

Advanced ICT Research Institute, National Institute of Information and Communications Technology, 588-2 Iwaoka, Kobe 651-2492, Japan ‡ Applied Electromagnetic Research Institute, National Institute of Information and Communications Technology, 4-2-1 Nukuikitamachi, Koganei, Tokyo 184-8795, Japan S Supporting Information *

ABSTRACT: The terahertz- and infrared-frequency vibrational modes of various roomtemperature imidazolium-based ionic liquids with molecular anions were examined extensively. We found that the molar-concentration-normalized absorption coefficient spectra in the low-wavenumber range for imidazolium cations with different alkyl-chain lengths were nearly identical for the same anion. Regarding the overall view of a wide range of imidazolium-based ionic liquids, we found that the reduced mass of the combination of an imidazolium-ring cation and the anion and the force constant play significant roles in determining the central frequency of the broad absorption band. In addition to these findings, we also discuss the correlation between the +C−H stretching vibrational modes in the 3000−3300 cm−1 range of the infrared spectra and the intermolecular vibrational band in the low-wavenumber range. Finally, we describe some interesting characteristics of the intermolecular vibrational band observed in a wide range of imidazolium-based ionic liquids.

■

low-frequency intermolecular vibrational modes was found.16,17 Regarding room-temperature imidazolium-based ILs with halogen anions, systematic studies of the intra- and intermolecular modes have been performed using THz-TD, FIR, and IR spectroscopies.27 The importance of the reduced mass (μ) calculated based on the mass of the methylimidazolium ring [mim+] and the mass of the halogen anion (Ahalogen−) and the force constant (k) between the cation and the anion in determining the central frequency of the intermolecular vibrational mode was pointed out. However, systematic studies using THz-TD, FIR, and IR spectroscopies on room-temperature ILs consisting of imidazolium cations with different alkylchain lengths and various molecular anions have not been performed. In addition, the molar-concentration-normalized absorption coefficients, including the absolute value and the complex dielectric constant, which are obtained from THz-TDS measurements, have not been sufficiently discussed. In this work, we used THz-TD, FIR, and IR spectroscopies to extensively study the vibrational modes of room-temperature ILs containing imidazolium cations with different alkyl-chain lengths and a series of molecular anions. Regarding the intermolecular vibrational mode observed in the low-wavenumber range, we found that the molar-concentrationnormalized absorption coefficient spectra in the low-wavenumber range for imidazolium cations with different alkyl-chain

INTRODUCTION Room-temperature ionic liquids (ILs) are salts with melting points below 373 K. Their attractive material properties include nonvolatility under ambient and vacuum conditions, low flammability, electrochemical stability, high ionic conductivity, and superb performance as solvents in many organic reactions and the dissociation of biomaterials.1−5 These properties have led to a wide range of innovative applications6,7 that have accompanied the enormous growth of interest in ILs in recent years. Understanding the noncovalent interactions in ILs, as well as the bulk structures of ionic liquids, is crucial for elucidating their nature.8−12 The low-frequency inter- and/or intramolecular modes of various room-temperature imidazolium-based ILs with molecular anions have been studied using terahertz time-domain spectroscopy (THz-TDS),13−16 farinfrared (FIR) spectroscopy,16−19 conventional infrared (IR) spectroscopy,20−23 and optical heterodyne-detected Ramaninduced Kerr effect spectroscopy (OKE-RIKES).24−26 Although each method has its advantages, FIR spectroscopy is most sensitive to the interaction between the imidazolium cation and the molecular anion.16 It was reported that the central frequency of the low-frequency intermolecular vibrational modes of imidazolium-based ILs with some different molecular anions (Amolecule−) is determined by the main contribution of the force constant k and the minor contribution of the reduced mass μ from the viewpoint of the simple harmonic oscillator, ω = (k/μ)1/2. This is based on the model of local hydrogen bonds (+C−H···Amolecule−), in which a correlation between the IR absorption bands of the +C−H stretching vibrations and the © 2015 American Chemical Society

Received: September 18, 2015 Revised: November 2, 2015 Published: December 1, 2015 15696

DOI: 10.1021/acs.jpcb.5b09101 J. Phys. Chem. B 2015, 119, 15696−15705

Article

The Journal of Physical Chemistry B

cm−1, respectively. Details concerning the experimental procedure were also given elsewhere.27 Density functional theory (DFT) calculations for [C2mim+], [C 4 mim + ], [C 6 mim + ], [C 8 mim + ], [C 10 mim + ], [TfO − ], [Tf2N−], [PF6−], [PF3(C2F5)3−], and [BF4−] were performed to obtain the calculated vibrational spectra. The molecular structures and vibrational spectra were calculated at the DFT level of B3LYP with the 6-311G(d,p) basis set.

lengths were nearly identical for the same anion. Experimental evidence of almost no dependence on the alkyl-chain length, which was satisfied with a wide range of imidazolium-based ILs, is particularly important in considering the interaction between the imidazolium cation and the anion. The significant contribution of the reduced mass of the combination of the imidazolium-ring cation [mim+] and the molecular anion was also noted for a wide range of imidazolium-based ILs. These findings were similar to those of our previous systematic studies concerning room-temperature imidazolium-based ILs with halogen anions. In this article, we also discuss the correlation between the IR absorption bands of +C−H vibrations and the low-frequency intermolecular vibrational modes. Finally, we describe some interesting characteristics of the intermolecular vibrational band observed for a wide range of imidazoliumbased ILs (with halogen anions and molecular anions).

