Infrared Spectroscopy in the Middle Frequency Range for Various Imidazolium Ionic Liquids—Common Spectroscopic Characteristics of Vibrational Modes with In-Plane ⁺C(2)–H and ⁺C(4,5)–H Bending Motions and Peak Splitting Behavior Due to Local Symmetry Breaking of Vibrational Modes of the Tetrafluoroborate Anion

Abstract

Various alkyl-methylimidazolium ionic liquids (ILs) were inspected using infrared spectroscopy in the middle frequency range. In the 1050–1200 cm^–1 range, there is a skeletal vibrational mode accompanied with a large in-plane ⁺C(2)–H bending motion and ⁺C(4)–H and ⁺C(5)–H motions, and in the 1500–1650 cm^–1 range, there are two skeletal vibrational modes with in-plane ⁺C(4,5)–H bending motions. Interestingly, in both ranges, we found that skeletal vibrational modes with a large in-plane ⁺C(2)–H bending motion and in-plane ⁺C(4,5)–H bending motions are insensitive to increases in the basicity of anions or the strengthening of hydrogen bond-type interactions, and the behaviors are completely different from those in the ⁺C–H stretching vibrational modes in the 3000–3200 cm^–1 range and the skeletal vibrational modes with large out-of-plane ⁺C–H motions in the 700–950 cm^–1 range. Furthermore, in alkyl-methylimidazolium tetrafluoroborate [C_nmim⁺][BF₄^–] ILs, we found that absorption due to the (threefold) degenerate vibrational mode of [BF₄^–] was observed as a broad absorption band with three splitting peaks in the 900–1150 cm^–1 range as a result of local symmetry breaking due to the cation–anion interactions.

Introduction

The importance of room-temperature ionic liquids (ILs) has rapidly spread in both scientific and technological fields. Thus, the unique material properties of ILs, including their wide liquid temperature ranges, nonvolatility, high electrical and ionic conductivity, excellent chemical and thermal stability, lubrication characteristics, and superb and unique solubility for a wide range of materials, have attracted one’s attention.¹⁻⁶ Clarifying the nature of the interactions between cations and anions is crucial for understanding the unique physical and chemical properties of ILs, and therefore, consideration of Coulombic interactions, hydrogen bond-type interactions, dispersion interactions, and their mutual relationships in ILs is particularly important.⁷ For this purpose, various spectroscopic techniques, such as NMR spectroscopy,^8,9 dielectric spectroscopy,^10,11 infrared and Raman spectroscopy,¹²⁻²⁶ far-infrared (FIR) spectroscopy, terahertz time-domain spectroscopy (THz-TDS),²⁷⁻³³ and various methods of nonlinear optical spectroscopy,³⁴⁻³⁸ have been applied to IL systems in conjunction with computer simulations.³⁹⁻⁴⁸

Previously, we performed systematic studies to understand the physical characteristics that hold for a wide variety of imidazolium cation-based ILs.^21,22,31,32 The intermolecular vibrations at low frequencies due to the cation–anion interactions in imidazolium cation-based ILs that originated in Coulomb interaction and hydrogen bond-type interactions were previously investigated using THz-TDS and FIR spectroscopy. We found that the central frequencies of intermolecular vibrations on a simple harmonic oscillator model [ω=(k/μ)^−1/2] are imposed on the essential contribution of the reduced mass μ calculated from the respective masses of the methylimidazolium ring cation [mim⁺] and the anion [A^–] as well as the intermolecular force constant k, and the intermolecular vibration absorption bandwidths are relatively broad at 60–85 cm^–1 among a wide variety of alkyl-methylimidazolium cation-based ILs.^31,32 These features of the intermolecular vibrational bands seem to phenomenologically reflect the “fundamental state” as Coulomb liquids of alkyl-methylimidazolium cation-based ILs, although the local and directional hydrogen bond-type interactions may modify the cation–anion interactions.^29,30 The correlation between the center frequency of intermolecular vibrational modes and intramolecular vibrational frequencies due to the ⁺C–H stretching mode and the ⁺C–H out-of-plane bending mode under the cation–anion interactions, which are described later, does not necessarily hold.^21,22,31,32

