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Infrared Spectroscopy of Ionic Liquids Consisting of Imidazolium Cations with Different Alkyl Chain Lengths and Various Halogen or Molecular Anions with and Without a Small Amount of Water Toshiki Yamada, Yukihiro Tominari, Shukichi Tanaka, and Maya Mizuno J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b01429 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 30, 2017

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The Journal of Physical Chemistry

Infrared Spectroscopy of Ionic Liquids Consisting of Imidazolium Cations with Different Alkyl Chain Lengths and Various Halogen or Molecular Anions with and without a Small Amount of Water 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

ABSTRACT: Infrared spectroscopy was performed on ionic liquids (ILs) that had imidazolium cations with different alkyl chain lengths and various halogen or molecular anions with and without a small amount of water. The molar concentration normalized absorbance due to +C−H vibrational modes in the range of 3000 to 3200 cm-1 was nearly identical for ILs that had

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imidazolium cations with different alkyl chain lengths and the same anions. A close correlation was found between the red-shifted +C−H vibrational modes, the chemical shift of +C(2)−H proton, and the energy stabilization of hydrogen-bonding interaction. The vibrational modes of the water molecules interacting with anions in the range between 3300 and 3800 cm-1 was examined. The correlation between the vibrational frequencies of water, the frequencies of +C−H vibrational modes, and the center frequency of intermolecular vibrational modes due to ion pairs was discussed.

INTRODUCTION Room temperature ionic liquids (ILs) are a class of novel materials comprised of cations and anions that exist in a liquid state below 373 K. Their unique characteristics include low melting points, a wide liquid temperature range, negligible vapor pressure, chemical stability, wide electrochemical windows, high electrical conductivity as well as excellence as a solvent for various materials.1-6 A wide range of applications have been explored using these unique features.7,8 Considerable efforts have been paid to understand the noncovalent interactions in ILs such as Coulombic interaction, hydrogen bonding, and dispersion forces as well as the bulk structures of ILs, which determine the physical properties of ILs. Numerous experimental and theoretical techniques have been utilized to elucidate the noncovalent interactions and the bulk structures in ILs.9-15 Among these studies, the intra- and/or intermolecular vibrational modes on prototype imidazolium-based ILs have been investigated by terahertz time-domain spectroscopy (THz-TDS),16,17 far infrared (FIR) spectroscopy,18-20 infrared (IR) spectroscopy,21-28 Raman spectroscopy,29,30 and optical heterodyne-detected Raman-induced Kerr effect spectroscopy.31,32


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The low-frequency intermolecular vibrational modes for the imidazolium-based ILs with some molecular anions [Amolecule−] have been investigated by using FIR spectroscopy and THz-TDS.1620

In determining the central frequency of intermolecular vibrational modes, the significant

contribution of the force constant k from the viewpoint of the simple harmonic oscillator, ω=(k/µ)1/2 was pointed out, and the significance of the local hydrogen bond in the +

C−H…Amolecule− interactions was recognized.18-20 In addition, a correlation between the

intramolecular absorption bands due to +C−H stretching vibrations in the IR frequency region and the low frequency intermolecular absorption band was discussed. Previously, we systematically studied the inter- and intra-molecular vibrational modes of ILs that have imidazolium cations with different alkyl-chain lengths and various halogen [Ahalogen−] or molecular [Amolecule−] anions by using THz-TDS and FIR spectroscopy.34,35 In regards to the intermolecular vibrational modes, we found that the molar concentration normalized absorption coefficient for ILs that have imidazolium cations with different alkyl chain lengths and the same anions was nearly identical, whose validity was surprisingly confirmed for a wide range of imidazolium-based ILs. The significant contribution of the reduced mass µ calculated from the masses of the methyl-imidazolium ring cation [mim+] and the anion [A−] as well as the force constant k was also noted for a wide range of imidazolium-based ILs.34,35 We also found that the correlation between the intramolecular vibrational frequencies due to +C−H stretching modes in the IR frequency region, the center frequency of intermolecular vibrational modes, and the total interaction energy of ion pairs does not necessarily hold for a wide range of ILs. The structures of some imidazolium-based ILs and their water mixtures were investigated using IR, Raman and NMR spectroscopy.18,21,28,35-38 Particularly,

+

C−H intramolecular

vibrational modes under the interactions of +C−H…Amolecule− or +C−H…Ahalogen−, which are in


