Mechanisms of solvation dynamics of polyatomic solutes in polar and nondipolar solvents: A simulation study

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Abstract
Molecular dynamics (MD) simulations of a benzenelike solute in acetonitrile and CO 2 (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 benzene solute are simulated in order to mimic the type of electrostatic solvation observed in typical experimental systems. From equilibrium MD simulations the solvation time correlation function [TCF; C(t)] and the corresponding solvation velocity TCF [G(t)] are used to study the mechanisms underlying time-dependent solvation within the linear response 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 perturbation. The contribution of translational solventmotions to both the short and long time dynamics of C(t) increases from about 10% for a monopolar perturbation (m=0; i.e., a change in net charge) to about 40% for 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 markedly on the charge symmetry of both the solvent molecule’s charge distribution and the solute perturbation. In the nondipolar solvent CO 2 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 acetonitrile pair contributions to C(t) greatly suppress the magnitude of the solvation response and as a consequence greatly increase the speed of the response over what it would be in their absence. The importance of these intermolecular correlations in acetonitrile decreases substantially with m, such that the “suppression factors” (α s ) vary from ∼9 for m=0 to ∼2 for m=3. The intermolecular correlations of primary importance in acetonitrile are of a static rather than a dynamic nature (i.e., pair effects on G(t) are of only secondary importance). This feature makes it possible to employ several approximate relationships to relate the collective dynamics of solvation in polar fluids to simpler single-solvent molecule dynamics.