Conformational preferences of two imidazolium-based ionic liquids at high pressures

Conformational stabilities of 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) at high pressures have been investigated by Raman spectroscopy. The trans (planar) conformer for the CNCC angle of the [emim]+ is predominant at high pressures, but the conformational change of the CNCC angle of the [bmim]+ is independent of pressure. The gauche–trans conformer for the butyl cation of the [bmim]+ is predominant at high pressures. Thus, the difference in the alkyl chain length between the cations causes the respective conformational preferences of the two imidazolium-based ionic liquids at high pressures.