| Abstract
| - The competition between stepwise and concerted (CPET) pathways in proton-coupled electron-transfer reactions in water is discussed on thermodynamic and kinetic bases. In the case where water isthe proton acceptor, the CPET pathway may compete favorably with the stepwise pathway. The mainparameter of the competition is pK of the oxidized form of the substrate being smaller or larger than 0. Thedriving force of the forward reaction is however independent of pH, despite the equilibrium redox potentialof the proton−electron system being a function of pH. At high pH values, CPET reactions involving OH-as proton acceptor may likewise compete favorably with stepwise pathways. The overall reaction rateconstant is an increasing function of pH, not because the driving force depends on pH but because OH-is a reactant. In buffered media, association of the substrate with the basic components of the buffer offersan alternative CPET route; the driving force comes closer to that offered by the pH-dependent equilibriumredox potential.
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