Cage and Entropy Effects in the Dynamics of Dissociative Electron Transfer

Abstract

Dissociative electron transfers in condensed phases occur in two steps. The fragments are first formed within a solvent cage from which they further diffuse. The formation of caged, rather than free-moving, fragments is taken into account in an improved version of the dissociative electron transfer theory where entropic aspects are emphasized. A more detailed treatment than previously available of the fragmentation and solvent reorganization factors is given in terms of both energies and free energies. The reason that the bond dissociation energy, rather than the bond dissociation free energy, represents the contribution of fragmentation to the intrinsic barrier ensues. The resulting equations that relate the activation free enthalpy and entropy, as well as the symmetry factor, to the standard free enthalpy and entropy of the reaction are given for electrochemical, bimolecular, and intramolecular reactions. Solvation radii change upon electron transfer triggered bond cleavage. An iterative procedure is proposed for adapting the estimation of the solvent reorganization factor to the ensuing coupling of the fragmentation and solvent reorganization coordinates. Experimental examples illustrating applications of the theory are discussed.