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FTIR Spectra and Normal-Mode Analysis of a Tetranuclear Manganese Adamantane-like Complex in Two Electrochemically Prepared Oxidation States:  Relevance to the Oxygen-Evolving Complex of Photosystem II

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journal contribution
posted on 22.08.2002, 00:00 by Hendrik Visser, Christopher E. Dubé, William H. Armstrong, Kenneth Sauer, Vittal K. Yachandra
The IR spectra and normal-mode analysis of the adamantane-like compound [Mn4O6(bpea)4]n+ (bpea = N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, MnIV4 and MnIIIMnIV3, that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn−O vibrational modes are identified with isotopic exchange, 16O→18O, of the mono-μ-oxo bridging atoms in the complex. IR spectra of the MnIIIMnIV3 species are obtained by electrochemical reduction of the MnIV4 species using a spectroelectrochemical cell, based on attenuated total reflection [Visser, H.; et al. Anal. Chem. 2001, 73, 4374−4378]. A novel method of subtraction is used to reduce background contributions from solvent and ligand modes, and the difference and double-difference spectra are used in identifying Mn−O bridging modes that are sensitive to oxidation state change. Two strong IR bands are observed for the MnIV4 species at 745 and 707 cm-1, and a weaker band is observed at 510 cm-1. Upon reduction, the MnIIIMnIV3 species exhibits two strong IR bands at 745 and 680 cm-1, and several weaker bands are observed in the 510−425 cm-1 range. A normal-mode analysis is performed to assign all the relevant bridging modes in the oxidized MnIV4 and reduced MnIIIMnIV3 species. The calculated force constants for the MnIV4 species are frIV = 3.15 mdyn/Å, frOr = 0.55 mdyn/Å, and frMnr = 0.20 mdyn/Å. The force constants for the MnIIIMnIV3 species are frIV = 3.10 mdyn/Å, frIII = 2.45 mdyn/Å, frOr = 0.40 mdyn/Å, and frMnr = 0.15 mdyn/Å. This study provides insights for the identification of Mn−O modes in the IR spectra of the photosynthetic oxygen-evolving complex during its catalytic cycle.