Proton-coupled electron transfer

Some of the important conclusions reached in this analysis of PCET are summarized below. The sections in which they are discussed are also cited. (1) PCET describes reactions in which there is a change in both electron and proton content between reactants and products. It originates from the influence of changes in electron content on acid-base properties and provides a molecular-level basis for energy transduction between proton transfer and electron transfer (section 1). (2) Coupled electron-proton transfer or EPT is defined as an elementary step in which electrons and protons transfer from different orbitals on the donor to different orbitals on the acceptor. There is (usually) a clear distinction between EPT and H-atom transfer (HAT) or hydride transfer, in which the transferring electrons and proton come from the same bond. Hybrid mechanisms exist in which the elementary steps are different for the reaction partners (sections 5.1 and 5.2). (3) EPT pathways such as PhO*/PhOH exchange have much in common with HAT pathways in that electronic coupling is significant, comparable to the reorganization energy with HDA ∼ λ. (4) Multiple-Site Electron-Proton Transfer (MS-EPT) is an elementary step in which an electron-proton donor transfers electrons and protons to different acceptors, or an electron-proton acceptor accepts electrons and protons from different donors. It exploits the long-range nature of electron transfer while providing for the short-range nature of proton transfer (section 5.1). (5) A variety of EPT pathways exist, creating a taxonomy based on what is transferred, e.g., le-12H+ WEPT (section 5.1). (6) PCET achieves "redox potential leveling" between sequential couples and the buildup of multiple redox equivalents, which is of importance in multielectron catalysis (section 2. 1). (7) There are many examples of PCET and pH-dependent redox behavior in metal complexes, in organic and biological molecules, in excited states, and on surfaces (section 2). (8) Changes in pH can be used to induce electron transfer through films and over long distances in molecules. Changes in pH, induced by local electron transfer, create pH gradients and a driving force for long-range proton transfer in Photosysem 11 and through other biological membranes (sections 3 and 7.2). (9) In EPT, simultaneous transfer of electrons and protons occurs on time scales short compared to the periods of coupled vibrations and solvent modes. A theory for EPT has been developed which rationalizes rate constants and activation barriers, includes temperature- and driving force (ΔG)-dependences implicitly, and explains kinetic isotope effects. The distance-dependence of EPT is dominated by the short range nature of proton transfer, with electron transfer being far less demanding (sections 4.2 and 5). (10) Changes in external pH do not affect an EPT elementary step. Solvent molecules or buffer components can act as proton donor acceptors, but individual H2O molecules are neither good bases (pKa(H3)O+) = - 1.74) nor good acids (pK(H2O) = 15.7) (section 5.5.3). (11) There are many examples of mechanisms in chemistry, in biology, on surfaces, and in the gas phase which utilize EPT (sections 6 and 7). (12) PCET and EPT play critical roles in the oxygen evolving complex (OEC) of Photosystem Il and other biological reactions by decreasing driving force and avoiding high-energy intermediates (section 7.2). © 2007 American Chemical Society.