Water (in Water) as an Intrinsically Efficient Proton Acceptor in Concerted Proton Electron Transfers

The oxidation of PhOH in water by photochemically generated Ru-III(bpy)(3) is taken as prototypal example disclosing the special character of water, in the solvent water, as proton acceptor in concerted proton-electron transfer reactions. The variation of the rate constant with temperature and driving force, as well as the variation of the H/D kinetic isotope effect with temperature, allowed the determination of the reaction mechanism characterized by three intrinsic parameters, the reorganization energy, a pre-exponential factor measuring the vibronic coupling of electronic states at equilibrium distance, and a distance-sensitivity parameter. Analysis of these characteristics and comparison with a standard base, hydrogen phosphate, revealed that electron transfer is concerted with a Grotthus-type proton translocation, leading to a charge delocalized over a cluster involving several water molecules. A mechanism is thus uncovered that may help in understanding how protons could be transported along water chains over large distances in concert with electron transfer in biological systems.