In the X–H···Y hydrogen bond (X and Y being electronegative atoms), in general liquid systems consisting of neutral molecules, the absorption band of the stretching mode of X–H displays the salient modifications such as a red shift, substantial spectral broadening and reshaping, a considerable increase in intensity, and a blue shift of the bending mode.^20,49 The spectral shifts reflect the changes in the force constant of the oscillator and/or the anharmonicity of the vibrational potential due to the hydrogen bond.^20,49 In alkyl-methylimidazolium cation-based ILs, the intramolecular stretching modes at high frequencies (3000–3200 cm^–1) that are related to ⁺C(2)–H···A– hydrogen bond-type interactions are very sensitive to local and directional interactions.^12,19,21 The increase of the basicity of anions or the strengthening of hydrogen bond-type interactions with anions resulted in remarkable changes in the absorption bands such as a red shift, spectral broadening and reshaping, and an increase in the oscillator strength. A close correlation between the red-shifted ⁺C(2)–H stretching vibrational modes, the chemical shift (δ ppm) of the ⁺C(2)–H proton in ¹H NMR spectra, and hydrogen bond interaction energy evaluated using the conductor-like screening model for real solvent (COSMO-RS) calculations was also found.^19,21,46 The characteristic spectroscopic features in the bending modes at the 750–950 cm^–1 range that are related to out-of-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions were previously investigated.²² The absorption band due to the bending mode with an out-of-plane ⁺C(2)–H bending motion was sensitive to the interactions with anions and, interestingly, both blue- and red-shifted tendencies with the strengthening of the anions’ basicity or hydrogen bond-type interactions were observed.²² All these spectroscopic aspects of intramolecular vibrational modes related to hydrogen bond-type interactions seem to reflect “variety” in the physical and chemical properties of ILs.

In this paper, infrared spectroscopy in the middle frequency range was performed for various alkyl-methylimidazolium cation-based ILs. We investigated the skeletal vibrational modes in middle frequency regions (1050–1200 and 1500–1650 cm^–1) that are accompanied with in-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions as well as their spectral change when increasing the basicity of anions or strengthening hydrogen bond-type interactions. Systematic studies for various alkyl-methylimidazolium cation-based ILs were performed to elucidate the characteristics of the vibrational modes. The skeletal vibrational mode with a large in-plane ⁺C(2)–H bending motion in the region of 1050–1200 cm^–1 and two overlapped skeletal vibrational modes with in-plane ⁺C(4,5)–H bending motions in the region of 1500–1650 cm^–1 are remarkably insensitive to increases in the basicity of anions or the strengthening of hydrogen bond-type interactions, although these modes have a strong oscillator strength. The information from the systematic studies on the two overlapped modes is significant because the absorption bands due to skeletal vibrational modes with the in-plane ⁺C(4,5)–H bending motions have roused an intensive discussion in the context of Fermi resonance and its overtones,^{13−15,26,38} which affect the absorption bands due to the ⁺C–H stretching modes in the 3000–3200 cm^–1 region. The skeletal vibrational modes accompanied with in-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions in the regions of 1050–1200 and 1500–1650 cm^–1 are compared with the skeletal vibrational modes accompanied with out-of-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions in the region of 750–950 cm^–1 and the ⁺C(2)–H and ⁺C(4,5)–H stretching vibrational modes from the viewpoints of the spectral changes when increasing the basicity of anions or strengthening hydrogen bond-type interactions.

An another topic of this paper and an important observation is that we clearly saw that the absorption due to a (threefold) degenerate vibrational mode of the tetrafluoroborate anion [BF₄^–] in alkyl-methylimidazolium tetrafluoroborate ILs [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10 was observed as a broad absorption band with three separated vibrational states in the region of 900–1150 cm^–1 as a result of local symmetry breaking due to the cation–anion interactions. The observation of the peak-splitting behavior is important for better understanding the nature of local and directional hydrogen bond-type interactions in ILs.

