Another recent mechanism (Figure 5.9), described by Messinger and colleagues, invokes a different arrangement of water. In this proposal, the O~O bond forms
Figure 5.9 The Messinger model for oxygen evolution. Contrary to the other three main models of water oxidation, Messinger and colleagues implicate a bridging oxygen radical, instead of a terminal Mn=O as the reactive species in the S4 state. (Figure reproduced from reference 110.)
between a bridging oxygen atom and a terminal hydroxide bound to Ca2 +. The primary rationale for this choice in the mechanism hinges on the kinetic data that has been obtained for water exchange, discussed in Section 5.3.3 [95, 110]. Messinger and colleagues suggest that the rapid exchanging water is bound to Ca2 +, with the other, more slowly exchanging water, occupying a bridging position between two Mn atoms. In addition, this mechanism follows the interpretation of the XANES data discussed in Section 5.3.2, and leads to the prediction that oxida tion on the S2-S3 transition occurs at the bridging oxygen position invoked to form the O_O bond, and not Mn as has been proposed in the other mechanisms as well as the time resolved XANES data. This mechanism has support from quantum mechanical calculations conducted by Siegbahn and colleagues in which the stability of an oxygen centered radical was demonstrated and suggested to be more likely than a Mn=O reactive species in S4 .
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