Scheme 1.

Reaction diagram illustrating the difference between stepwise ETPT and PTET processes (edges) and CPET process (diagonal) for a generic Mⁿ⁺-oxo species. The literature contains several distinctions between CPET and hydrogen-atom transfer (HAT) steps. While HAT is commonly, but not always, taken to mean transfer of an electron and proton from one donor to one acceptor, CPET can also encompass transfer of an electron to one donor and transfer of a proton to a separate donor (so-called multi-site concerted CPET, or MS-CPET). See ref. ²⁷ for a deeper discussion of these terms. Alternatively, Hammes-Schiffer has distinguished these pathways on the basis of proton transfer occurring by an electronically adiabatic (HAT) or nonadiabatic (CPET) process.²⁹