Experimental Section

1-Alkyl-3-methylimidazolium cation-based ILs with different halogen or molecular anions were in use, as shown in Figure 1. Thus, IL samples with methylimidazolium cations with different alkyl-chain lengths and a variety of anion species were investigated in order to systematically obtain information. All sample ILs are in liquid states at 25 °C. The common abbreviations for anions and cations are used in this paper. All the highly pure (>98%) ILs were purchased from Kanto Chemical Co., Tokyo, Japan, or Merck Ltd., Tokyo, Japan. The density of sample ILs was measured (see Table S1, Supporting Information). The detailed information on IL samples and experimental procedures and apparatus were described elsewhere.^21,22,31,32 An FTIR spectrometer (HORIBA, Ltd., FT-720) with an attenuated total reflection (ATR) unit (Smiths detection, DuraScope) was employed to record infrared (IR) spectra. IR spectra were obtained with a resolution of 2 or 4 cm^–1 and a scan number of 10. In density functional theory (DFT) calculations, geometry was optimized at the B3LYP/6-311+G(d,p) level of theory with a charge of +1 (−1) for cations (anions) and a multiplicity of the singlet, and then, vibrational modes and frequencies were calculated at the same level of theory. The level of theory used in this study is one of the commonly used ones.²⁴

Figure 1.

ILs with the 1-alkyl-3-methylimidazolium cation and halogen or molecular anions used in this study.

Results and Discussion

Figure 2 shows the vibrational modes with displacement vectors and vibrational frequencies for the methylimidazolium cation [C₆mim⁺] in calculations. In addition to the skeletal vibrational modes accompanied with in-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions that are the topic of this paper, the skeletal vibrational modes accompanied with out-of-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions and ⁺C(2)–H and ⁺C(4,5)–H stretching vibrational modes are also depicted for reference.

Figure 2.

Vibrational modes with displacement vectors and vibrational frequencies for the methylimidazolium cation [C₆mim⁺] in calculations.

The skeletal vibrational mode with a large in-plane ⁺C(2)–H bending motion and ⁺C(4)–H and ⁺C(5)–H motions is 1174 cm^–1 in calculations. The vibrational mode with a large in-plain ⁺C(2)–H bending motion hardly depends on the alkyl-chain length (C_n), and the vibrational mode is essentially the mode related to the methylimidazolium ring cation [mim⁺] (see Figure S1, Supporting Information). Experimentally, this can be confirmed by obtaining absorption spectra normalized by the molar concentration,^{21,22,27,31,32} which will be discussed later. The skeletal vibrational mode with an in-plane ⁺C(4)–H motion and ⁺C(5)–H and ⁺C(2)–H motions is 1595 cm^–1 in calculations, and the skeletal vibrational mode with an in-plane ⁺C(5)–H motion and ⁺C(4)–H and ⁺C(2)–H motions is 1603 cm^–1 in calculations. The frequencies of these two modes are very close to each other. The oscillator strength at 1595 cm^–1 is a little larger than that at 1603 cm^–1. These two modes are also hardly dependent on the alkyl-chain length (C_n), and the vibrational modes are essentially the modes related to the methylimidazolium ring cation [mim⁺] (see Figure S2, Supporting Information). In Figure 2, the displacement vector in the in-plane ⁺C(2)–H bending motion at 1174 cm^–1 is larger than that in the in-plane ⁺C(4)–H motion and in-plane ⁺C(5)–H motion at 1595 and 1603 cm^–1, respectively. As we previously showed,^21,22 the skeletal vibrational modes with large out-of-plane ⁺C(2)–H and large out-of-plane ⁺C(4,5)–H motions and the stretching modes of ⁺C(2)–H and ⁺C(4,5)–H are also hardly dependent on the alkyl-chain length (C_n). All vibrational modes of the imidazolium cation depicted in Figure 2 have relatively large oscillator strengths, which are preferable for discussing the influence on hydrogen bond-type interactions with various anions.

