Dynamics and Structure of Room Temperature Ionic Liquids: Ultrafast IR Experiments and MD Simulations of the Ion Cage Model

Dynamics in room-temperature ionic liquids (ILs) and the solvation of charged and neutral molecules are important to applications of ILs in battery technology, organic synthesis, and many other areas. Using ultrafast IR spectroscopy, pulse-field-gradient (pfg) NMR, and MD simulations, we investigated three ionic liquids that share the same cation but differ in their anions. IR polarization-selective pump-probe and 2D IR experiments were performed on a negatively charged probe, thiocyanate, to investigate IL dynamics. It was found that thiocyanate is an excellent surrogate for the IL anions. We employed a neutral probe, methylthiocyanate, to determine how a neutral solute's dynamics and interactions differ from those of the RTIL ions. The pfg-NMR experiments, performed on all liquids and species, measured the translational diffusion constants. Extensive molecular dynamics simulations were performed to understand the experimental observables within the IL cage model, in which cations surround an anion (anion cage), and anions surround a cation (cation cage) for all ionic species, including the ionic probe. The neutral probe is surrounded by a combination of cations and anions (neutral probe cage). The cage model shows that the dynamical properties, including translational diffusion, orientational diffusion, and viscosity, are determined by the cage escape time, te, i.e., the time for an ion to leave the group of ions surrounding an ion of the opposite charge or the neutral probe. It is observed that te is correlated with the slowest component of the cage structural fluctuations measured by 2D IR.