Proton-coupled electron transfer: classification scheme and guide to theoretical methods

Proton-coupled electron transfer (PCET) reactions, which involve the transfer of both electrons and protons, are essential for many energy conversion processes. The relative timescales of the electrons, transferring proton(s), solute, and solvent, as well as the couplings among these components, vary widely among PCET processes. This paper presents a scheme for the classification of PCET reactions in terms of these timescales and couplings, which are framed in the context of electronic, vibrational, and vibronic nonadiabaticity. A guide to the theoretical methods that are applicable in the various regimes is also provided. In addition, theoretical calculations and physical properties that can be used to classify specific experimentally relevant systems are discussed. The ability to classify PCET reactions will assist in the development of more accurate theoretical treatments and will aid in the interpretation of experimental data. This progress will lead to a more comprehensive understanding of energy conversion processes that rely on PCET reactions.