Molecular dynamics are computed for a modelSN2 reaction Clminus;+CH3Clrarr;ClCH3+Clminus;in water and are found to be strongly dependent on the instantaneous local configuration of the solvent at the transition state barrier. There are significant deviations from the simple picture of passage over a free energy barrier in the reaction coordinate, and thus, a marked departure from transition state theory occurs in the form of barrier recrossings. Factors controlling the dynamics are discussed, and, in particular, the rate of change of atomic charge distribution along the reaction coordinate is found to have a major effect on the dynamics. A simple frozen solvent theory involving nonadiabatic solvation is presented which can predict the outcome of a particular reaction trajectory by considering only the interaction with the solvent of the reaction system at the gashyphen;phase transition barrier. The frozen solvent theory also gives the transmission coefficient kgr; needed to make the transition state theory rate agree with the outcome of the molecular dynamics trajectories. This theoretical kgr; value, which is the implementation for theSN2 reaction of the van der Zwanndash;Hynes nonadiabatic solvation transmission coefficient, is in good agreement with the trajectory results. In contrast, a Kramers theory description fails dramatically.
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