Two-electron transfer reactions in polar solvents

Chemical, biological, and electrode based electron transfer (ET) processes involve multielectron events. However, an adequate framework in which to describe these complex reactions does not yet exist. A theory for two-electron transfer reactions in Debye solvents is developed. The theory is formulated by generalizing Zusman's model of ET reactions [L. D. Zusman, Chem. Phys. 49, 295 (1980)] to those involving three parabolic wells: One for the doubly reduced donor, one for the singly reduced donor/singly reduced acceptor, and one for the doubly reduced acceptor. 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 expressions for two-electron transfer are obtained. When the stepwise mechanism dominates, the free energy of activation is predicted to depend upon the driving forces of the two sequential steps but is independent of the overall driving force of the reaction. When concerted two-electron transfer dominates, the Marcus relation is obtained with a reorganization energy associated with the shift of two electrons. The dynamical solvent effect in two-electron ET processes is predicted to be unusual. Two distinct regimes exist, each with essentially linear 1/γL dependence (with γL the solvent longitudinal relaxation time): one for slow solvents and one for fast solvents. A combination of solvent and free energy studies could be used to elucidate the mechanism of multielectron processes in chemical and biological systems. © 1996 American Institute of Physics.

Duke Scholars

Author David N. Beratan Chemistry