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Spectroscopically consistent Mn oxidation state assignments of the natural water splitting catalyst

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Krewald,  Vera
Research Department Neese, Max Planck Institute for Chemical Energy Conversion, Max Planck Society;

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Retegan,  Marius
Research Department Neese, Max Planck Institute for Chemical Energy Conversion, Max Planck Society;

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Neese,  Frank
Research Department Neese, Max Planck Institute for Chemical Energy Conversion, Max Planck Society;

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Pantazis,  Dimitrios A.
Research Department Neese, Max Planck Institute for Chemical Energy Conversion, Max Planck Society;

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Citation

Krewald, V., Retegan, M., Neese, F., & Pantazis, D. A. (2014). Spectroscopically consistent Mn oxidation state assignments of the natural water splitting catalyst. Talk presented at 12th European Biological Inorganic Chemistry Conference. Zurich, Switzerland. 2014-08-24 - 2014-08-28. doi:10.1007/s00775-014-1160-3.


Cite as: http://hdl.handle.net/21.11116/0000-0007-A3AF-C
Abstract
Studying nature’s water splitting machinery is not only of fundamental interest, but also driven by the vision that the understanding of its geometric and electronic construction principles could guide the design of artificial water oxidizing catalysts, an essential requirement for a hydrogen-based energy economy. In nature, sunlight is converted into chemical energy by oxidizing water and sequentially releasing electrons, protons and O2 at different steps of the catalytic cycle, known as the Kok cycle of S0–S4 intermediates (see scheme) [1]. Despite decades of research, a central aspect of the electronic structure of the natural catalyst is still debated: the oxidation states of the four Mn ions. Two paradigms exist that differ by two electrons, the ‘‘low’’ oxidation state scheme with Mn(III)3Mn(IV) for the S2 state, and the ‘‘high’’ oxidation state scheme with Mn(III)Mn(IV)3 for the S2 state [2-4]. We use quantum chemical methods to evaluate two sets of models for all S-states of the Kok cycle, conforming to both oxidation state schemes, analyzing for the first time the two formulations with a common methodological approach. Comparison of calculated properties with state-specific experimental information available from EXAFS (Mn–Mn distances) and EPR/ENDOR (spin states, 55Mn hyperfine couplings) reveals that the ‘‘low’’ oxidation state scheme models are incompatible with experiment. On the other hand the sequence of models in the ‘‘high’’ oxidation state scheme is fully consistent with experimental data, and furthermore consistent with the electron/proton release events of the catalytic cycle.