Solvent Dynamical Effects in Electron Transfer: Electrochemical versus Self-Exchange Kinetics of Tris(Hexafluoroacetylacetonato)Ruthenium(III/II) and Comparison with Other Probe Reactants

摘要

A number of recent experimental studies have focused on the manner and extent towhich solvent dynamics ('solvent friction') can influence the barrier-crossing frequency for electron-transfer reactions. Our initial examinations of solvent-dependent electrochemical kinetics of metallocenium-metallocene and other redox couples have been supplemented more recently by detailed studies of a number of related homogeneous self-exchange reactions. The latter class of systems offer several advantages for this purpose; in particular, the availability of optical electron-transfer energies for related binuclear systems provides direct experimental estimates of the required solvent-dependent barrier heights, Delta G. A question of central significance concerns the degree to which the influence of solvent dynamics upon the electron-transfer rates can be sensitive to the reactant structure and environment as a consequence of variations in the donor-acceptor orbital interactions. When this 'orbital overlap', as gauged by the electronic matrix coupling element, H12, is relatively small the barrier-crossing frequency is predicted to be controlled primarily instead by the 'electron-hopping probability' so that the reaction rates will be insensitive to, and ultimately independent of, the solvent dynamics. Keywords: Solid state physics, Electrochemistry, Ruthenium, Refractory metals, Kinetics, Reprints. (jg)