■

RESULTS AND DISCUSSION Complex dielectric spectra (Re ε and Im ε) and the absorption coefficients (α) for the [Cnmim+][TfO−] imidazolium-based ILs with n = 2, 4, 6, and 8 are shown in Figure 2a−d, and the molar-concentration-normalized absorption coefficients (α/M) are shown in the insets of Figure 2c,d. In the low-wavenumber range (13−130 cm−1), it was very interesting to see that the relatively large and broad absorption bands with a peak at 89 cm−1 in the α/M spectra (insets of Figure 2c,d) were nearly the same for all [Cnmim+][TfO−] imidazolium-based ILs in terms of the absorption intensity, central absorption frequency, and absorption bandwidth. This absorption band can be attributed to an intermolecular absorption band whose intensity, center frequency, and bandwidth are determined by the number of pairs due to the electrostatic interaction between methylimidazolium ring [mim+] and the molecular anion [TfO−], because there are no strong intramolecular vibrational modes for any of the [Cnmim+] cations [see Figure S1 in the Supporting Information (SI)] and no intramolecular vibrational modes for [TfO−] (see Figure S2, SI) in the low-wavenumber range. In the midwavenumber range (130−500 cm−1) in the α (α/M) spectra, [TfO−] exhibits some intramolecular absorption bands (see Figure S2, SI). In the midwavenumber range, as seen in our previous report on imidazolium-based ILs with halogen atomic anions, there are some weak but discernible intramolecular absorption bands, in which the accordion-like motion of the alkyl-chain parts of the imidazolium cations is included.27 On the other hand, in the present study, it is difficult to discriminate these weak absorption bands because of the influence of the intramolecular absorption bands of [TfO−]. In the high-wavenumber range (500−670 cm−1), there are two relatively strong absorption bands between 600 and 670 cm−1 that stem from the intramolecular vibrational modes of the skeletal vibration of the imidazolium ring (see Figure S1, SI). Two strong absorption bands between 500 and 600 cm−1 and a very strong absorption band between 600 and 670 cm−1 originate from intramolecular vibrational modes of [TfO−] (see Figure S2, SI). However, it was difficult to distinguish between these absorption bands from 600 to 670 cm−1 because of the influence of the very strong intramolecular absorption band of [TfO−]. In the low-frequency range of the Re ε spectra in Figure 2a, [Cnmim +][TfO−] ILs with smaller n values tend to exhibit larger magnitudes of Re ε. This, in turn, means that the magnitude of the electric susceptibility in this range follows the same trend. Similar features were found in [Cnmim+][Tf2N−], [Cnmim+][PF6 −], and [Cnmim+][BF4−], as shown later in Figures 3a−6a and was previously found for [Cnmim+][I−], [Cnmim+][Br−], and [Cnmim+][Cl−].27 Therefore, the features hold true for a wide range of imidazoliumbased ILs with molecular anions and halogen atomic anions. In Figure 2b, the existence of a dielectric relaxation mode or librational motion of the cations24,28 in [Cnmim+][TfO−] is indicated from the behavior of Im ε spectra below 30 cm−1. Im ε spectra with similar behavior below 30 cm−1 were also found

■

EXPERIMENTAL SECTION A relatively large number of room-temperature ILs consisting of alkyl methylimidazolium cations and molecular anions, whose structures and abbreviations are shown in Figure 1, were

Figure 1. Room-temperature imidazolium-based ionic liquids used in our study.

studied. Ionic-liquid samples of high purity (>98%) were used in our study. Hereinafter, abbreviations such as [C6mim+][TfO−] are utilized, where C6 denotes the hexyl group and mim represents the methylimidazolium-ring part of the molecular cation. The experimental apparatus for terahertz time-domain spectroscopy (THz-TDS), Fourier transform far-infrared (FIR) spectroscopy, and attenuated-total-reflection Fourier transform infrared (ATR-FTIR) spectroscopy was described elsewhere.27 A brief outline of the apparatus is presented here. The THz-TDS spectrometer (Tochigi Nikon, Rayfact RS01020), FIR spectrometer (JASCO Corporation, VIR-F), and ATR-FTIR spectrometer (HORIBA, Ltd., FT-720 and Smiths detection, DuraScope) enabled us to measure complex dielectric constants or complex refractive indexes with a frequency range of 0.4−3.6 THz (13−120 cm−1), FIR absorption spectra with a frequency range of 30−670 cm−1, and IR absorption spectra with a frequency range of 550−3300 15697


Article


Figure 2. (a) Re ε and (b) Im ε spectra and (c,d) absorption coefficients (α) obtained by (c) THz-TDS and (d) FIR spectroscopy for the [Cnmim+][TfO−] imidazolium-based ILs with n = 2, 4, 6, and 8. The molar-concentration-normalized absorption coefficients (α/M) are also shown in the insets of panels c and d.

Figure 3. (a) Re ε and (b) Im ε spectra and (c,d) absorption coefficients (α) obtained by (c) THz-TDS and (d) FIR spectroscopy for the [Cnmim+][Tf2N−] imidazolium-based ILs with n = 2, 4, 6, 8, and 10. The molar-concentration-normalized absorption coefficients (α/M) are also shown in the insets of panels c and d.

for [Cnmim+][Tf2N−], [Cnmim+][PF6 −], [Cnmim+][PF3(C + − − 2F5)3 ], and [Cnmim ][BF4 ] as shown later in Figures 3b−6b and was previously found for [Cnmim+][I−], [Cnmim+][Br−] and [Cnmim+][Cl−].27 Therefore, the features also hold true for a wide range of imidazolium-based ILs with molecular anions and halogen atomic anions.

