Datensatz

Zusammenfassung

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.