We have investigated the nonspherical anion effect on the trans-trans (TT) and gauche-trans (GT) equilibrium in pure 1-butyl-3-methylimidazolium ([bmim]+)-based room temperature ionic liquids (RTILs) by the Raman spectroscopy. The intensity ratio of the [bmim]+ cation in [bmim]+-based RTILs having nonspherical anions changes with nature of the anions. However, the enthalpy change of the [bmim]+ cation is approximately ？1.0？kJ/mol for all [bmim]+-based RTILs used in this study and is independent of the anionic species. The present results indicate that the conformational stability of the [bmim]+ cation in [bmim]+-based RTILs including nonspherical anions is driven by the entropic contribution associated with the orientation and configuration of the [bmim]+ cation with respect to the counteranion. 1. 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 [1–5]. The conformational equilibria of imidazolium cations, such as 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium ([emim]+) of the imidazolium-based RTILs, the so-called prototype ionic liquids, have been investigated by the Raman spectroscopy combined with density functional theory calculations [2, 5–12]. The [bmim]+ cation has a trans-trans and gauche-trans ( - ) equilibrium for NCCC and CCCC angles of the butyl group, as shown in Figure 1. Thermodynamic studies of the conformational equilibrium of RTILs are useful to clarify the relationship between the conformation and the complicated phase transition behavior of these RTILs [3–5]. Figure 1: Optimized structures of the (a) trans-trans ( ) and (b) gauche-trans ( ) conformers of 1-butyl-3-methylimidazolium ([bmim] +) cation by B3LYP/6-311+(d) level. The effect of spherical anions, such as halide anions (Cl？, Br？, and I？), on the - equilibrium of the [bmim]+ cation in pure [bmim]+-based RTILs has been studied [3, 10–12]. 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
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