The Re ε, Im ε, and α (α/M) spectra for [Cnmim+][Tf2N−] imidazolium-based ILs with n = 2, 4, 6, 8, and 10 are shown in Figure 3a−d and the insets of Figure 3c,d. In the lowwavenumber range (13−130 cm−1), the relatively large and broad absorption bands with a peak at 84 cm−1 in the α/M spectra (insets of Figure 3c,d) were nearly the same for all [Cnmim+][TfO−] imidazolium-based ILs in terms of the 15698


Article


Figure 4. (a) Re ε and (b) Im ε spectra and (c,d) absorption coefficients (α) obtained by (c) THz-TDS and (d) FIR spectroscopy for the [Cnmim+][PF6−] imidazolium-based ILs with n = 4, 6, and 8. The molar-concentration-normalized absorption coefficients (α/M) are also shown in the insets of panels c and d.

Figure 5. (a) Re ε and (b) Im ε spectra and (c,d) absorption coefficients (α) obtained by (c) THz-TDS and (d) FIR spectroscopy for the ionic liquid [C6mim+][PF3(C2F5)3−]. The molar-concentration-normalized absorption coefficient spectra (α/M) are also shown in the insets of panels c and d.

cm−1), there are two relatively strong absorption bands between 600 and 670 cm−1 that stem from the intramolecular vibrational modes of the imidazolium ring (see Figure S1, SI). Two very strong absorption bands between 500 and 600 cm−1 and a very strong absorption band between 600 and 670 cm−1 originated from the intramolecular vibrational modes of [Tf2N−] (see Figure S3, SI). However, it was difficult to

absorption intensity, central absorption frequency, and absorption bandwidth. This absorption band can be attributed to the intermolecular absorption band based on the electrostatic interaction between [mim+] and [Tf2N−]. In the midwavenumber range (130−500 cm−1) in the α (α/M) spectra, [Tf2N−] exhibits some intramolecular absorption bands (see Figure S3, SI). In the high-wavenumber range (500−670 15699


Article


Figure 6. (a) Re ε and (b) Im ε spectra and (c,d) absorption coefficients (α) obtained by (c) THz-TDS and (d) FIR spectroscopy for the [Cnmim+][BF4−] imidazolium-based ILs with n = 2, 4, 6, 8, and 10. The molar-concentration-normalized absorption coefficients (α/M) are also shown in the insets of panels c and d.

relatively large and broad band with a peak at 64 cm−1 in the α (α/M) spectrum was found. Drawing an analogy with the other room-temperature imidazolium-based ILs, we presume that the absorption band originated from the electrostatic interaction between [mim+] and [PF3(C2F5)3−]. In the midwavenumber range (130−500 cm−1) in the α (α/M) spectra, there were many intramolecular absorption bands of [PF3(C2F5)3−] (see Figure S5, SI). In the high-wavenumber range (500−670 cm−1), the two relatively strong absorption bands between 600 and 670 cm−1 stem from the intramolecular vibrational modes of the imidazolium ring (see Figure S1, SI). The two very strong absorption bands between 500 and 600 cm−1 and the very strong absorption band between 600 and 670 cm−1 originate from the intramolecular vibrational mode of [PF3(C2F5)3−] (see Figure S5, SI). However, it was difficult to distinguish between these absorption bands from 600 to 670 cm−1 because of the influence of the very strong intramolecular absorption band of [PF3(C2F5)3−]. The Re ε, Im ε, and α (α/M) spectra of the [Cnmim+][BF4−] imidazolium-based ILs with n = 2, 4, 6, 8, and 10 are shown in Figure 6a−d and the insets of Figure 6c,d. In the lowwavenumber range (13−130 cm−1), the relatively large and broad absorption bands with a peak at 100 cm−1 in the α/M spectra (insets of Figure 6c,d) were nearly the same for all of the [Cnmim+][BF4−] imidazolium-based ILs in terms of the absorption intensity, central absorption frequency, and absorption bandwidth. This absorption band can be attributed to the intermolecular absorption band based on the electrostatic interaction between [mim+] and [BF4−]. In the midwavenumber range (130−500 cm−1) in the α (α/M) spectra, weak absorption bands due to intramolecular vibrations of the imidazolium cations were observed because there were no intramolecular absorption bands of [BF4−]. The most discernible absorption band at about 250 cm−1 for [C2mim+] observed in other imidazolium ILs16 was also found. In the

distinguish between these absorption bands from 600 to 670 cm−1 because of the influence of the very strong intramolecular absorption band of [Tf2N−]. The Re ε, Im ε, and α (α/M) spectra for the [Cnmim+][PF6−] imidazolium-based ILs with n = 4, 6, and 8 are shown in Figure 4a−d and the insets of Figure 4c,d. In the lowwavenumber range (13−130 cm−1), the relatively large and broad absorption bands with a peak at 83 cm−1 in the α/M spectra (insets of Figure 4c,d) were nearly the same for all of the [Cnmim+][PF6−] imidazolium-based ILs in terms of the absorption intensity, central absorption frequency, and absorption bandwidth. This absorption band can be attributed to the intermolecular absorption band based on the electrostatic interaction between [mim+] and [PF6−]. In the midwavenumber range (130−500 cm−1) in the α (α/M) spectra, weak absorption bands due to intramolecular vibrations were discerned. Because there were no intramolecular absorption bands of [PF6−] in this range, these weak absorption bands can be attributed to intramolecular absorption bands, in which the accordion-like motion of the alkyl-chain parts of the imidazolium cations is included.27 In the high-wavenumber range (500−670 cm−1), the two relatively strong absorption bands between 600 and 670 cm−1 stem from the intramolecular vibrational modes of the imidazolium ring (see Figure S1, SI). The very strong absorption band between 500 and 600 cm−1 originates from the intramolecular vibrational mode of [PF6−] (see Figure S4, SI). The Re ε, Im ε, and α (α/M) spectra for [C6mim+][PF3(C2F5)3−] are shown in Figure 5a−d and the insets of Figure 5c,d. In the anion [PF3(C2F5)3−], three of the F atoms in the anion [PF6−] are replaced by three C2F5 groups. The other ionic liquids, which consist of imidazolium cations with different alkyl-chain lengths and the anion [PF3(C2F5)3−], were not measured, and these ionic liquids are not commercially available. In the low-wavenumber range (13−130 cm−1), a 15700