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the frequency range between 3000 and 3200 cm-1, were extensively studied. For very diluted aqueous mixtures, the IR spectra of the imidazolium-based ILs were very similar, compared with those without water. Mixing of halide imidazolium-based ILs with heavy water led to marked changes of the IR spectra in the frequency range, while mixing of imidazolium ILs with BF4− and heavy water did not induce apparent spectral changes.21,28 The difference was attributed to the difference of the strength of hydrogen-bond type interaction as well as the difference of the position of anion. The association of water in some imidazolium-based ILs was also investigated by measuring the vibrational modes of H2O in the frequency range between 3300 and 3700 cm-1, or the vibrational modes of D2O in the frequency range between 2300 and 2800 cm-1.35,36 It was found that the association of water molecules with the anions is an indicator for the miscibility of water with imidazolium-based ILs, and the vibrational modes for single water molecules associated in imidazolium-based ILs (by adding a small amount of water) can be used as a reliable measure of the polarity of imidazolium-based ILs.36 Furthermore, the relationship between the vibrational frequencies of water molecules and the low-frequency intermolecular vibrational frequency for some imidazolium-based ILs was also discussed.18 In this paper, we systematically studied +C−H intramolecular vibrational frequencies of ILs that have imidazolium cations with different alkyl chain lengths and various halogen or molecular anions. We found that the molar concentration normalized absorbance in the frequency range between 3000 and 3200 cm-1 for ILs that have imidazolium cations with different alkyl chain lengths and the same anions were nearly identical in terms of the absorption frequency and absorption intensity, which was surprisingly established for a wide range of imidazolium ILs. The vibrational modes of the water molecules interacting with anions in the frequency range between 3300 and 3800 cm-1 were systematically investigated by adding a small


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amount of water. We discuss the correlation between the vibrational frequencies of water interacting with anions [A−], the frequencies of +C−H stretching vibrations under the interactions of +C−H…A−, and the center frequencies of intermolecular vibrational modes due to ion pairs. We also discuss the correlation between the red-shifted

+

C−H intramolecular vibrational

frequencies, the chemical shift (δ ppm) obtained from NMR spectra, and the energy stabilization of hydrogen-bonding interaction evaluated by COSMO-RS (COnductor-like Screening MOdel for Realistic Solvents) calculations.38

EXPERIMENTAL SECTION We examined a relatively large number of room-temperature ILs consisting of alkylmethylimidazolium cations and halogen or molecular anions, as shown in Figure 1. The abbreviations for some of the molecular anions are also shown in Figure 1.

Figure 1. Room-temperature ILs consisting of alkyl-methylimidazolium cations and halogen or molecular anions used in this study.


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Hereinafter, abbreviations of [Cnmim+] are utilized for alkyl-methylimidazolium cations, where Cn and mim+ represent the carbon number of the alkyl group and methylimidazolium cation part, respectively. All highly pure (>98%) IL samples were used in our study. Particularly, IL samples with iodine anion (I−) with a high purity (>99%) were colorless and very transparent. IL samples having a miscibility with water were dried in a vacuum for 24 hours prior to use in the experiments. For preparation of IL samples including a small amount of water, 10 µl of H2O was added to pure IL samples of 500 µl and stirred for 6 hours prior to use in the experiments. The typical water concentration is about 1.5 wt%, which was at the same level of water concentration used to investigate the association of water molecules to the anions.36 In these experiments, we performed THz-TDS, FIR and IR spectroscopy measurements for IL samples, for which the experimental procedures and apparatus were described elsewhere.33,34 The THzTDS spectrometer (Tochigi Nikon, Rayfact RS-01020), FIR spectrometer (JASCO Corporation, VIR-F), and ATR-FTIR spectrometer (HORIBA, Ltd., FT-720 and Smiths detection, DuraScopeTM) were used to measure complex dielectric constant spectra with a frequency range of 13 cm-1 to 130 cm-1, FIR absorption spectra with a frequency range of 30 to 670 cm-1, and IR absorption spectra with a frequency range of 550 cm-1 to 3800 cm-1, respectively. Density functional theory (DFT) calculation was performed for the molecular anions of [SCN−] and [N(CN)2−], in which calculation, the geometry was optimized at the B3LYP/6-311G(d,p) level of theory and the infrared vibrational spectra were calculated at the same level of theory. DFT calculations for the other molecular anions and alkyl-methylimidazolium cations were presented elsewhere. 33,34


