Theory of chemical bonds in metalloenzymes. XVII. Symmetry breaking in manganese cluster structures and chameleonic mechanisms for the O-O bond formation of water splitting reaction

摘要

Symmetry breaking in cluster structures of manganese oxides by doping of Ca(II) ion is examined in relation to chameleonic mechanisms of water splitting reaction. The orbital and spin correlation diagrams have been depicted to clarify one-electron transfer and electron-pair transfer mechanisms for the reaction. The spin-polarized molecular orbital models have been applied to elucidate correspondence between magnetic-coupling mode and reaction mechanism of the oxygen-oxygen (O-O) bond formation and oxygen evolution catalyzed by multicenter Ca(II) manganese oxides and related systems. The present UB3LYP calculations followed by the natural orbital analyses have been performed to elucidate electronic structures of the key intermediates and the transition state structure for the O-O bond formation. The results indicate that the reaction proceeds through the continuous diradicaloid mechanism without discreet free radical fragments and/or electron-pair transfer mechanism induced by symmetry breaking with Ca(II) in the pure low-spin singlet state. The computational results are compatible with local singlet and triplet diradical-coupling mechanisms for the O-O bond formation in the low- and high-spin states, respectively. Thus, magnetic (exchange) coupling modes in the oxygen evolution complex are directly related to the local singlet and triplet diradical mechanisms as in the case of soluble methane monooxygenase.