| Abstract
| - High-valent Mn-oxo species have been suggested to have a catalytically important role in thewater splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopyin order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species.Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectorswith respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of themetal site was then studied by measuring the polarization dependence of X-ray absorption near-edgespectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of densityfunctional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinatedin a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along theMn⋮N axis that can be assigned to a strong 3dz2−4pz mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edgepeak. This component was interpreted as a 1s to 3dxz,yz transition with 4px,y mixing, due to the displacementof the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations,and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of theseresults to understanding the mechanism of the photosynthetic water oxidation is discussed.
|