Article

The Journal of Physical Chemistry B high-wavenumber range (500−670 cm−1), the two relatively strong absorption bands between 600 and 670 cm−1 stem from the intramolecular vibrational modes of the imidazolium ring (see Figure S1, SI). The relatively strong absorption band between 500 and 600 cm−1 originates from the intramolecular vibrational mode of [BF4−] (see Figure S6, SI). The α/M spectra of [C6mim+][BF4−], [C6mim+][TfO−], [C 6 mim + ][PF 6 − ], [C 6 mim + ] [Tf 2 N − ], and [C 6 mim + ][PF3(C2F5)3−] in the low-wavenumber range are shown in Figure 7. Relatively strong and broad absorption bands were

[C6mim+][Tf2N−], and 64 cm−1 for [C6mim+][PF3(C2F5)3−]. The broad absorption bands due to the intermolecular vibration between the imidazolium cation and the molecular anion clearly have frequency and intensity dependencies on the molecular anion species. Taking an overall view of Figure 7, the imidazolium-based ionic liquids with heavier molecular anions tend to show smaller molar-concentration-normalized absorptions at lower frequencies and the imidazolium-based ionic liquids with lighter molecular anions tend to show stronger molar-concentration-normalized absorptions at higher frequencies, although the arrangement order is incomplete. We discuss this issue in detail at the end of this section. ATR-FTIR spectra from 2800 to 3300 cm−1 for [C6mim+][TfO−], [C6mim+][Tf2N−], [C6mim+][PF6−], [C6mim+][PF3(C2F5)3−], and [C6mim+][BF4−] are shown in panels a− e, respectively, of Figure 8. In Figure 8a−e, several peaks in the frequency range between 2800 and 3000 cm−1 are assigned to the symmetric and asymmetric stretching vibrations of methylene and methyl groups in the alkyl-chain part. The significance of the vibrational modes in the frequency range between 3000 and 3300 cm−1 was noted in the discussion of the cation−anion interaction strength, because the +C−H stretching vibrational modes [+C(2)−H stretching and +C(4/ 5)−H antisymmetric and symmetric modes]20,27 in the imidazolium-ring cation exist in this range. We found that [C6mim+][PF6−] (Figure 8c) and [C6mim+][PF3(C2F5)3−] (Figure 8d) exhibited two identical main absorption bands in terms of frequency and shape, reflecting nearly identical levels of interaction between the cation and anion molecules because of the similarities in their chemical structures. [C6mim+]-

Figure 7. Molar-concentration-normalized absorption coefficient (α/ M) spectra obtained from THz-TDS for the imidazolium-based ILs [C6mim+][BF4−], [C6mim+][TfO−], [C6mim+][PF6−], [C6mim+] [Tf2N−], and [C6mim+][PF3(C2F5)3−].

found at 100 cm−1 for [C6mim+][BF4−], 89 cm−1 for [C6mim+][TfO−], 83 cm−1 for [C6mim+][PF6−], 84 cm−1 for

Figure 8. FTIR spectra between 2800 and 3300 cm−1 of (a) [C6mim+][TfO−], (b) [C6mim+] [Tf2N−], (c) [C6mim+][PF6−], (d) [C6mim+][PF3(C2F5)3−], and (e) [C6mim+][BF4−]. 15701


Article

The Journal of Physical Chemistry B [Tf2N−] (Figure 8b) and [C6mim+][BF4−] (Figure 8e) also exhibited almost the same two main absorption bands. [C6mim+][PF6−] (Figure 8c) and [C6mim+][BF4−] (Figure 8e) exhibited two similar absorption bands, although [C6mim+][BF4−] (Figure 8e) exhibited slightly red-shifted absorption bands compared to [C6mim+][PF6−] (Figure 8c). On the other hand, [C6mim+][TfO−] (Figure 8a) produced clearly red-shifted and broader absorption bands, compared to the other imidazolium-based ILs used in this study. One of the characteristics of [TfO−] is that it is a strong proton acceptor.29 We previously showed that imidazolium-based ILs with halogen anions exhibit much more red-shifted spectra.27 The halogens exhibit characteristics of much stronger proton acceptors.29 Yamamoto et al.13 measured the complex dielectric spectra of the imidazolium-based ILs [C2mim+][TfO−], [C4mim+][TfO−], C2mim[BF4−], and C4mim[BF4−] using THz-TDS and indicated that the intermolecular vibrations, and not intramolecular vibrations, are what mainly contribute to the dielectric spectra. Ludwig and co-workers16−18 examined imidazolium-based ILs with a series of anion molecules ([C 2 mim + ][SCN − ], [C 2 mim + ][N(CN) 2 − ], [C 2 mim + ][EtSO4−], and [C2mim][Tf2N−]) using FIR and FTIR spectroscopies and DFT calculations. They noted that the peak positions of the intermolecular absorption band for imidazolium-based ILs with a series of anion molecules in the low-wavenumber range were mainly determined by the force constant k from the equation of a simple harmonic oscillator, ω = (k/μ)1/2. They also discussed the correlation between the frequency of the intramolecular +C−H stretching vibrational mode in the frequency range between 3000 and 3300 cm−1 and the intermolecular vibrational frequency in the low-wavenumber range. Increasing the strength of hydrogen bonding in + C−H···Amolecule− resulted in a lengthened covalent bond +C− H (weaker force constant for the intramolecular +C−H stretching vibration) and a shortened hydrogen bond (stronger force constant for the intermolecular vibration). Essentially, a stronger the hydrogen bond results in a red-shifted intramolecular +C−H stretching vibrational mode and a blue-shifted intermolecular vibrational mode. They also reported the direct observation of a local and directional hydrogen bond as well as the correlation between the interaction energy and the redshifted intramolecular +C−H stretching vibrational mode.30 On the other hand, Lassègues et al.22,31 claimed that there is no need to invoke the hydrogen-bond interaction between the + C−H unit and molecular anion Amolecule− for certain imidazolium-based ILs with weakly coordinating molecular anions, and they gave different assignments for the stretching vibrational modes in this range. Johnson et al.32 suggested local structural motifs for [C2mim+][BF4−], in which they are not strongly hydrogen bonded but, nonetheless, considerable intensity of the +C(2)−H stretching vibrational mode is induced. Recently, the existence or nonexistence of hydrogen bonding in +C(2)−H···Amolecule− was heavily discussed for [Cnmim+][Tf2N−].33−35 DFT or ab initio calculations have been performed for various imdazolium-based ILs to calculate the cation−anion interaction energies.36−40 For imidazolium-based ILs with molecular anions, the typical level of the interaction energy is about 75−90 kcal/mol for isolated ion pairs.36 Tsuzuki et al.36 showed by ab initio calculations that the magnitudes of the interaction energies of imidazolium complexes follow the trend of BF4− > TfO− (CF3SO3−) > Tf2N− [(CF3SO2)N−] ≈ PF6− (−85.2, −82.6, −78.8, and −78.4 kcal/mol, respectively) and