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RESULTS AND DISCUSSION First we exhibit the complex dielectric spectra (Re ε and Im ε) and absorption coefficient (α) for [Cnmim+][SCN−] ILs with n = 2, 4 in Figure 2a−d, and [Cnmim+][N(CN)2−] ILs with n = 2, 4 in Figure 3a−d, respectively. The molar concentration normalized absorption coefficients (α/M) are also shown in the insets of Figure 2c,d and Figure 3c,d, respectively. Although absorption spectra for [C2mim+][SCN−] and [C2mim+][N(CN)2−] have been studied by FIR,18,20 Re ε, Im ε spectra and molar concentration normalized absorption coefficients with the absolute value for [Cnmim+][SCN−] and [Cnmim+][N(CN)2−] ILs with different alkyl chain lengths have not been studied. The other imidazolium-based ILs expect for [Cnmim+][SCN−] and [Cnmim+][N(CN)2−] in Figure 1 were previously investigated in detail by THz-TDS and FIR spectroscopy, and various interesting characteristics were found.33,34 Therefore, it is worthwhile to present the data of [Cnmim+][SCN−] and [Cnmim+][N(CN)2−] that will be mentioned later as well as to confirm the consistency with the statements in our previous papers.33,34 Furthermore, the molar concentration normalized absorption coefficients at the central absorption frequency of the intermolecular vibrational band are discussed later.


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Figure 2. (a) Re ε spectra, (b) Im ε spectra, (c) Absorption coefficients (α) obtained by THzTDS, and (d) Absorption coefficients (α) obtained by FIR, for the [Cnmim+][SCN−] imidazoliumbased ILs with n=2 and 4. The molar concentration normalized absorption coefficient (α/M) is also shown in the insets.


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Figure 3. (a) Re ε spectra, (b) Im ε spectra, (c) Absorption coefficients (α) obtained by THzTDS, and (d) Absorption coefficients (α) obtained by FIR, for the [Cnmim+][N(CN)2−] imidazolium-based ILs with n=2 and 4. The molar concentration normalized absorption coefficient (α/M) is also shown in the insets.

In the low THz frequency range (13 to 130 cm-1), α/M spectra (insets in Figure 2c,d for [Cnmim+][SCN−] ILs and insets in Figure 3c,d for [Cnmim+][N(CN)2−] ILs) are nearly the same for two [Cnmim+][SCN−] ILs and two [Cnmim+][N(CN)2−] ILs, respectively. The relatively large absorption bands with peaks at 117 cm-1 for [Cnmim+][SCN−] ILs and 114 cm-1 for [Cnmim+][N(CN)2−] ILs can be attributable to the intermolecular absorption bands, whose


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intensity, center frequency, and bandwidth are determined by the number of pairs due to the electrostatic interaction between the methylimidazolium ring cation and molecular anion, because there are no strong intramolecular absorption band of [Cnmim+] cations, [SCN−] anion, and [N(CN)2−] anion [see Figures S1, S2, and S3 in the supporting information (SI)]. Ludwig and co-workers have found that the center frequencies of intermolecular absorption bands are 117.6 cm-1 for [C2mim+][SCN−] and 113.5 cm-1 for [C2mim+][N(CN)2−] using FIR spectroscopy.20 We previously found that α/M for ILs that have [Cnmim+] with different alkyl-chain lengths and the same anion (Ahalogen− or Amolecule−) were nearly identical, which was surprisingly true for a wide range of imidazolium ILs.33,34 In the mid THz frequency range (130 to 500 cm-1) in α (α/M), there are some weak but discernible intramolecular absorption bands originating from the imidazolium cations. Particularly, the most discernible absorption band at about 240 to 250 cm-1 is related to the intramolecular vibrational mode of [C2mim+] (see Figure S1, SI).20 In the case of [Cnmim+][SCN−] ILs, there is an intramolecular absorption band at about 460 cm-1 due to an intramolecular out-of-plane bending mode of [SCN−] (see Figure S2, SI), although the peak is barely identifiable due to the low signal-to-noise ratio in this region. In the case of [Cnmim+][N(CN)2−] ILs, the absorption band at about 170 to 180 cm-1 due to the intramolecular bending mode of [N(CN)2−] was observed (see Figure S3, SI).20 In the high THz frequency range (500 to 670 cm-1), there are two relatively strong absorption bands between 600 and 670 cm-1 due to the intramolecular vibrational modes of the skeletal vibration of the imidazolium ring (see Figure S1, SI). In the case of [Cnmim+][N(CN)2−], there is a strong absorption band between 500 and 600 cm-1 which originates from intramolecular vibrational mode of [N(CN)2−] (see Figure S3, SI). In Figures 2a and 3a, the magnitude of Re ε in the low-frequency region follows the trend of [C2mim+][SCN−] > [C4mim+][SCN−], and [C2mim+][N(CN)2−] > [C4mim+][N(CN)2−],


