Theory of highly exothermic electron transfer reactions

Abstract

The theory of highly exothermic homogeneous outer-sphere electron transfer reactions is discussed for transfers occurring over a range of distances. A finite rate of diffusion of reactants and their long-range force are treated by solving the diffusion equation numerically for the reactant pair distribution function. Steady-state solutions for the bimolecular rate constant are compared with experimental data as well as with our recent approximate analytic solution, which is found to agree in the present case. On the basis of short-time solutions, it is proposed that experiments which measure electron transfer rates at short times following the onset of reaction improve the possibility of observing the inverted effect in bimolecular systems. The chance of seeing it in linked systems (unimolecular reactions) is even greater. The relation between the prediction of an “inverted region” in the rate constant vs. ΔG° plot and the existence of a maximum in charge transfer spectral plots of intensity vs. absorption frequency is pointed out.