2-ELECTRON TRANSFER-REACTIONS IN POLAR-SOLVENTS

Chemical, biological, and electrode based electron transfer (ET) proce sses involve multielectron events. However, an adequate framework in w hich to describe these complex reactions does not yet exist. A theory for two-electron transfer reactions in Debye solvents is developed. Th e theory is formulated by generalizing Zusman's model of ET reactions [L. D. Zusman, Chem. Phys. 49, 295 (1980)] to those involving three pa rabolic wells: One for the doubly reduced donor, one for the singly re duced donor/singly reduced acceptor, and one for the doubly reduced ac ceptor. The ET processes are described in terms of diffusional motion along a one-dimensional reaction coordinate with tunneling transitions at the intersection points of the parabolas. Two competing mechanisms of two-electron,transfer arise. One is a process with two sequential single electron steps D(=)A-->D(-)A(-)-->DA(=). The other involves ET in one concerted two-electron step (D(=)A-->DA(=)). The general rate e xpressions for two-electron transfer are obtained. When the stepwise m echanism dominates, the free energy of activation is predicted to depe nd upon the driving forces of the two sequential steps but is independ ent of the overall driving force of the reaction. When concerted two-e lectron transfer dominates, the Marcus relation is obtained with a reo rganization energy associated with the shift of two electrons. The dyn amical solvent effect in two-electron ET processes is predicted to be unusual. Two distinct regimes exist, each with essentially linear 1/ta u(L) dependence (with tau(L) the solvent longitudinal relaxation time) : one for slow solvents and one for fast solvents. A combination of so lvent and free energy studies could be used to elucidate the mechanism of multielectron processes in chemical and biological systems. (C) 19 96 American Institute of Physics.