Nonequilibrium solvent effects on the S N2 reaction using a self-consistent reaction field continuum model based on multipole expansions

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Abstract
A simple model has been developed that allows analysis of nonequilibrium solvent effects on chemical processes. It is based on the use of a self-consistent reaction field approach using a multipole development of the solvation energy and on the separation of the inertial and noninertial polarization of the solvent. The solute’s wave function is computed at the ab initio level. The main advantage with respect to previously reported models is that the inclusion of nonequilibrium or dynamic solvent effects are introduced through the definition of a single solvent coordinate which is related to the chemical system coordinates. Besides, inclusion of polarization effects is straightforward. Results are presented for the SN2 reaction F−+CH3F→FCH3+F−. The frozen-solvent hypothesis and the role of solvent fluctuations are discussed. It is shown that the climb to the transition barrier must be preceded by a convenient fluctuation of the solvent so that its inertial polarization component is suitable to solvate the transition state. Other solvent fluctuations, energetically less favorable, could decrease or even suppress the transition barrier. Nonequilibrium solvation effects on the value of the transmission coefficient are discussed. The methodology proposed in this work may be extended to the study of other rapid processes in solution such as proton transfers or electronic excitations.