MECHANISMS OF SOLVATION DYNAMICS OF POLYATOMIC SOLUTES IN POLAR AND NONDIPOLAR SOLVENTS - A SIMULATION STUDY

Molecular dynamics (MD) simulations of a benzenelike solute in acetoni trile and CO2 (298 K and 52.18 cm(3)/mol) are used to investigate the molecular basis of solvation dynamics in polar and nondipolar solvents . The solvation response to various charge rearrangements within the b enzene solute are simulated in order to mimic the type of electrostati c solvation observed in typical experimental systems. From equilibrium MD simulations the solvation time correlation function [TCF; C(t)] an d the corresponding solvation velocity TCF [G(t)] are used to study th e mechanisms underlying time-dependent solvation within the linear res ponse limit. Decomposition of G(t) into contributions from rotational and translational solvent velocities reveals that the relative mix of these two types of motion is quite similar in the two solvents but is strongly dependent on the multipolar order (m) of the solute perturbat ion. The contribution of translational solvent motions to both the sho rt and long time dynamics of C(t) increases from about 10% for a monop olar perturbation (m = 0; i.e., a change in net charge) to about 40% f or a perturbation of octopolar (m = 3) symmetry. Decomposition of both C(t) and G(t) into single-molecule and molecular-pair : contributions shows that the collective nature of the solvation response depends ma rkedly on the charge symmetry of both the solvent molecule's charge di stribution and the solute perturbation. In the nondipolar solvent CO2 neither C(t) nor G(t) differ significantly from their single-molecule counterparts-collective effects are therefore of little consequence to solvation in this solvent. However, in the highly dipolar solvent ace tonitrile pair contributions to C(t) greatly suppress the magnitude of the solvation response and as a consequence greatly increase the spee d of the response over what it would be in their absence. The importan ce of these intermolecular correlations in acetonitrile decreases subs tantially with m, such that the ''suppression factors'' (alpha(s)) var y from similar to 9 for m = 0 to similar to 2 for m = 3. The intermole cular correlations of primary importance in acetonitrile are of a stat ic rather than a dynamic nature (i.e., pair effects on G(t) are of onl y secondary importance). This feature makes it possible to employ seve ral approximate relationships to relate the collective dynamics of sol vation in polar fluids to simpler single-solvent molecule dynamics. 0 1998 American Institute of Physics. [S0021-9606(98)03420-5]