Computer simulations of solvation dynamics in acetonitrile

Computer simulations of the solvation of monatomic ions in acetonitrile are used to investigate dynamical aspects of solvation in polar aprotic solvents. The observed dynamics depend significantly on solute charge and on which multipole moment of the solute is perturbed. In all cases, the solvation response has a two-part character. One part consists of a fast initial relaxation and attendant oscillations, both of which occur on a time scale of 0.1-0.2 ps. The initial response is well fit by a Gaussian function and accounts for ∼80% of the total relaxation. The second dynamical component occurs on a much slower, ∼1 ps time scale, and accounts for the remainder of the relaxation. The fast response results from small amplitude inertial dynamics of solvent molecules within the confines of their instantaneous environment. The slow component reflects larger amplitude motions involving the breakup and reorganization of these local environments, especially in the first solvation shell of the solute. Comparison of the observed solvation dynamics to predictions of available theories points out the inadequacy of the latter due to their neglect of inertial parts of the solvent dielectric response.