Stable Structure in Fluorinated Imidazolium Ionic Liquids

Abstract

One of the most important characteristics required for ionic liquids (ILs) is the capability to realize low-viscosity. Although ionic liquids are sometimes called "designer solvents", no rational design has yet been proposed for preparing ILs with low viscosity. Among recently developed ionic liquids, we were fascinated by alkyl ether substituted ILs due to their low-viscous nature. We discovered that 1-(3,3'-difluoro)butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide showed a significant low viscosity compared to the corresponding non-fluorinated version of [bmim][TFSI]. To gain the insight for the rational design of ionic liquids, we attempted to investigate the origin of this change in the viscosity by the difluorination using a DFT calculation. The calculations suggest that there is a clear difference in the population pattern of the stable conformers (probability of occurrence) between bmim cation 1 and 1-(3,3'-difluoro)butyl-3-methylimidazolium cation 3f. Moreover, an intramolecular hydrogen bond is suggested for cation 3f. We will discuss the origin of viscosity change of ILs by the difluorination using DFT calculation method.