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respectively, which means that the magnitude of electric susceptibility in this region follows the same trend. In Figures 2b and 3b, the behavior of Im ε in the low-frequency region indicates the existence of some sort of dielectric relaxation or librational modes of the cations for these ILs. These features for Re ε and Im ε are also true for a wide range of imidazolium ILs.33,34 After the THz and FIR spectroscopic data of [Cnmim+][SCN−] and [Cnmim+][N(CN)2−] ILs, the main subject of this paper will be discussed. Figure S4a-j in SI shows ATR-IR spectra in the 2800 to 3300 cm-1 range for [Cnmim+][Cl−] ILs with n = 6, 8, [Cnmim+][Br−] ILs with n = 6, 8, 10, [Cnmim+][I−] ILs with n = 3, 4, 6, [Cnmim+][SCN−] ILs with n = 2, 4, [Cnmim+][N(CN)2−] ILs with n = 2, 4, [Cnmim+][TfO−] ILs with n = 2, 4, 6, 8, [Cnmim+][Tf2N−] ILs with n = 2, 4, 6, 8, 10, [Cnmim+][BF4−] ILs with n = 2, 4, 6, 8, 10, [Cnmim+][PF6−] ILs with n = 4, 6, 8, and the [C6mim+][PF3(C2F5)3−] IL, respectively (see Figure S4a-j, SI). The corresponding molar concentration normalized absorbance spectra are shown in Figure 4a-j.


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Figure 4. Molar concentration normalized absorbance between 2800 and 3300 cm-1 for (a) [Cnmim+][Cl−] ILs with n = 6, 8, (b) [Cnmim+][Br−] ILs with n = 6, 8, 10, (c) [Cnmim+][I−] ILs with n = 3, 4, 6, (d) [Cnmim+][SCN−] ILs with n = 2, 4, (e) [Cnmim+][N(CN)2−] ILs with n = 2, 4, (f) [Cnmim+][TfO−] ILs with n = 2, 4, 6, 8, (g) [Cnmim+][Tf2N−] ILs with n = 2, 4, 6, 8, 10, (h) [Cnmim+][BF4−] ILs with n = 2, 4, 6, 8, 10, (i) [Cnmim+][PF6−] ILs with n = 4, 6, 8, and (j) [C6mim+][PF3(C2F5)3−] IL.

Kim et al. found that the penetration depth change arising from differences in refractive indices under the ATR optical geometry is not serious and IR spectra in transmission mode yield very similar results for some imidazolium-based ILs with molecular or halogen anions.28 Therefore, we could compare ATR-IR spectra (corresponding molar concentration normalized absorbance spectra) among different imidazolium-based ILs. Several peaks in the frequency range between 2800 and 3000 cm−1 are mainly assigned to the symmetric and asymmetric stretching vibrations of methylene and methyl groups in the alkyl chain part. In this region, the absorption frequencies and absorption intensities depends on alkyl chain length of the imidazolium cations, as found in Figures 4a-j and S4a-j. The absorption bands in the frequency range between 3000 cm-1 and 3200 cm-1 are mainly assigned to +C(2)−H stretching mode, +

C(4/5)−H asymmetric stretching mode, and +C(4/5)−H symmetric stretching mode, under some

portion of the contribution of Fermi resonances and overtones.23,25 Although absorption bands due to these modes tend to overlap and their assignment is different in the literatures,21,23-25,28 as a general consensus, +C(2)−H stretching mode exists in lower frequency, and +C(4/5)−H asymmetric and symmetric modes exist in higher frequencies. We found that, in the frequency range between 3000 cm-1 and 3200 cm-1 of Figure 4a-j, the molar concentration normalized


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absorption bands for ILs that have imidazolium cations with different alkyl chain lengths and the same anions were nearly identical in terms of the absorption intensities and absorption frequencies, which was surprisingly established for a wide range of imidazolium ILs. The data indicate that +C(2)−H stretching mode and +C(4/5)−H asymmetric and symmetric stretching modes are essentially the intramolecular vibrations within the imidazolium ring cation, and the intensities and frequencies are determined by the number of ion pairs and the interactions with the counter anions. The molar concentration normalized absorption spectra of Figure 4a-j are useful to distinguish the intramolecular vibrations whose intensity and frequencies are determined by the number of ion pairs from the intramolecular vibrations whose intensity and frequencies are not determined by the number of ion pairs, i.e., intramolecular vibrations depending on alkyl chain length of imidazolium cations. For ILs with halide anions (Ahalogen−), there are two usages for the assignments of +C(2)−H stretching mode and +C(4/5)−H asymmetric and symmetric stretching modes. For one usage,21,28 +

