Dynamics of electron transfer in the oxidation of water by chlorophyll a dimer

The observed temperature dependence of the rate constant for reduction of (Chl a.2H2O)2+. by water is fitted to a theoretical expression for the rate of an intramolecular electronic radiationless transition. The theory assumes a single effective mediating mode and the best-fit value for the frequency of this mode is omega(m) = 680 cm(-1), which corresponds to high-frequency, intermolecular librations in water. From an explicit calculation of the nonadiabatic-coupling matrix element, an average electron-transfer distance of 11 A is obtained, consistent with molecular dimensions available for this system.