Monte Carlo simulation of the diabatic free energy curves for a dissociative electron transfer reaction in a polar solvent

In the present work we have carried out a Monte Carlo simulation of a dissociative electron transfer reaction in a polar solvent. In particular, we have chosen as a very simple model the electrochemical reduction of hydrogen fluoride to give a hydrogen atom and a fluoride anion in a dipolar solvent. From a classical point of view, the electron transfer occurs at the intersection region S* of the diabatic potential hypersurfaces H_pp and H_ss, corresponding to the precursor and successor complexes, respectively. We have evaluated both diabatic surfaces using potential functions that have been built up with ab initio methods by us. For each of the obtained configurations the parameter ΔE = H_ss – H_pp has been calculated. This parameter is then used as the reaction coordinate for obtaining the diabatic free energy curves of the reaction. Because the activation energy is high, a suitable mapping potential along with the statistical perturbation theory is employed to force the system to evolve toward the intersection region S*. A total of 68,340,000 configurations have been generated. The main conclusion of this article is that Marcus' relationship seems to fail for this kind of inner-sphere processes. © 1992 by John Wiley & Sons, Inc.