Solvent Dynamical Effects on Electron-Transfer Kinetics: Influence of Electronic Coupling on Weak-Overlap Exchange Processes

摘要

A numerical integration treatment of the appropriate Kramers' diffusion equation, involving overdamped solvent motion, is employed to estimate adiabatic barrier-crossing frequencies NU(os) for weak-overlap electron-exchange processes featuring only outer-shell (solvent) reorganization as a function of the electronic matrix coupling element H12. Comparisons are made with estimates of NU(os) obtained from limiting analytical expressions. The competing influence of reaction nonadiabaticity and solvent motion upon the barrier-crossing frequency is also considered as a function of H12 using a Landau-Zener treatment. The results indicate that the dominant influence of overdamped solvent motion on the preexponential factor, that yields the observed inverse correlation between the reaction rate and the solvent longitudinal relaxation time, requires reactions that feature moderate degrees of electronic coupling, say H12 approx. or = 0.1 kcal/mol, in most common polar solvents. These factors are briefly illustrated with reference to recent measurements of the solvent-dependent kinetics of cobalticinium-cobaltocene electrochemical exchange.