SHORT-TIME DYNAMICS OF SOLVATION - LINEAR SOLVATION THEORY FOR POLAR-SOLVENTS

Almost all of the current effort at understanding the dynamics of solvation in polar liquids has been focused on the dielectric properties of the solvent. For times under 300 fs, however, such a perspective tends to hide a feature which is common to both polar and nonpolar solvents, namely, that short-time solvation is governed by the instantaneous normal modes (INMs) of the solvent. We illustrate this point here by applying the simplest level of INM theory-a linear theory-to a dipolar solute dissolved in acetonitrile. What we find is that the solvation process can be represented in the frequency domain as a solvation spectrum, which in turn can be dissected into portions resulting from a variety of different molecular processes. This analysis leads inexorably to the conclusions that short-time solvation, at least in this polar solvent, is primarily the result of solvent librations and overwhelmingly the result of the first solvation shell. At somewhat longer times, the solvent motions responsible for the solvation are far more intertwined, but most of the basic events in polar solvation, ranging from the earliest purely inertial dynamics through the onset of the more highly coupled behavior, can apparently be thought of in instantaneous-normal-mode terms. Within this framework one finds that polar liquids have a surfeit of high-frequency modes available and do have a somewhat specialized weighting of the various modes, but, aside from such issues, it is probably fair to say that short-time solvation is largely independent of dielectric considerations.