C(2)−H stretching mode was assigned to the shoulder-like structure (3015 to 3040 cm-1), and

the adjacent large peak was assigned to the +C(4/5)−H asymmetric stretching mode, and these strongly overlapped. Another absorption band at a higher frequency was assigned to +C(4/5)−H symmetric stretching mode. The frequencies of +C(2)−H stretching mode and +C(4/5)−H asymmetric stretching mode are strongly red-shifted, and the red-shifted +C−H vibrational modes under +C−H…Ahalogen− interactions is an indicator of the weakening of these +C−H bonds, which indicates the strengthening of hydrogen bond type interactions with the proton acceptors in ILs. Although the frequencies of +C(2)−H stretching mode are strongly red-shifted, the difference is relatively minor between Cl−, Br−, and I− but the red-shifted order is [Cnmim+][Cl−], [Cnmim+][Br−], [Cnmim+][I−].21,28 The frequencies of +C(4/5)−H asymmetric stretching mode are


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also strongly red-shifted, and the red-shifted order is clearly [Cnmim+][Cl−], [Cnmim+][Br−], [Cnmim+][I−].21,28 For another usage,29 the broad absorption band at the lower frequency, including the shoulder structure and adjacent large peak was assigned to the +C(2)−H stretching mode, and another absorption band at the higher frequency was assigned to +C(4/5)−H asymmetric and symmetric stretching modes. For both usages, the frequencies of +C(2)−H stretching mode are strongly red-shifted and the red-shifted order is [Cnmim+][Cl−], [Cnmim+][Br−], [Cnmim+][I−]. The halogen anions ([Cl−], [Br−], and [I−]) are strong proton acceptors, with a large contribution to the energy stabilization of hydrogen-bonding interaction with [Cnmim+], and the order is [Cnmim+][Cl−], [Cnmim+][Br−], [Cnmim+][I−], as evaluated by COSMO-RS calculations.38 The molecular anions ([SCN−] and [N(CN)2−]) are strong proton acceptors, and [TfO−] is a relatively strong proton acceptor. We note that the spectral shape of [Cnmim+][SCN−], [Cnmim+][N(CN)2−], and [Cnmim+][TfO−] is very similar to that of [Cnmim+][I−] in the frequency range between 3000 cm-1 and 3200 cm-1. For ILs with molecular anions (Amolecule−), there are also two usages for the assignments of +C(2)−H stretching mode and +

C(4/5)−H asymmetric and symmetric stretching modes, as are the cases with ILs with halide

anions (Ahalogen−).21,23-25,28 The red-shifted order of +

+

C−H vibrational modes under

C−H…Amolecule− interactions, which are included in the broad absorption band at lower

frequency, is [Cnmim+][SCN−], [Cnmim+][N(CN)2−], and [Cnmim+][TfO−]. [Tf2N−], [BF4−], [PF6−], and [PF3(C2F5)3−] are classified to the weakly coordinated molecular anions.23-25 The absorption band with the weak shoulder-like structure at the lower frequency was observed for [Cnmim+][Tf2N−], [Cnmim+][BF4−], [Cnmim+][PF6−], and [Cnmim+][PF3(C2F5)3−]. In contrast to the cases of ILs with strong proton acceptors, the absorption intensity of this band is relatively weaker than another absorption band at the higher frequency. The red-shifted order of +C−H


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vibrational modes under

+

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C−H…Amolecule− interactions is [Cnmim+][Tf2N−]≈[Cnmim+][BF4],

