抄録
Pressure effect on the conformational equilibrium of 1-butyl-3-methylimidazolium cation ([bmim]+) in the [bmim]-based ionic liquids (ILs) with three different anionic species (I, BF4, and PF6) have been studied using Raman spectroscopy. The gauche conformer for the NCCC angle of [bmim]+ increases with increasing pressure at all cases, and the trans conformer decreases. From the pressure dependence of the Raman intensity, we determined the difference in the partial molar volume (ΔVtrans→gauche) between the conformers of [bmim][X](X= I, BF4, and PF6). The negative ΔVtrans→gauche is due to the molecular volume difference (ΔVM) which consists of the van der Waals volume and void volume differences. This means that the pressure-induced [bmim]+ conformational preference of [bmim]-based ILs is dependent of the configuration of [bmim]+ against each anionic species. Thus, we proposed that the increment of the gauche conformer of [bmim]+ on the pressure-induced phase transitions is related to the change in the initial configuration of [bmim]+ against anion by compression.
