Choice of the reaction coordinate in electron-transfer reactions in solution

The electrochemical reduction of methyl chloride in water to give methyl radical and chloride anion has been studied by the Monte Carlo method. The carbon-chlorine distance (dC-Cl) has been chosen to define the reaction coordinate, and a standard statistical perturbation theory has been used to obtain the Helmholtz energy change between the dividing surfaces defined by each particular value of dC-Cl. The corresponding transition state, in the sense of the variational transition-state theory, has been located and analysed. The results have been compared with those obtained as a function of the reaction coordinate ΔE in a previous work, defined as the difference between the diabatic energy hypersurfaces for each configuration of the system. We conclude that the non-geometrical parameter, ΔE, is the best choice for the reaction coordinate.