Figure 3 shows absorption spectra normalized by the molar concentration for all IL samples in the 1050–1200 cm^–1 region. The normalization was performed using the density of the IL samples (see Table S1, Supporting Information). The absorption spectra before normalization can be referred (see Figure S3, Supporting Information). Although there is some uncertainty about the validity of molar concentration normalization for ATR-IR absorption spectra due to the penetration depth differences arising in relation to differences in refractive indices, we previously showed the practical usefulness of molar concentration normalization for ATR-IR absorption spectra in the 2800–3300 cm^–1 and 750–950 cm^–1region for various alkyl-methylimidazolium ILs.^21,22 Cha et al. reported that the penetration depth change arising in relation to differences in refractive indices under the ATR geometry is not serious, and IR measurements in transmission geometry bring in very similar results for some alkyl-methylimidazolium ILs.¹⁹ Molar concentration normalization is equivalent to normalization by the number of ion pairs, the number of cations, and the number of anions. As an important example of its usefulness, the molar concentration-normalized absorption spectra enable us to distinguish the vibrational modes whose absorption intensities and frequencies are decided by the number of ion pairs from the vibrational modes whose absorption intensities and frequencies are not decided by the number of ion pairs, such as vibrational modes depending on the alkyl-chain length and their conformation of imidazolium cations.

Figure 3.

Absorption spectra normalized by the molar concentration in the 1050–1200 cm^–1 region: (a) [C_nmim⁺][Cl^–] with n = 6 and 8; (b) [C_nmim⁺][Br^–] with n = 6, 8, and 10; (c) [C_nmim⁺][I^–] with n = 3, 4, and 6; (d) [C_nmim⁺][SCN^–] with n = 2 and 4; (e) [C_nmim⁺][N(CN)₂^–] with n = 2 and 4; (f) [C_nmim⁺][TfO^–] with n = 2, 4, 6, and 8; (g) [C_nmim⁺][Tf₂N^–] with n = 2, 4, 6, 8, and 10; (h) [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10; (i) [C_nmim⁺][PF₆^–] with n = 4, 6, and 8; and (j) [C₆mim⁺][PF₃(C₂F₅)₃^–]. The dashed area in (f,g,j) corresponds to the 1165–1170 cm^–1 region, in which a peak in (a–e, h,i) is observed.

