DFT Study on Conformation of 1-Alkyl-3-methylimidazolium with Ethyl, Propyl, Butyl, Pentyl, and Hexyl Group

Comprehensive conformational analyses of 1-alkyl-3-methylimidazolium cations ([C(n)mim](+) where n = 2-6) were performed by DFT calculations with dispersion correction in the gas phase. The energy difference (Delta E) of each conformer from the non-planar and all-trans (t) form was calculated. The planar forms have 2-3 kJ mol(-1) higher energy than the corresponding non-planar form. A significant difference between these two forms is that unlike the non-planar form, the planar form cannot take gauche (g) or gauche' (g') in the second dihedral angle of the alkyl group. The conformational behavior for the alkyl group somewhat resembles that of alkane while interactions between the alkyl group and the imidazolium ring play an important role for [C(n)mim](+) in both attractive and repulsive ways. For example, when the conformers contain g/g' in the inner dihedral position, they tend to provide low Delta E due to the attractive interaction. Meanwhile, the strong repulsive interactions coming from steric hindrance do not allow some non-planar forms to stably exist. Instead, such a conformer takes the planar form that experiences less steric hindrance. The calculated results were compared with those from the Cambridge Structural Database, and Raman bands of the calculated cations were displayed and discussed.