Electron Transfer Reactions in Solution

Abstract

In this section, the general theories of electron transfer reactions with a stress on the relation between adiabaticity and the reaction coordinate are described. Although adiabaticity of reactions has been rarely explained in chemistry textbooks, proper knowledge of adiabaticity and the reaction coordinate is essential for understanding chemical reactions. Generally, chemical reactions proceed along the diabatic surface of the reactant which intersects with the diabatic surface of the product at the transition state. Perturbation theory for a degenerate system along the reaction coordinate defines whether the degeneration is resolved (lifted) at the crossing point or not: degeneration is resolved and two adiabatic surfaces emerge at the crossing point only when the symmetries of two diabatic surfaces are identical to each other. Thermal reactions are allowed to proceed along the lower-energy adiabatic surface, while no thermal reaction is allowed to occur when two diabatic surfaces cross at the transition state. The degree of separation between two adiabatic surfaces determines the probability of successful thermal electron transfer reactions, and therefore the adiabaticity of electron transfer reactions depends on the degree of coupling between the orbitals of the reactants which is related to the distance between the reactants.