We found an absorption band with a peak between 1165 and 1170 cm^–1 in [C_nmim⁺][Cl^–], [C_nmim⁺][Br^–], [C_nmim⁺][I^–], [C_nmim⁺][SCN^–], [C_nmim⁺][N(CN)₂^–], and [C_nmim⁺][PF₆^–]. In addition, the intensities normalized by the molar concentration are almost identical in each alkyl-methylimidazolium IL, which indicates that the skeletal vibrational mode with a large in-plane ⁺C(2)–H motion is hardly dependent on the alkyl-chain length (C_n) and the mode is essentially related to the methylimidazolium ring cation [mim⁺]. In [C_nmim⁺][TfO^–], [C_nmim⁺][Tf₂N^–], and [C₆mim⁺][PF₃(C₂F₅)₃^–], a shoulder-like structure between 1165 and 1170 cm^–1 in the absorption band affected by the vibrational modes of the anion was found, while in [C_nmim⁺][BF₄^–], a peak between 1165 and 1170 cm^–1 on the tail of the absorption band of [BF₄^–] was found. Note that the scale of vertical axes in [C_nmim⁺][TfO^–], [C_nmim⁺][BF₄^–], [C_nmim⁺][Tf₂N^–], and [C₆mim⁺][PF₃(C2F₅)₃^–] is different from that in [C_nmim⁺][Cl^–], [C_nmim⁺][Br^–], [C_nmim⁺][I^–], [C_nmim⁺][SCN^–], [C_nmim⁺][N(CN)₂^–], and [C_nmim⁺][PF₆^–]. Interestingly, this shows that the absorption band due to the skeletal vibrational mode with a large in-plane ⁺C(2)–H bending motion hardly depends on the anion species. Since spectral shifts generally reflect the changes in the force constant of the oscillator and/or the anharmonicity of the vibrational potential due to the hydrogen bond,⁴⁹ the data in Figure 3 indicate that the anharmonicity of this mode and its influence on the hydrogen bond-type interaction with anions are very small regardless of the large in-plane ⁺C(2)–H bending motion. The tendency for anion species is essentially different from tendencies in the absorption band in the 2800–3300 cm^–1 region due to the stretching ⁺C(2)–H vibrational mode and the absorption band in the 750–950 cm^–1 region due to the skeletal vibrational mode with a large out-of-plane ⁺C(2)–H bending motion. It is important to focus on differences by the direction of the ⁺C(2)–H motion. In the case of the stretching ⁺C(2)–H vibrational mode in Figure 2, the oscillator strength of the corresponding absorption band tends to increase with the strengthening of hydrogen bond-type interactions or the anion’s basicity and the absorption frequency is red-shifted as hydrogen bond-type interactions are strengthened or the anion’s basicity is increased.²¹ The red shift reflects the reduced force constant of the oscillator and/or the enhanced anharmonicity in [⁺C(2)–H]···A^– interactions. In the case of the skeletal vibrational mode with a large out-of-plane ⁺C(2)–H bending motion in Figure 2, the corresponding absorption band was sensitive to interactions with anions, and tendencies for both blue and red shifts were observed with increases of the anions’ basicity or strengthening of hydrogen bond-type interactions.²² For the skeletal vibrational mode with a large out-of-plane ⁺C(2)–H bending motion, the significance of [⁺C(2)–H]_{out-of-plane-bending}···A^– interactions and their anharmonic character has been identified in both experiments and anharmonic calculations.²⁶ On the other hand, the skeletal vibrational mode with a large in-plane ⁺C(2)–H bending motion in Figure 2 is remarkably insensitive to the strengthening of hydrogen bond-type interactions or anion’s basicity, as shown in Figure 3. It was also reported that the wavenumbers of in-plane ⁺C(2)–H bending in [C₆mim⁺][Cl^–] and [C₆mim⁺][PF₆^–] with ab initio MP2 calculations were nearly identical, while the wavenumbers of ⁺C(2)–H stretching with ab initio MP2 calculations were largely different from each other.²³ Here, [Cl^–] is one of the strongest proton acceptors and [PF₆^–] is one of the most weakly coordinating anions.^15,46

Figure 4 shows absorption spectra normalized by the molar concentration for all IL samples in the 1500–1650 cm^–1 region. The absorption spectra before the normalization can be referred (see Figure S4, Supporting Information).

Figure 4.

Absorption spectra normalized by the molar concentration in the 1500–1650 cm^–1 region: (a) [C_nmim⁺][Cl^–] with n = 6 and 8; (b) [C_nmim⁺][Br^–] with n = 6, 8, and 10; (c) [C_nmim⁺][I^–] with n = 3, 4, and 6; (d) [C_nmim⁺][SCN^–] with n = 2 and 4; (e) [C_nmim⁺][N(CN)₂^–] with n = 2 and 4; (f) [C_nmim⁺][TfO^–] with n = 2, 4, 6, and 8; (g) [C_nmim⁺][Tf₂N^–] with n = 2, 4, 6, 8, and 10; (h) [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10; (i) [C_nmim⁺][PF₆^–] with n = 4, 6, and 8; and (j) [C₆mim⁺][PF₃(C₂F₅)₃^–].