stated that the electrostatic interaction through charge−charge interactions is the major source of the attraction between the ions and that the hydrogen bond in +C(2)−H···Amolecule− is not essential for attraction in the ion pair. In contrast to the significant directionality of the hydrogen bond between neutral molecules such as water dimers, in which the main source is the orientation-dependent dipole−dipole interaction, the interaction energy between an imidazolium cation and a molecular anion is more than 10 orders of magnitude of the interaction energy of water dimers as neutral molecules and that the isotropic charge−charge interaction (pure Coulombic interaction) is the main source. In this respect, the hydrogen bond in +C(2)−H···Amolecule− is different from a conventional hydrogen bond. One example is TfO−, which is a stronger proton acceptor than BF4−. Compared with BF4−, TfO− leads to red-shifted absorption bands in the range between 3000 and 3300 cm−1, as shown in Figure 8, whereas the interaction energy of [C2mim+][TfO−] is smaller than that of [C2mim+][BF4−].36 The correlation between the red shift of the IR absorption bands in the range between 3000 and 3300 cm−1 and the interaction energy does not necessarily hold. For imidazolium-based ILs with halogen atomic anions, the typical level of interaction energy is about 80−100 kcal/mol for the ion pairs.37−39 Thus, the typical level of interaction energy between the imidazolium cation and a halogen anion is larger than that between the imidazolium cation and a molecular anion. However, the difference might not be as large as indicated by Tsuzuki et al.,37 who demonstrated through the use of ab initio calculations that the magnitude of the interaction energy of imidazolium complexes close to the +C(2)−H follows the trend Cl− > Br− > BF4− > PF6− (−82.0, −80.9, −77.3, and −71.5 kcal/mol, respectively). More recently, it was demonstrated through the use of ab initio calculations that the strength of the interionic interaction between [Cnmim+] and [anion−] follows the trend Cl− > Br− > BF4− > N(CN)2− > PF6− > Tf2N− [(CF3SO2)N−].40 In the present work, we mainly considered the interaction energy for ion pairs.36−40 These calculations can provide the right trend for the interaction energies in ILs. However, a detailed description can be provided by considering large clusters or by Car−Parrinello molecular dynamics (CPMD) simulations, including the dispersion correction.12,17 We return to the discussion of the data in Figures 2−8 and the data in our previous systematic study of imidazolium-based ILs with halogen anions.27 In our previous studies, we found that the α/M spectra in the low-wavenumber range were nearly identical for imidazolium cations with different alkyl-chain lengths, so long as the halogen anion (I−, Br−, or Cl−) was the same, in terms of the absorption intensity, absorption frequency, and absorption bandwidth. The data indicate that the intermolecular vibrational band is phenomenologically determined by the number of cation−anion pairs, where the alkyl-chain parts in the imidazolium cation have almost no influence. We examined the intermolecular vibrational bands from the fundamental viewpoint of ω = (k/μ)1/2 (1/μ = 1/ μ[mim+] + 1/μ[halogen anion−]), where the effective mass of the cation for the intermolecular vibration is the mass (m = 83) of the methylimidazolium-ring part [mim+]. Considering not only experimental data but also the charge-rich range and electrostatic potential map of the imidazolium cation, the idea of [mim+] seemed to be convincing.27 The data in Figures 2−7 in the present study also indicate that the α/M spectra in the lowwavenumber range are nearly the same in terms of the absorption intensity, central absorption frequency, and 15702


Article


Table 1. Values of the Reduced Mass μ, 1/μ1/2, Wavenumber at the Absorption Peak (cm−1), Vibration Energy (meV), and Bandwidth (meV) for the ILs Examined in This Work and Comparison with Data for [C2mim+][N(CN)2−] and [C2mim+][SCN−] +

−

[mim ][PF3(C2F5)3 ] [mim+][PF6−] [mim+][(CF3SO2)2N−]half (full)a [mim+][CF3SO3−] [mim+][BF4−] [mim+][l−]b [mim+][Br−]b [mim+][Cl−]b [mim+][N(CN)2−] ([C2mim+][N(CN)2−]c) [mim+][SCN−] ([C2mim+][SCN−]c)

reduced mass μ

1/μ1/2

wavenumber (cm−1)

energy (meV)

bandwidth (fwhm) (meV)

