Effect of Counteranions on the Conformational Equilibrium of 1-Butyl-3-methylimidazolium-Based Ionic Liquids

We have investigated the nonspherical anion effect on the trans-trans (TT) and gauche-trans (GT) equilibrium in pure 1-butyl-3-methylimidazolium ([bmim]


Introduction
Room temperature ionic liquids (RTILs) consisting of organic cations and anions remain in the liquid state at room temperature [1]. The conformational behavior of RTILs has been studied to identify correlations with chemical and physical properties of RTILs, such as conductivity, viscosity, and melting point [1][2][3][4][5].
The effect of spherical anions, such as halide anions (Cl − , Br − , and I − ), on the TT-GT equilibrium of the [bmim] + cation in pure [bmim] + -based RTILs has been studied [3,[10][11][12]. Katayanagi et al. [3] reported that the conformational preference of the [bmim] + cation varies with different halide anions. In relation to this, using the Raman spectroscopy and molecular dynamics (MD) simulations, Umebayashi et al. [12] showed that the thermodynamic stability of the conformational equilibrium of the [bmim] + cation is affected by the halide anion. Important conclusion is that anions localized near the C2-H atoms of the [bmim] + cation stabilize the gauche conformer, and the effect is stronger with smaller anions. On the other hand, Lopes and Pádua [13] reported that the populations of the conformers of the [bmim] + cation in [bmim][X] (X = BF 4 , PF 6 , or TFSA, i.e., other than the halide anions) are nearly independent of the associated anions by a molecular simulation. In this situation, we feel that a general view of the effect of counteranions on the conformational equilibrium of the [bmim] + cation has not been established yet.
In this study, we have investigated the thermal stability of the TT-GT equilibrium of the [bmim] + cation in pure [bmim][X] RTILs (X = CH 3 COO, SCN, NO 3 , BF 4 , and PF 6 ) including nonspherical anions using the Raman spectroscopy to help a further understanding of the dependence of the conformational isomerism in [bmim] + -based ionic  liquids on the nature of the anion. Our results showed that the conformational preference of the [bmim] + cation varies with different counteranions studied, and the conformational stability is entropically (configuration) driven.

Experimental
In this study, as samples we selected RTILs . Just in case, water contents in the samples were checked by a Karl-Fischer titration method and found to be less than 120 ppm. Generally, RTILs are easily contaminated by vacuum drying to reduce H 2 O; thus, we used the samples without further purification, and special care was taken to avoid further atmospheric contamination. The Raman spectra were measured by a JASCO NR-1800 Raman spectrophotometer equipped with a single monochromator and a charge-coupled device detector. The exposure time for each run and the spectral resolution were 3.0 s and 8.6 cm −1 , respectively. The 514.5 nm line for [bmim][X] (X = BF 4 and PF 6 ) and the 647.1 nm line for [bmim][X] (X = CH 3 COO, SCN, and NO 3 ) from Lexel Ar + and Kr + ion lasers were used as excitation sources with a power of 250 mW. For thermal conformational variation experiments, the temperature was increased from 273 K to 368 K in 10 K increments using a Japan High-Tech LK-600PH temperature controller. The resulting Raman spectra were fitted with Gaussian-Lorentzian mixing functions using the GRAMS/386 software (Galactic Ind. Co. Ltd.).  [2,6,7]. According to previous studies [2,7], the Raman bands at 600 and 620 cm −1 arising from the CH 2 rocking mode of the NCCC of these RTILs were assigned to the GT and TT conformers of the [ To show detailed conformational changes for the [bmim] + cation, we determined the intensity ratio (I GT /I TT ) of the two conformations of the [bmim] + cation. Because the observed Raman band intensity (I) is proportional to the product of the Raman cross-section (σ) and the concentration of the conformer (c) (I ∝ σc) [14], the change in I GT /I TT of the [bmim] + cation as a function of the anionic Interestingly, we have found that I GT /I TT seems to be related to the Hofmeister series of anions [15][16][17][18], which was originally an indication of the effects of ions on the water structure, although the order of the I GT /I TT values is not perfectly match with the Hofmeister series of anions, that is, at the BF 4 . Unfortunately, there has been no conclusive idea on the origin of the Hofmeister series. Recently Zhang and Cremer [16] insisted that changes in bulk water structure by added salts cannot explain specific ion effects. Hofmeister phenomena need to be understood in terms of direct interactions between the ions and macromolecules, in addition to the ability of the ions to stabilize the structure of proteins. At any rate, the molecular-level pictures of the Hofmeister phenomena for example, how ions operate, are important, because of their relevance to a broad range of fields [18]. In view of the results, it looks like the Raman intensity of the GT conformer was higher than that of the TT conformer when a relatively chaotropic anion such as PF 6 − was present, while the Raman intensities of the GT and TT conformers were almost identical when a more kosmotropic anion such as CH 3 COO − was present. To investigate the difference in the partial molar enthalpy between the TT and GT conformers (ΔH TT → GT ), we subsequently measured the temperature dependence of the Raman spectra of the pure [bmim] + -based RTILs. Figure 4 shows representative Raman spectra of pure [bmim][X] (X = SCN and NO 3 ) as a function of temperature. The Raman intensity of the TT conformer increases as the temperature increases for all [bmim] + -based RTILs used in this study, while that of the GT conformer decreases. Recently, Endo et al. [5] reported that the Raman intensity of the GT conformer of the [ [5] and Holomb et al. [10].

Results and Discussion
We can determine ΔH TT → GT of the [bmim] + cation from the temperature dependence of the relative Raman intensities, as shown in Figure 5. Assuming that the ratio of the Raman scattering cross-sections between the conformers A and B is independent of temperature, ΔH A → B is given by (1) [19] In this equation, R, T, and p are the gas constant, temperature, and pressure, respectively. I A and I B indicate the relative Raman intensity of conformers A and B, respectively, and "A" and "B" correspond to the conformers of the [bmim] + cation. From the slopes of the lines in Figure 5, ΔH TT → GT is determined to be −0. 9    In summary, the counteranion effect on the trans-trans (TT) and gauche-trans (GT) equilibrium in 1-butyl-3-methylimidazolium ([bmim] + )-based RTILs has been studied using the Raman spectroscopy. The value of I GT /I TT , which is proportional to ΔG TT → GT , of the [bmim] + cation shows a linear correlation with the Hofmeister series of the anions. On the other hand, the value of the enthalpy change (ΔH TT → GT ) of the [bmim] + cation is approximately −1.0 kJ/mol and is independent of the anionic species. We conclude that the conformational preference of the [bmim] + International Journal of Spectroscopy 5 cation and anions the conformational stability are driven by entropic contributions such as the orientation between the [bmim] + cation and nonspherical anions.