Comparison of Hydride, Hydrogen Atom, and Proton-Coupled Electron Transfer Reactions

A comparison of hydride, hydrogen atom, and proton-coupled electron transfer reactions is presented. Herein, hydride and hydrogen atom transfer refer to reactions in which the electrons and protons transfer between the same donor and acceptor, while proton-coupled electron transfer (PCET) refers to reactions in which the electrons and protons transfer between different centers. Within these definitions, hydride and hydrogen atom transfer reactions are typically electronically adiabatic, hence evolving on a single electronic surface. In contrast, PCET reactions are often electronically nonadiabatic since the electron transfers a longer distance through a proton transfer interface. For all three types of reactions, solute reorganization is important, particularly the hydrogen donor--acceptor mode. Solvent reorganization is critical for hydride transfer and PCET, which involve significant solute charge redistribution, but not for hydrogen atom transfer. Theoretical descriptions and simulation methodology for all three types of reactions are presented, as well as experimentally relevant applications to hydride transfer in enzymes and PCET in solution.