We found an absorption band between 1550 and 1590 cm^–1 for all alkyl-methylimidazolium ILs. The absorption band has been differently assigned by different authors.²⁰ Looking at the data in Figure 4 carefully, we discern that the absorption band originates from two adjacent bands with different vibrational modes. The two vibrational modes draw attention in the context of Fermi resonance by their overtones and the combination of the two modes.^{13−15,26,38} In calculations, as shown in Figure 2, the skeletal vibrational modes with in-plane ⁺C(4)–H, ⁺C(5)–H, and ⁺C(2)–H motions are in 1595 and 1603 cm^–1. The frequencies of these two modes are very close to each other. The oscillator strength at 1595 cm^–1 is a little larger than that at 1603 cm^–1. These two modes are also hardly dependent on the alkyl-chain length (C_n), and the vibrational modes are essentially the modes related to the methylimidazolium ring cation [mim⁺] (see Figure S2, Supporting Information). As experimentally shown in Figure 4, the intensities normalized by the molar concentration are almost identical for each alkyl-methylimidazolium IL, which indicates that the two skeletal vibrational modes with an in-plane ⁺C(4)–H motion and in plane ⁺C(5)–H motion are hardly dependent on the alkyl-chain length (C_n) and the two modes are essentially related to the methylimidazolium ring cation [mim⁺]. As far as normalized absorbance with different anions is concerned, the intensities of alkyl-methylimidazolium ILs with the weakly coordinating anions such as [Tf₂N^–], [BF₄^–], [PF₆^–], and [PF₃(C₂F₅)₃^–] are a little weaker, compared with those with a strong proton acceptor such as [Cl^–], [Br^–], [I^–], [SCN^–], and [N(CN)₂^–]. Thus, we found that two skeletal vibrational modes with the in-plane ⁺C(4)–H motion and in-plane ⁺C(5)–H motion are remarkably insensitive to the strengthening of hydrogen bond-type interactions or the increase of anion basicity. In the anharmonic calculations for [C₂mim⁺][Tf₂N^–], it is pointed out that the two vibrational modes have pronounced harmonic characters,³⁸ which support our systematic experimental data.

Thus, apart from the skeletal vibrational modes accompanied with out-of-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions and the ⁺C(2)–H and ⁺C(4,5)–H stretching vibrational modes, the anharmonicity of skeletal vibrational modes accompanied with in-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions is inherently small as pointed out by the harmonic and anharmonic frequency calculations. Therefore, the spectral shifts for the anharmonicity of the vibrational potential due to the hydrogen bond-type interactions are small.

Figure 5 shows absorption spectra normalized by the molar concentration in the 900–1150 cm^–1 region for [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10. The absorption spectra before the normalization can be referred (see Figure S5, Supporting Information).

Figure 5.

Absorption spectra normalized by the molar concentration in the 900–1150 cm^–1 region for [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10.