70.0 52.8 52.2 (64.0) 53.8 42.5 50.2 40.8 24.9 36.8 (41.6) 34.2 (38.1)

0.119 0.138 0.138 (0.125) 0.137 0.153 0.14 0.16 0.20 0.165 (0.155) 0.171 (0.162)

64 83 84 89 100 72.5 92 122 113.5 117.6

7.93 10.3 10.4 11.0 12.4 9.0 11.4 15.1 14.1 14.6

∼9.5 ∼10.5 ∼10.0 ∼10.0 ∼10.0 ∼7.0 ∼8.0 ∼8.0 d d

“Half” denotes that one-half of the mass of [(CF3SO2)2N−] is utilized to calculate the reduced mass, while “full” denotes that the mass of [(CF3SO2)2N−] is utilized to calculate the reduced mass. bValues for [mim+][l−], [mim+][Br−], and [mim+][Cl−] from ref 27. cValues of the wavenumber of the absorption peak for [C2mim+][N(CN)2−] and [C2mim+][SCN−] from ref 16. dNot available due to difficulty evaluating these values from ref 16. a

stretching vibration bands, compared with those with [TfO−] and [N(CN)2−].27,30,31 The ionic liquid with [Br−] has a largely red-shifted intermolecular absorption band compared with [N(CN)2−] and almost the same intermolecular absorption band as the ionic liquid with [TfO−]. The ionic liquid with [I−] has a largely red-shifted intermolecular absorption band, compared with [TfO−] and [N(CN)2−]. Regarding the interaction energy between ion pairs, the interaction energies of the ionic liquids with [TfO−] and [N(CN)2−] are smaller than those with [Br−] and [I−]. Thus, the correlation between the intramolecular +C−H stretching vibrational modes, the intermolecular vibrational mode, and the interaction energy of ion pairs does not necessarily hold for a wide range of imidazolium-based ILs. Figure 9 shows the intermolecular vibration frequency plotted as a function of 1/μ1/2, as obtained from Table 1.

absorption bandwidth for imidazolium cations with different alkyl-chain lengths, so long as the molecular anion ([TfO−], [Tf2N−], [PF6−], [PF3(C2F5)3−], or [BF4−]) is the same. The data also indicate that the intermolecular vibrational band is phenomenologically determined by the number of cation− anion pairs, where the alkyl-chain parts in the imidazolium cation have almost no influence. Therefore, we examined the intermolecular vibrational bands from the fundamental viewpoint of ω = (k/μ)1/2 (1/μ = 1/μ[mim+] + 1/μ[molecular anion−]). The values of the reduced mass μ, 1/μ1/2, central absorption frequency (wavenumber and energy), and bandwidth (full width at half-maximum) of the absorption band are summarized in Table 1. These values were all obtained from the data in Figures 2−7, our previous study, and the aforementioned analysis. In Table 1, we also include16 the data for [C2mim+][N(CN)2−] and [C2mim+][SCN−] as examples with molecular anions as strong proton acceptors, and the data for their corresponding C+−H stretching vibration bands in the range between 3000 and 3300 cm−1 were given.30,31 We discuss the correlation between the red-shifted intramolecular +C−H stretching vibrational modes and the blue-shifted intermolecular vibrational mode, and vice versa. We compare the imidazolium-based ILs with molecular anions of [TfO−] and [BF4−]. Compared to that with [BF4−], the ionic liquid with [TfO−] gives rise to largely red-shifted C+−H stretching vibration bands and a red-shifted intermolecular absorption band. Regarding the interaction energy between ion pairs, the interaction energy of the ionic liquid with [TfO−] is smaller than that with [BF4−].36 However, one should notice that the interaction due to hydrogen-bond basicity and hydrogen bonding for the ionic liquid with [TfO−] is stronger than that with [BF4−], as evaluated by Cláudio et al. using COSMO-RS (conductor-like screening model for real solvent) calculations.41 We also compare the imidazolium-based ILs with the molecular anions of [PF3(C2F5)3−] and [PF6−]. The ionic liquid with [PF3(C2F5)3−] has nearly the same C+−H stretching vibration bands as that with [PF6−]. Compared to [PF6−], it has a largely red-shifted intermolecular absorption band. We compare the imidazolium-based ILs with the anions of [Br−], [I−], [TfO−], and [N(CN)2−], which are classified as strong proton acceptors, although halogen atomic anions are stronger proton acceptors than [TfO−] and [N(CN)2−]. Ionic liquids with [Br−] and [I−] have largely red-shifted C+−H

Figure 9. Plot of intermolecular vibration frequency versus 1/μ1/2 obtained from the data in Table 1.

Regarding the molecular anion Tf2N− [(CF3SO2)N−], we assumed one-half of the mass. It has been indicated that Tf2N− contains two Tfs and that Tf2N− is partially involved in the intermolecular vibrational motion.16 We found a correlation between the intermolecular vibration frequency and 1/μ1/2 for a wide range of imidazolium-based ILs, although there was a degree of dispersion in the data. Note that we plotted the data from zero on both the x and y axes. In general, a significant contribution made by the reduced mass to the intermolecular vibrational frequency was found in a wide range of imidazolium-based ILs. The dispersion in the data indicates that the contribution of k1/2 also exists when the data among 15703


