Non-Adiabatic Electron Transfer: Some Dynamical and Electronic Extensions of Standard Rate Expressions

Abstract

The standard Marcus-Hush approach for computing rates of non-adiabatic electron transfer processes is based upon a combination of perturbation theory and transition state theory. While the standard approach is both extremely powerful and widely applicable, there are certain experimental situations in which the transition state theory argument, in particular, is no longer valid. We discuss a number of situations in which extensions of Marcus-Hush theory are required to deal with electron transfer rate phenomena. These situations include: 1) nuclear tunneling reactions, in which a vibronic treatment, related to standard small polaron theory, can be used; 2) control of fast reactions by solvent dynamics, in which case the relaxation time spectrum of the solvent must be included in the rate discussion; 3) secondary stable minima on the potential energy surface, in which case gating phenomena may occur because alternative barriers and steepest descent pathways can be found for trajectories moving from reactant to product; 4) highly anisotropic diffusion or friction along different coordinates, in which case trajectories wander far from the steepest descents pathway and nonexponential transient behavior may be observed; 5) specific electronic effects, including modified initial states, the importance of coincidence events, choice of the initial state for electron transfer, breakdown of perturbation theory and generalized superexchange behavior.