[Cnmim+][PF6−]≈[Cnmim+][PF3(C2F5)3−]. In summary, the red-shifted order for anions is [Cl−], [Br−], [I−], [SCN−], [N(CN)2−], [TfO−], [Tf2N−]≈[BF4], [PF6−]≈[PF3(C2F5)3−]. We focus on the two main molar concentration normalized absorption bands in the frequency range between 3000 cm-1 and 3200 cm-1 of Figure 4a-j. Overall, the oscillator strength in the lower frequency absorption band tends to increase with the strengthening of hydrogen bond type interactions with anions, while the absorption frequency shifts to a lower frequency with the strengthening of hydrogen bond type interactions with the anions. The oscillator strength in the higher frequency absorption band almost unchanged among IL samples, and the absorption frequency is slightly shifted or almost unchanged with the strengthening of hydrogen bond type interactions with the anion. Therefore, the ratio of the oscillator strength of the lower frequency absorption band to that of the higher-frequency absorption band increase with the strengthening of hydrogen bond interactions with the anion. The chemical shift (δ ppm) of +C(2)−H proton of 1H-NMR for imidazolium-based ILs was investigated in the literature. The experimental values are 10.22, 9.78, 9.28, 9.04, 8.99, 8.72, 8.39, 8.37, 8.11 for [C4mim+][Cl−], [C4mim+][Br−], [C4mim+][I−], [C4mim+][SCN−], [C4mim+][N(CN)2−], [C4mim+][TfO−], [C4mim+][Tf2N−], [C4mim+][BF4−], and [C4mim+][PF6−], respectively.28,38 We found a very close correlation between the red-shifted +

C−H vibrational modes and the chemical shift (δ ppm) of +C(2)−H proton, because both

measurements are sensitive to the local interactions and both are indications of the hydrogen bond type interactions. The hydrogen bond interaction energy EHB (kJ/mol), which is not the total interaction energy, was evaluated by COSMO-RS calculations.38 The calculated values are 30.72, -25.60, -19.97, -17.01, -22.60, -17.11, -9.86, -9.79, -2.88, and -0.74 for [C4mim+][Cl−], [C4mim+][Br−],

[C4mim+][I−],

[C4mim+][SCN−],

[C4mim+][N(CN)2−],

[C4mim+][TfO−],


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[C4mim+][Tf2N−],

[C4mim+][BF4−],

[C4mim+][Tf2N−],

[C4mim+][PF6−],

and

[C4mim+][PF3(C2F5)3−], respectively. Overall, we found a close correlation between the redshifted +C−H vibrational modes and the calculated hydrogen-bond interaction energy. In addition, a good correlation between hydrogen bond basicity (β) data was found in the same literature.38 Figure 5a-i shows ATR-IR spectra in the 3300 to 3800 cm-1 range for ILs samples with tiny amount of water, which were recorded to investigate the association of water molecules to the ions in ILs. The typical water concentration was 1.5 wt%. We confirmed that the spectral shape did not change with different concentrations of 1.0 wt% and 2.0 wt%. In addition, the spectral shape and band positions in the 2800 to 3300 cm-1 range did not change with the addition of a tiny amount of water. So far, this level of water concentration has been used to investigate the association of water molecules with the ions.18,35,36 Water molecules tend to associate with the anions in ILs, and the vibrational frequencies of the single water molecules associated in ILs has been used to investigate hydrogen bonding and the polarity of ILs. In Figure 5a-i, we systematically investigated the vibrational frequencies of single water molecules in ILs that have imidazolium cation with different alkyl-chain lengths and various halogen [Ahalogen−] or molecular [Amolecule−] anions.


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Figure 5. Infrared absorption spectra of IL samples with a small amount of water in the 3300 to 3800 cm-1 range for (a) [Cnmim+][Cl−][H2O] with n = 6, 8, (b) [Cnmim+][Br−][H2O] with n = 6, 8, (c) [Cnmim+][I−][H2O] with n = 3, 4, 6, (d) [Cnmim+][SCN−][H2O] with n = 2, 4, (e) [Cnmim+][N(CN)2−][H2O] with n = 2, 4, (f) [Cnmim+][TfO−][H2O] with n = 2, 4, 6, 8, (g) [Cnmim+][Tf2N−][H2O] with n = 2, 4, 6, 8, 10, (h) [Cnmim+][BF4−][H2O] with n = 2, 4, 6, 8, 10, (i) [Cnmim+][PF6−][H2O] with n = 4, 6, 8.

