ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S
4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of N
O. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)
4–O
(5) bond and the π*-LUMO of the Mn(V)
1=O
(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn
4O
6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)
1=O
(6) bond to the σ*-LUMO of the Mn(IV)
4–O
(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)
4–O
(5)]
-? part is relaxed to the ?Mn(III)
4?…?O
(5)- structure and the cation radical [O
(6)=Mn(V)
1]
+ ? part is relaxed to the
+O
(6)–Mn(IV)
1? structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)
4->Mn(III)
4 and Mn(V)
1->Mn(IV)
1. On the other hand, the O
(5)- and O
(6)+ sites generated undergo the O–O bond formation in the CaMn
4O
6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn
4O
6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)
1=O
(6) and σ*-LUMO of Mn(IV)
4–O
(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism.
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