Ultrafast structure and dynamics in ionic liquids: 2D-IR spectroscopy probes the molecular origin of viscosity

The viscosity of imidazolium ionic liquids increases dramatically when the strongest hydrogen bonding location is methylated. In this work, ultrafast two-dimensional vibrational spectroscopy (2D-IR) of dilute thiocyanate ion ([SCN]−) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1C21im][NTf2]) shows that the structural reorga- nization occurs on a 26 ± 3 ps timescale and on a 47 ± 15 ps timescale, respectively. The results suggest that the breakup of local ion cages is the fundamental event which activates molecular diffusion and determines the viscosity of the fluids