Article


sively examined. The molar-concentration-normalized absorption coefficient spectra in the low-wavenumber range for imidazolium cations with different alkyl-chain lengths were found to be nearly identical for the same anion, which is surprisingly true for a wide range of imidazolium-based ILs with molecular anions and halogen atomic anions. In general, for a wide range of imidazolium-based ILs, we noted a significant contribution of the reduced mass, calculated based on the imidazolium-ring cation and anion, as well as the force constant, for determining the center frequency of the intermolecular absorption band. Regarding the bandwidth of the intermolecular absorption band, the bandwidth is broad, and the energy width is nearly the same for the imidazolium cation with different alkyl-chain lengths as long as the anion is the same. In addition, the energy width is about ∼10 meV for imidazolium-based ILs with molecular anions and ∼8 meV for imidazolium-based ILs with halogen atomic anions, regardless of the difference in anions. We also discussed the correlation between the frequency of the intramolecular +C−H stretching vibrational mode and the frequency of the intermolecular vibrational frequency and the interaction energy. The correlation between these is not necessarily established for a wide range of imidazolium-based ILs. We found that the molarconcentration-normalized intermolecular vibration absorption coefficient at the central frequency is almost inversely proportional to the molecular weight of ion pair ([mim+][anion−]). Because the molecular weight of the ion pair is related to the physical size of the ion pair, such as the size of charge distributions and the average distance of ion pairs, this relation is essential. The systematic data we obtained from THz-TD, FIR, and FTIR spectroscopies and the corresponding analysis could prove useful in other experimental studies and help to further detailed modeling and simulation of various ILs to elucidate their nature.

some different anions are compared. In addition, one should be aware of the accuracy of the effective mass in each system, which is determined phenomenologically. In the case of certain alkylammonium ionic liquids, the significant contributions of both the reduced mass and force constant were noted.42 For tetraalkylammonium halide crystals, the importance of the reduced mass in determining the interior vibration frequency was demonstrated.43 For some protic ionic liquids, the influence on the far-infrared spectra of hydrogenbonding and dispersion forces in a Coulomb-dominated system was discussed previously.44,45 Figure 10 shows α/M at the central absorption frequency of the intermolecular vibrational band as a function of the

Figure 10. Molar-concentration-normalized absorption coefficient (α/ M) at the central absorption frequency of the intermolecular vibrational band versus the molecular weight of the ion pair ([mim+][anion−]).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b09101. Calculated vibrational spectra for the alkyl methylimidazolium cations (C2mim+, C4mim+, C6mim+, C8mim+, and C10mim+) and molecular anions ([TfO−], [Tf2N−], [PF6−], [PF3(C2F5)3−], and [BF4−]) in the low-, mid-, and high-terahertz frequency ranges (PDF)

molecular weight of the ion pair ([mim+][anion−]) obtained from the data in Figures 2−7 in the present study and the data in our previous studies concerning room-temperature ILs consisting of imidazolium cations and halogen anions. We found that the concentration-normalized intermolecular vibration absorption intensity at its peak was nearly inversely proportional to the molecular weight of the ion pair ([mim+][anion−]). Because the molecular weight of the ion pair is related to the physical size of the ion pair, such as the size of the charge distributions and the average distance of ion pairs, the relation obtained from Figure 10 is essential. Here, we briefly discuss the bandwidth of the intermolecular absorption band. The energy width (full width at halfmaximum) is essentially broad for a wide range of imidazolium ILs, which also seems to be characteristic for Coulomb fluids. The energy width is nearly the same for imidazolium cations with different alkyl-chain lengths as long as the molecular anion is the same. In addition, the energy width is about ∼10 meV for imidazolium-based ILs with molecular anions and ∼8 meV for imidazolium-based ILs with halogen atomic anions, regardless of the difference in anions, as summarized in Table 1.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Wasserscheid, P., Welton, T., Eds. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, Germany, 2007. (2) Hallett, J. P.; Welton, T. Room-temperature ionic liquids. Solvents for synthesis and catalysis 2. Chem. Rev. 2011, 111, 3508− 3576. (3) Castner, E. W., Jr.; Wishart, J. F. Spotlight on ionic liquids. J. Chem. Phys. 2010, 132, 120901-1−120901-9. (4) Angell, C. A.; Ansari, Y.; Zhao, Z. Ionic Liquids: Past, present and future. Faraday Discuss. 2012, 154, 9−27. (5) Wang, H.; Gurau, G.; Rogers, R. D. Ionic liquid processing of cellulose. Chem. Soc. Rev. 2012, 41, 1519−1537.

■

CONCLUSIONS The terahertz- and infrared-frequency vibrational modes of various room-temperature imidazolium-based ILs were exten15704


