Solvent Dynamical Effects in Electron Transfer: Numerical Predictions of Molecularity Effects Using the Mean Spherical Approximation

摘要

The influence of solvent molecularity upon the adiabatic barrier-crossing frequency, nu sub n, and barrier height, Delta G*, for electron-exchange reactions involving only solvent reorganization is examined numerically on the basis of a mean spherical approximation (MSA) treatment of the reaction coordinate time-correlation function, Delta(t) for Debye solvents. The calculated Delta G* values for a spatially isolated redox couple fall increasingly below the corresponding dielectric continuum quantity, Delta G* sub con, as the ratio of the solvent to reactant radii, (r sub sol/r sub re), increases. Thus for (r sub sol/r sub re) somewhat < 2 with the above dielectric parameters, nu sub n is calculated to be up to ca 2- and 5-fold smaller than (nu sub n) con, on the basis of the GLE and diffusion model, respectively. These differences can be understood in terms of the relative influences upon nu sub n of the slower relaxation components arising from short-range solvation, responsible for the deviations from (nu sub n)con, in comparison with the faster dynamics associated with more distant solvent molecules. Slightly larger deviations from the continuum predictions are obtained using the GLE approach in the presence of barrier-top curvature. At least for (r sub sol/r sub re) somewhat < 2, the MSA-prescribed rate constants also do not deviate greatly from the corresponding continuum prediction, the corresponding decreases in nu sub n and Delta G* being partly compensatory. Reprints. (jhd)