Solvent effects on outersphere electron transfer reactions in mixed dipolar liquids

Abstract

A microscopic theory of outersphere electron transfer reactions occurring in mixed dipolar liquids is presented. Both static and dynamic effects of solvent mixture on rates of electron transfer processes are calculated by employing molecular models. The donor–acceptor system is composed of two spheres between which the electron is transferred. The sizes of the donor and acceptor can be different. The solvent mixture is composed of different kinds of spherical molecules with embedded dipoles. The molecules of the different components are distinguished by their sizes and dipole moments. A microscopic expression for the free energy of activation is derived by using density functional theory and constrained variational approach. The dynamic effects are calculated by using a molecular hydrodynamic theory which properly includes the finite wavevector modes of relaxation of dipolar mixture. In the numerical calculations, the solvent molecules of different components are taken to be of equal size but different polarity and mole fraction. Explicit numerical results are obtained for the activation free energy and the rate constant of electron transfer between a donor and an acceptor of varying sizes and the importance of preferential solvation is elucidated. The effects of changing the donor–acceptor separation on the activation free energy and the rate constant are also discussed.