Effects of Methylation at the 2 Position of the Cation Ring on Phase Behaviors and Conformational Structures of Imidazolium-Based Ionic Liquids

The proton at the 2 position of the cation ring in imidazolium-based ionic liquids (ILs) strongly interacts with anions; therefore, the methylation at this position (C(2) methylation) causes significant changes in the physicochemical properties of these liquids. We investigated the C(2) methylation effects on the phase behaviors and cation conformations of ILs by calorimetric and Raman spectroscopic measurements, focusing on the pairs of 1-butyl-3-methylimidazolium salt ([C(4)mim]X) and 1-butyl-2,3-dimethylimidazolium salt ([C(4)C(1)mim]X), where X(-) is Cl(-), Br(-), I(-), BF(4)(-), and PF(6)(-). The melting and freezing points of all pairs increased after the C(2) methylation, as reported previously, and the reason for the increase was the overcompensation of the DeltaS(trans) decrease for the DeltaH(trans) decrease. The C(2) methylation also affected the phase behaviors of the ILs. With Raman spectroscopic measurements, all cation conformations in crystalline phases were assigned to trans-trans (TT), gauche-trans (GT), or gauche'-trans (G'T) conformers of the butyl group. Except in [C(4)C(1)mim]BF(4), all crystal-crystal phase transitions of the present samples occurred accompanied by conformational changes among TT, GT, and G'T. For the gas states of [C(4)mim](+) and [C(4)C(1)mim](+), DFT calculations showed that there were hardly any differences in the structures of the butyl group for each set of paired conformers or in the energetic orders among the conformers. On the other hand, the conformer adopted in the crystalline phase differed between [C(4)mim]X and [C(4)C(1)mim]X. In addition, the population of the conformers in the liquid state also differed in each pair. The data from higher frequency Raman spectra suggested that the difference in cation conformation in each pair, for the crystalline and liquid states, was due to the shift in the position of the anion relative to that of the cation. By C(2) methylation, the relative distance between the anion and cation decreased for Cl(-), Br(-), and I(-) salts, but it increased for BF(4)(-) and PF(6)(-) salts.