Activation energy of photosynthetic oxygen evolution: An attempt at theoretical analysis

Abstract

In this study, a first attempt is made to calculate theoretically the activation energies of the different steps of water oxidation. The analysis has led to the following conclusions about the process mechanism: (1) the barrier produced by the repulsion of unbound O-atoms during their mutual approach may be partly surmounted at the expense of the binding energy of water molecules by manganese ions; (2) the concerted transfer of electrons and protons involving the participation of bases stronger than water improves essentially the process energetics; (3) multi-electron steps require the use of several one-electron acceptors; (4) four-electron oxidation of water in a single elementary act is unlikely: (5) the most likely reaction path is the rate-determining two-electron water oxidation to hydrogen peroxide (the possibility of this process occurring in two consecutive one-electron steps is not clear), followed by two fast oxidation steps of H 2O 2 to HO 2 and further to O 2.