The Role of Collective Solvent Coordinates and Nonequilibrium Solvation in Charge-Transfer Reactions

In this article we discuss how the solvent energy-gap reaction coordinate arising in weak-overlap charge-transfer theory may be generalized to include nonequilibrium solvation effects in variational transition state theory (VTST) for arbitrary reactions of neutral or charged species. The discussion also indicates how the parameters of weak-overlap charge-transfer theory may be related to the potential energy surface of a reaction and the potential of mean force. This synthesis of aspects of weak-overlap charge-transfer theory and variational transition state theory provides a clearer picture of the usefulness and limitations of the energy-gap coordinate in Marcus theory or in more general contexts. Furthermore, it furnishes a new view of the role of solvent coordinates in electronically adiabatic reactions. As an example, we show how VTST provides a way to treat solute vibrational coordinates and collective solvation coordinates in a comparable way in the computational modeling of aqeuous proton-transfer reactions including quantum mechanical vibrations and multidimensional tunneling effects.