Rotational dynamics of a diatomic solute in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate

摘要

Reorientational time correlation functions C_l(t)(ident to) for a diatomic solute in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI~+PF_6~-) are analyzed via molecular dynamics computer simulations,where <...> denotes an equilibrium ensemble average,P_l the lth order Legendre polynomial and theta{t) the angle between the solute orientation at time t and its initial direction.Overall results are indicative of heterogeneous dynamics in EMI~+PF_6~-.For a small nondipolar solute,C_l(t) are well-described as stretched exponential functions in wide time ranges.One striking feature is that after rapid initial relaxation,C_2(t) decays more slowly than C_1(t).As a result,the correlation time associated with the former is considerably longer than that with the latter.This is ascribed to solvent structural fluctuations,which allow large-amplitude solute rotations.As the solute size grows,relaxation of C_l(t) approaches exponential decay.