Nonequilibrium solvation effects for a polyatomic reaction in solution

We present a general linear-response method for including nonequilibrium so lvation effects (solvent friction effects) in variational transition state theory with multidimensional tunneling (VTST/MT) for calculating reaction r ate constants in solution. The generalized Langevin approach is used to inc lude a collective solvent coordinate into VTST/MT, and a general prescripti on is suggested for coupling this collective solvent coordinate to the solu te, which is treated in its full dimensionality. The new formalism is illus trated by application to the aqueous free radical reaction H + CH3OH --> H- 2 + CH2OH at 298 K. This reaction is treated with a linear mixing of Hartre e-Fock theory and Austin Model 1 with specific reaction parameters (HF para llel to AM1-SRP). The results with nonequilibrium solvation (NES) an compar ed to those obtained earlier with the separable equilibrium solvation (SES) and the equilibrium solvation path (ESP) approximations. We focus on the s peedup due to solvation and on the kinetic isotope effects (KIEs). We calcu late that nonequilibrium solvation decreases the rate constant by a factor of 2 but changes the KIEs by less than 2%. We also present results that sho w how the nonequilibrium effect depends on the solvation time and the stren gth of the solute-solvent coupling.