Development of broken-symmetry (BS) methods in chemical reactions. A theoretical view of water oxidation in photosystem II and related systems

Abstract

Fundamental concepts and basic theories for the broken-symmetry (BS) methods have been reviewed in relation to theoretical elucidation and understanding of the mechanism for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII). The HOMO-LUMO mixings by the BS method have provided the BS orbitals which are mainly localized on the metal and oxygen sites of the high-valent transition-metal oxo (MO) bonds, respectively. The oxyl-radical character (M–O) is responsible for radical reactivity such as the radical coupling in accord with various experimental results. The Lewis acids play important roles for reduction of the oxyl-radical character, indicating the participation of the water-coordinated Ca(II) ion of the CaMn3O4 cubane to stabilization of the Mn(V)=O…Ca(II) bond for essentially non-radical reactions. Several chemical indices have been calculated to elucidate the radical character for quantitative purpose. Implications of the computational results are discussed in relation to possible mechanisms of water oxidation in OEC of PSII.