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Published December 2, 2019 | Supplemental Material + Published
Journal Article Open

Calcium Valence-to-Core X-ray Emission Spectroscopy: A Sensitive Probe of Oxo Protonation in Structural Models of the Oxygen-Evolving Complex

Abstract

Calcium is an abundant, nontoxic metal that finds many roles in synthetic and biological systems including the oxygen-evolving complex (OEC) of photosystem II. Characterization methods for calcium centers, however, are underdeveloped compared to those available for transition metals. Valence-to-core X-ray emission spectroscopy (VtC XES) selectively probes the electronic structure of an element's chemical environment, providing insight that complements the geometric information available from other techniques. Here, the utility of calcium VtC XES is established using an in-house dispersive spectrometer in combination with density functional theory. Spectral trends are rationalized within a molecular orbital framework, and Kβ_(2,5) transitions, derived from molecular orbitals with primarily ligand p character, are found to be a promising probe of the calcium coordination environment. In particular, it is shown that calcium VtC XES is sensitive to the electronic structure changes that accompany oxo protonation in Mn₃CaO₄-based molecular mimics of the OEC. Through correlation to calculations, the potential of calcium VtC XES to address unresolved questions regarding the mechanism of biological water oxidation is highlighted.

Additional Information

© 2019 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Received: September 26, 2019; Published: November 19, 2019. The authors acknowledge the Max Planck Society for funding. Z.M. thanks Yvonne Brandenburger and Fabian Strunk for technical expertise with the LabXES spectrometer, as well as Dr. Ragnar Bjornsson and Dr. Vijay Chilkuri for discussions of MO theory. Z.M. acknowledges the Deutscher Akademischer Austauschdienst for M.Sc. scholarship funding. T.A. is grateful to the NIH (Grant R01-GM102687B) for supporting the synthetic model studies. The authors declare no competing financial interest.

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Published - acs.inorgchem.9b02866.pdf

Supplemental Material - ic9b02866_si_001.pdf

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