In Figure 5, the absorption bands in the 900–1150 cm^–1 region can be attributed to the vibrational modes of [BF₄^–] because alkyl-methylimidazolium cations do not have strong absorption bands in the 900–1150 cm^–1 region. Since normalization by the molar concentration is equivalent to the normalization by the number of ion pairs, the number of cations, and the number of anions, the absorption bands in Figure 5 are normalized by the number of [BF₄^–]. Thus, the absorption bands of [BF₄^–] in [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10 are nearly identical with the same molar-normalized intensities but slightly blue-shifted in an increasing order by n of [C_nmim⁺][BF₄^–]. In [C₄mim⁺][BF₄^–], [C₆mim⁺][BF₄^–], [C₈mim⁺][BF₄^–], and [C₁₀mim⁺][BF₄^–], we clearly identified three peaks in the broad absorption band, while in [C₂mim⁺][BF₄^–], the center of the three peaks was unclear. [BF₄^–] has a T_d symmetry, and in DFT calculations [B3LYP/6-311+G(d,p)], a threefold degenerate vibrational mode [BF₄^–] with an extremely strong oscillator strength at 1033 cm^–1 was found in this region (see Figure S6, Supporting Information). The data in Figure 5 indicate that a broad absorption band with three separate vibrational states is observed as a result of symmetry breaking due to the local interaction between the cation and anion. The studies on cluster formation, local assembly motifs, and hydrogen bond networks under the hydrogen bond-type interactions in [C₂mim⁺][BF₄^–] have been conducted.^17,18,35 Bulk FTIR spectra for [C₂mim⁺][BF₄^–] or [C₄mim⁺][BF₄^–] have been measured in literature studies.^{17,18,25,35,50} However, the clear peak splitting shown in Figure 5 has not been clearly observed or discussed in most literature studies,^17,18,35,50 although there was a literature in which split peaks for the vibrational mode of [C₄mim⁺][BF₄^–] are mentioned.²⁵ One of the causes might be absorption saturation by the intense broad band in bulk FTIR spectra. Taking advantage of attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and molar concentration normalization, we successfully and systematically observed broad intense bands with clear peak splitting in [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10. The peak wavenumbers of the three separate peaks in the 900–1050 cm^–1 region are listed (see Figure S7, Supporting Information). Although there are slight differences in the peak wavenumbers of three separate peaks observed when analyzing [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10, the average of the energy splitting between the lowest wavenumber peak and the middle wavenumber peak is about 16 cm^–1 (0.046 kcal/mol) and the average of the energy splitting between the middle wavenumber peak and the highest wavenumber peak is about 14 cm^–1 (0.04 kcal/mol) and the average of total energy splitting is about 30 cm^–1 (0.086 kcal/mol). The interactions between the imidazolium cation and anion have been studied, and the interaction energies of the ion pair of various imidazolium ILs at ab initio MP2 calculations were −71 to −89 kcal/mol and the anion dependence of the interaction energy was moderate.^41,44 The main contribution in the interaction energy is the electrostatic interaction energy. The electrostatic interaction (E_es) can be described as E_es = E_{charge–charge} + E_{charge–dipole} + E_{dipole–dipole} + ... by multipole expansion.⁴⁴ The isotropic charge–charge interaction is the leading term in the electrostatic interaction between ions, while the local and directional dipole–dipole interactions are the leading term in the electrostatic interaction in hydrogen bonding in neutral molecules. The charge–charge interaction is the leading term in the electrostatic interaction in imidazolium ILs, and the hydrogen bond-type interaction in imidazolium ILs is the dipole–dipole interaction under the existence of the charge–charge interaction. In that respect, their difference from conventional hydrogen bonds is pointed out.⁴⁴ For the split peaks experimentally observed in actual [C_nmim⁺][BF₄^–] systems, we mainly consider the influence of hydrogen bond-type interactions as the local interaction between the imidazolium cation [C_nmim⁺] and the anion [BF₄^–] because the charge–charge interaction, that is, the long-range Coulombic interaction has an isotropic character in actual [C_nmim⁺][BF₄^–] systems. In the ab initio MP2 calculations, the portion of local and direction dipole–dipole interaction in imidazolium ILs was not evaluated.^41,44 On the other hand, the hydrogen bond interaction energy (E_HB) has been evaluated by COSMO-RS calculations, in which E_HB were −7.342, −6.119, −4.773, −4.065, −5.402, −4.089, −2.357, −2.340, −0.688, and −0.177 kcal/mol for [C₄mim⁺][Cl^–], [C₄mim⁺][Br^–], [C₄mim⁺][I^–], [C₄mim⁺][SCN^–], [C₄mim⁺][N(CN)2^–], [C₄mim⁺][TfO^–], [C₄mim⁺][Tf₂N^–], [C₄mim⁺][BF₄^–], [C₄mim⁺][PF₆^–], and [C₄mim⁺][PF₃(C₂F₅)₃^–], respectively.⁴⁶ Thus, E_HB shows large anion dependence, and the E_HB of [C₄mim⁺][BF₄^–] is −2.340 kcal/mol, whose magnitude is smaller than that of water. The peak split of 30 cm^–1 (0.086 kcal/mol) with a broad band may be caused by the fluctuation of hydrogen bond-type interaction for the vibrational transition in the 900–1150 cm^–1 region. It is pointed out that the hydrogen bonding in ILs is not static but fluctuates and has dynamic characteristics.⁴⁷ It is interesting to note that peak splits by local symmetry breaking due to local hydrogen bond-type interactions were observed in [C_nmim⁺][BF₄^–] systems, although [BF₄^–] is a weakly coordinating molecular anion. In cryogenic ion spectroscopy, the shape and complicated spectroscopic structures in the 950–1150 region for the clusters of [C₂mim⁺]_m[BF₄^–]_n, where m and n are natural numbers and m + n are odd numbers, have been observed and the spectroscopic structure largely depends on the cluster size denoted by (m, n).¹⁷ Studies on the correlation between cryogenic ion vibrational predissociation data and our data would be interesting.

