Modeling the free energy surfaces of electron transfer in condensed phases

A three parameter model for the free energy surfaces of electron transfer (ET) in condensed phases was presented. This model is based on the Hamiltonian of a two-state solute linearly coupled to a harmonic, classical solvent mode with different force constants in the initial and final states. It extends the two parameter Marcus-Hush model by introducing the third parameter α₁ reflecting the variation of the force constant of the classical solvent mode driving ET. Its validity is equivalent to the condition of thermodynamic stability of the driving solvent mode. The model parameters are fully defined by the first two equilibrium cumulants of the ET reaction coordinate.