In the 3300 to 3800 cm-1 range, there are two main bands that are assigned to the symmetric and asymmetric stretching vibrations, ν1 and ν3, respectively of H2O molecules.18,35,36 These bands tend to overlap and the lower frequency band is attributed to the symmetric stretching vibration ν1 and the higher frequency band is attributed to the asymmetric stretching vibrations ν3. We found that, in the frequency range between 3300 cm-1 and 3800 cm-1, the absorbance of water molecules in ILs that had imidazolium cations with different alkyl chain lengths and the same anions were nearly identical in terms of the spectral shape and absorption frequencies. The missing data from [Cnmim+][PF3(C2F5)3−] was due to non-miscibility of water with [Cnmim+][PF3(C2F5)3−]. The overlapping absorption bands due to symmetric (ν1) and asymmetric (ν3) stretching vibrational modes of water molecules in [Cnmim+][Cl−], [Cnmim+][Br−], [Cnmim+][I−] are strongly red-shifted. The red-shift originates from the hydrogen bonding of water to the anions, which reflects the strength of the hydrogen bonding interaction.18,35 The halogen anions ([Cl−], [Br−], and [I−]) are strong proton acceptors. The red-shifted order of the vibrational absorption bands of water is clearly [Cl−]…[H2O], [Br−]…[H2O], [I−]…[H2O]. The molecular anions ([SCN−] and [N(CN)2−]) are strong proton acceptors. We note that the frequency range of absorption bands of water in [Cnmim+][SCN−], [Cnmim+][N(CN)2−] is very


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similar

to

[Cnmim+][I−].

The

red-shifted

order

is

Page 20 of 29

[SCN−]…[H2O],

[I−]…[H2O],

[N(CN)2−]…[H2O], although the differences are minor. [TfO−] is a relatively strong acceptor. The red-shifted order of vibrational absorption bands of water is clearly [N(CN)2−]…[H2O], [TfO−]…[H2O]. The molecular anions ([Tf2N−], [BF4−], and [PF6−]) are categorized as weakly coordinated molecular anions. The red-shifted order of vibrational absorption bands of water is [Tf2N−]…[H2O] ≈ [BF4]…[H2O], [PF6−]…[H2O]. In the experiments for IL samples with a tiny amount of water, the red-shifted order is [Cl−]…[H2O], [Br−]…[H2O], [SCN−]…[H2O], [I−]…[H2O],

[N(CN)2−]…[H2O],

[TfO−]…[H2O],

[Tf2N−]…[H2O]

≈

[BF4]…[H2O],

[PF6−]…[H2O]. As the overall view, the ratio of the lower-frequency vibrational band (ν1 band) to the higher-frequency vibrational band (ν3 band) increase with the strengthening of hydrogen bond interactions between water and the anions. As an important fact extracted from the data in Figure 4a-j (Figure S4a-j) and Figure 5a-i, it is very interesting to note that the frequency-shift of intramolecular +C−H vibrations caused by the strength of hydrogen bond type interaction under +

C−H…A− interactions between the imidazolium cation and the counter anion has a strong

correlation with the frequency-shift of intramolecular stretching vibrations of the neutral water molecule (H2O) caused by the strength of hydrogen bond interaction between A− and H2O in imidazolium-based ILs, which was established for a wide range of imidazolium-based ILs. Fumino et al. found a linear relation between the average of ν1 and ν3 band frequencies of water molecules (H2O) dissolved in [C2mim+][SCN−], [C2mim+][N(CN)2−], [C2mim][EtSO4−], and [C2mim+][Tf2N−], and the center frequency of intermolecular vibrational modes between molecular cations and anions of the corresponding neat ionic liquids, indicating that the ν1 and ν3 band frequencies of water molecules in these ionic liquids and the center frequency of intermolecular vibrational modes in the neat ionic liquids both reflect the ionic strength of the


20

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anion, and the shift of the intermolecular vibrational bands originates from decreasing force constants k and not from different reduced masses µ due to the anions.18 However, such correlation between the ν1 and ν3 band frequencies of water molecules and the center frequency of intermolecular vibrational modes does not necessarily hold for a wide range of imidazoliumbased ILs, as found in Figure 5a-i of the present paper and the data in previous papers,33,34 in which the blue-shifted order of the center absorption frequency of the intermolecular vibrational mode in neat imidazolium-based ILs was found to be [Cnmim+][Cl−], [Cnmim+][SCN−], [Cnmim+][N(CN)2−],