In ion and solvent systems such as KNO₃ dissolved in water, the peak broadening or splitting by about 60 cm^–1 of the asymmetric N–O stretching vibrational mode of [NO₃^–] (D_3h symmetry) due to symmetry breaking by the local solvent environment was reported, which was observed by resonant Raman scattering experiments.^51,52 The mode of [NO₃^–] is also IR active and has been used as a sensitive probe to elucidate the nature of the ion—water molecule interaction in both experiments and computer simulations.^53,54 Two-dimensional infrared spectroscopy measurements for the mode of [NO₃^–] in water solution have been performed to reveal hydration dynamics of [NO₃^–] in water in combination with molecular dynamics simulations.⁵⁵

The clear peak splitting of the mode of [BF₄^–] observed in [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10 could also be a useful probe, which would promote various experiments including into dynamic aspects and computer simulations, providing insights into the nature of noncovalent interactions between cations and anions in imidazolium ILs.

Conclusions

The IR spectroscopic studies in the middle frequency range for a wide variety of alkyl-methylimidazolium-based ILs were performed. We systematically studied the skeletal vibrational modes in the 1050–1200 cm^–1 and 1500–1650 cm^–1 regions that are accompanied with large in-plane + C(2)–H bending motions and +C(4,5)–H bending motions as well as their spectral change for the strengthening of the basicity of anions or the strengthening of hydrogen bond-type interactions. In contrast to ⁺C–H stretching vibrational modes in the 3000–3200 cm^–1 region and the skeletal vibrational modes with large out-of-plane ⁺C–H motions in the 700–950 cm^–1 region, the skeletal vibrational modes with large in-plane + C(2)–H bending motions and with the in-plane ⁺C(4,5)–H bending motions are insensitive to increases of the basicity of anions or the strengthening of hydrogen bond-type interactions, although the modes have strong oscillator strengths. This insensitivity may originate from the anharmonicity of skeletal vibrational modes with in-plane ⁺C(2)–H and ⁺C(4,5)–H bending motions that are inherently small, which has been identified in the harmonic and anharmonic frequency calculations; accordingly, the spectral shifts for the anharmonicity of the vibrational potential due to the hydrogen bond-type interactions are very small.

As an another important finding in alkyl-methylimidazolium tetrafluoroborate [BF₄^–] ILs, a broad absorption band with three splitting peaks in the 900–1150 cm^–1 range was observed in [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10. The absorption due to a (threefold) degenerate vibrational mode of [BF₄^–] was observed as a broad absorption band with three splitting peaks in the 900–1150 cm^–1 range that can be recognized as a result of the local symmetry breaking of a threefold degenerate vibrational mode of [BF₄^–] due to the hydrogen bond-type interaction in [C_nmim⁺][BF₄^–] systems. Taking advantage of ATR-FTIR and molar concentration normalization, the clear peak splitting in broad intense bands was successfully and systematically observed. By the experimental observation of the peak splitting of the mode of [BF₄^–], it is expected that this will promote various experiments and computer simulations including dynamic and assembled structure modeling for the better understanding of the nature of noncovalent interactions between cations and anions in imidazolium ILs.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c05769.

• Density values of IL samples used in this study; calculated vibrational spectra for alkyl-methylimidazolium cations, C₂mim⁺, C₃mim⁺, C₄mim⁺, C₆mim⁺, C₈mim⁺, and C₁₀mim⁺, in the frequency range of 1050–1200 cm^–1; calculated vibrational spectra for alkyl-methylimidazolium cations, C₂mim⁺, C₃mim⁺, C₄mim⁺, C₆mim⁺, C₈mim⁺, and C₁₀mim⁺, in the frequency range of 1500–1650 cm^–1; original ATR-FTIR spectra in the 1050–1200 cm^–1 range without molar concentration normalization; original ATR-FTIR spectra in the 1500–1650 cm^–1 range without molar concentration normalization; original ATR-FTIR spectra in the 900–1150 cm^–1 region for [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10; calculated vibrational spectrum for [BF₄^–] in the 900–1150 cm^–1 region; and peak wavenumbers of three separate peaks in the 900–1050 cm^–1 region for [C_nmim⁺][BF₄^–] with n = 2, 4, 6, 8, and 10 (PDF)

The authors declare no competing financial interest.