Article

The Journal of Physical Chemistry B (6) Plechkova, N. V.; Seddon, K. R. Applications of ionic liquids in the chemical industry. Chem. Soc. Rev. 2008, 37, 123−150. (7) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 2009, 8, 621−629. (8) Hayes, R.; Warr, G. G.; Atkin, R. Structure and nanostructure in ionic liquids. Chem. Rev. 2015, 115, 6357−6426. (9) Weingärtner, H. Understanding ionic liquids at the molecular level: Facts, problems, and controversies. Angew. Chem., Int. Ed. 2008, 47, 654−670. (10) Castner, E. W., Jr.; Margulis, C. J.; Maroncelli, M.; Wishart, J. F. Ionic Liquid: Structure and Photochemical Reactions. Annu. Rev. Phys. Chem. 2011, 62, 85−105. (11) Fumino, K.; Ludwig, R. Analyzing the interaction between cation and anion in ionic liquids: The subtle balance between Coulomb forces and hydrogen bonding. J. Mol. Liq. 2014, 192, 94− 102. (12) 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. (13) Yamamoto, K.; Tani, M.; Hangyo, M. Terahertz time-domain spectroscopy in imidazolium ionic liquids. J. Phys. Chem. B 2007, 111, 4854−4859. (14) Koeberg, M.; Wu, C.-C.; Kim, D.; Bonn, M. THz dielectric relaxation of ionic liquid: water mixture. Chem. Phys. Lett. 2007, 439, 60−64. (15) Krüger, M.; Bründermann, E.; Funkner, S.; Weingärtner, H.; Havenith, M. Polarity fluctuations of the protic ionic liquid ethylammonium nitrate in the terahertz regime. J. Chem. Phys. 2010, 132, 101101-1−101101-4. (16) Wulf, A.; Fumino, K.; Ludwig, R.; Taday, P. F. Combined THz and Raman spectroscopy studies of imidazolium-based ionic liquids covering the frequency range 2−300 cm−1. ChemPhysChem 2010, 11, 349−353. (17) 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. (18) Wulf, A.; Fumino, K.; Ludwig, R. Spectroscopic evidence for an enhanced anion−cation interaction from hydrogen bonding in pure imidazolium ionic liquid. Angew. Chem., Int. Ed. 2010, 49, 449−453. (19) 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. (20) 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. (21) Fumino, K.; Wulf, A.; Ludwig, R. Strong, localized, and directional hydrogen bonds fluidize ionic liquids. Angew. Chem., Int. Ed. 2008, 47, 8731−8734. (22) 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, 6419−6412. (23) 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. (24) Xiao, D.; Hines, L. G., Jr.; Li, S.; Bartsch, R. A.; Quitevis, E. L.; Russina, O.; Triolo, A. Effect of cation symmetry and alkyl chain length on the structure and intermolecular dynamics of 1,3dialkylimidazolium Bis(trifluoromethanesulfonyl)amide ionic liquids. J. Phys. Chem. B 2009, 113, 6426−6433. (25) Shirota, H.; Nishikawa, K.; Ishida, T. Atom substitution effect of [XF6]− in ionic liquids. 1. Experimental study. J. Phys. Chem. B 2009, 113, 9831−9839. (26) 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 mix-

ture: Molecular dynamics simulations and OKE spectroscopic measurements. J. Chem. Phys. 2014, 141, 044506-1−044506-13. (27) Yamada, T.; Tominari, Y.; Tanaka, S.; Mizuno, M.; Fukunaga, K. Vibrational modes at terahertz and infrared frequencies of ionic liquids consisting of an imidazolium cation and a halogen anion. Materials 2014, 7, 7409−7422. (28) Stoppa, A.; Hunger, J.; Buchner, R.; Hefter, G.; Thoman, A.; Helm, H. Interactions and Dynamics in Ionic Liquids. J. Phys. Chem. B 2008, 112, 4854−4858. (29) Ueno, K.; Tokuda, H.; Watanabe, M. Ionicity in ionic liquids: correlation with ionic structure and physicochemical properties. Phys. Chem. Chem. Phys. 2010, 12, 1649−1658. (30) 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. (31) 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. (32) Johnson, C. J.; Fournier, J. A.; Wolke, C. T.; Johnson, M. A. Ionic liquids from the bottom up: Local assembly motifs in [EMIM][BF4] through cryogenic ion spectroscopy. J. Chem. Phys. 2013, 139, 224305-1−224305-7. (33) Chatzipapadopoulos, S.; Zentel, T.; Ludwig, R.; Lütgens, M.; Lochbrunner, S.; Kühn, O. Vibrational dephasing in ionic liquids as a signature of hydrogen bonding. ChemPhysChem 2015, 16, 2519−2523. (34) Roth, C.; Chatzipapadopoulos, S.; Kerlé, D.; Friedriszik, F.; Lütgens, M.; Lochbrunner, S.; Kühn, O.; Ludwiq, R. Hydrogen bonding in ionic liquids probed by linear and nonlinear vibrational spectroscopy. New J. Phys. 2012, 14, 105026-1−105026-15. (35) 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. (36) Tsuzuki, S.; Tokuda, H.; Hayamizu, K.; Watanabe, M. Magnitude and directionality of interaction in ion pairs of ionic liquids: Relationship with ionic conductivity. J. Phys. Chem. B 2005, 109, 16474−16481. (37) Tsuzuki, S.; Tokuda, H.; Mikami, M. Theoretical analysis of the hydrogen bond of imidazolium C2−H with anions. Phys. Chem. Chem. Phys. 2007, 9, 4780−4784. (38) Turner, E. A.; Pye, C. C.; Singer, R. D. Use of ab initio calculations toward the rational design of room temperature ionic liquids. J. Phys. Chem. A 2003, 107, 2277−2288. (39) Wang, Y.; Li, H.; Han, S. The chemical nature of the +C−H···X− (X = Cl or Br) interaction in imidazolium halaide ionic liquids. J. Chem. Phys. 2006, 124, 044504-1−044504-8. (40) Izgorodina, E. I.; Golze, D.; Maganti, R.; Armel, V.; Taige, M.; Schubert, T. J. S.; MacFarlane, D. R. Importance of dispersion forces for prediction of thermodynamic and transport properties of some common ionic liquids. Phys. Chem. Chem. Phys. 2014, 16, 7209−7221. (41) Cláudio, A. F. M.; Swift, L.; Hallett, J. P.; Welton, T.; Coutinho, J. A. P.; Freire, M. G. Extended scale for the hydrogen-bond basicity of ionic liquids. Phys. Chem. Chem. Phys. 2014, 16, 6593−6601. (42) 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. (43) Aimone, L.; Badiali, J.-P.; Cachet, H. Far infrared spectroscopic properties and interionic potential energy for tetra-alkylammonium halides. J. Chem. Soc., Faraday Trans. 2 1977, 73, 1607−1615. (44) Ludwig, R. Phys. Chem. Chem. Phys. 2015, 17, 13790−13793. (45) Fumino, K.; Fossog, V.; Stange, P.; Paschek, D.; Hempelmann, R.; Ludwig, R. Angew. Chem., Int. Ed. 2015, 54, 2792−2795.

15705


Terahertz and Infrared Spectroscopy of Room-Temperature

Recommend Documents