[Cnmim+][BF4],

[Cnmim+][Br−],

[Cnmim+][TfO−],

[Cnmim+][Tf2N−],

[Cnmim+][PF6−], [Cnmim+][I−], [Cnmim+][PF3(C2F5)3−]. Similarly, we previously found that the correlation between the intramolecular vibrational frequency due to +C−H stretching vibrations in the IR frequency region and the center frequency of intermolecular vibrational modes does not necessarily hold for a wide range of imidazolium-based ILs.33,34 We should bear in mind that the red-shifted +C−H stretching vibrations in the frequency range between 3000 cm-1 and 3200 cm-1 and the red-shifted vibrational absorption bands of water in the frequency range between 3300 cm-1 and 3800 cm-1 both originate from the change in frequency of intramolecular vibrational modes interacting with anions, which is expected to be more sensitive to the local interactions such as hydrogen bond type interactions in ILs. On the other hand, THz-TDS and FIR spectroscopy can directly probe the low frequency intermolecular vibrational modes of cations and anions in the imidazolium-based ILs, which originate from long-range pure Coulombic interactions (charge-charge interactions) and local and directional hydrogen bond type interactions. As the overall view for a wide range of imidazolium-based ILs, we previously found the significant contribution of the reduced mass µ calculated from the masses of the methyl-imidazolium ring cation [mim+] and the anion as well as the force constant k,


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Page 22 of 29

phenomenologically. As far as the value of molar concentration normalized absorption coefficient at the central absorption frequency of the intermolecular vibrational band is concerned, we previously found that it is inversely proportional to the effective molecular weight of ion pair ([mim+][A−]).34 The effective molecular weight of the ion pair is associated with the physical size of the ion pair, such as the size of the charge distribution and the average distance of ion pair. Using the data of the insets in Figure 2c and Figure 3c, the values for [mim+][SCN−] and [mim+][N(CN)2−] were added (see Figure S5, SI). The correlation between the absorption intensity due to +C−H stretching vibrations in the IR frequency region and the absorption intensity due to the intermolecular vibrational mode in the low THz frequency range does not necessarily establish for a wide range of imidazolium-based ILs.

CONCLUSIONS The +C−H intramolecular vibrational frequencies of ILs that have imidazolium cations with different alkyl chain lengths and various halogen or molecular anions with and without a small amount of water were systematically studied. We found that the molar concentration normalized absorbance in the frequency range between 3000 and 3200 cm-1 for ILs that have imidazolium cations with different alkyl chain lengths and the same anions were nearly identical in terms of the absorption frequency and absorption intensity, whose validity was surprisingly demonstrated for a wide range of imidazolium-based ILs. The oscillator strength in the lower-frequency absorption band in the two main absorption bands tends to increase with the strengthening of hydrogen bond type interactions with the anions, while the absorption frequency is shifted to a lower frequency with the strengthening of hydrogen bond type interactions with the anions. We found a close correlation between the red-shifted +C−H vibrational modes and the chemical shift


22

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(δ ppm) of +C(2)−H proton, and the energy stabilization of hydrogen-bonding interaction evaluated by COSMO-RS calculations. The vibrational modes of the water molecules interacting with anions in the frequency range between 3300 and 3800 cm-1 was also systematically investigated by adding a small amount of water. A strong correlation between the frequencies of +

C−H stretching vibrations under the interactions +C−H…A− and the vibrational frequencies of

water interacting with anions was found, while the correlation between the frequencies of +C−H stretching vibrations (or the vibrational frequencies of water interacting with anions) and the center frequencies of intermolecular vibrational modes due to the ion pairs does not necessarily hold for a wide range of ILs. The systematic experimental data we obtained and the comparisons with the other experimental and theoretical studies are useful to extract the overall view of noncovalent interactions in ILs, which could help to further detailed modelling of the bulk structures and simulations to elucidating nature.39-41

Supporting Information Available: Calculated vibrational spectra for the alkyl-methylimidazolium

cations

(C2mim+,

and

C4mim+)

and

molecular

anions

([SCN−]

and

[N(CN)2−])[Tf2N-] in the low, mid and high THz frequency regions, ATR-IR spectra in the 2800 to 3300 cm-1 range for [Cnmim+][Cl−] ILs with n = 6, 8, [Cnmim+][Br−] ILs with n = 6, 8, 10, (c) [Cnmim+][I−] ILs with n = 3, 4, 6, [Cnmim+][SCN−] ILs with n = 2, 4, [Cnmim+][N(CN)2−] ILs with n = 2, 4, [Cnmim+][TfO−] ILs with n = 2, 4, 6, 8, [Cnmim+][Tf2N−] ILs with n = 2, 4, 6, 8, 10, [Cnmim+][BF4−] ILs with n = 2, 4, 6, 8, 10, [Cnmim+][PF6−] ILs with n = 4, 6, 8, and [C6mim+][PF3(C2F5)3−] IL, and α/M at the central absorption frequency of the intermolecular vibrational band against the molecular weight of the ion pair ([mim+][A-]). These materials are available free of charge via the Internet at http://pubs.acs.org.


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Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.

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