X-Ray Absorption And Emission Spectroscopy Of Manganese Complexes Relevant To Photosystem Ii

Abstract

A series of high-spin Mn monomers has been investigated by Mn K[beta] x-ray emission spectroscopy (XES) and by Mn K-edge x-ray absorption spectroscopy (XAS). The XES and XAS were coupled with density functional theory (DFT) calculations. The XES K[beta] main lines are dominated by 3p-3d exchange correlation effects and the DFT calculations show that the valence to core spectra are dominated by Mn np to 1s electric dipole allowed transitions, and are sensitive to metal spin state, oxidation state, ligand identity, and metal-ligand bond lengths. The time-dependent DFT (TDDFT) calculations reveal that all experimentally observed XAS preedge features correspond either to Mn 1s to 3d transitions, or to metal-to-ligand charge transfer features that correspond to transitions into empty [pi]* orbitals of either [pi]-donor or [pi]-acceptor ligands. The ability of TDDFT to reproduce the experimentally observed features at the correct relative energies depends on the nature of the transition. XAS, XES, and DFT calculations were also used to evaluate the protonation states of the ([MICRO SIGN]-O)2 bridges in a [Mn2(IV)([MICRO SIGN]-O)2(salpn)2] dimer. The XAS spectra exhibit distinct differences in the pre-edge region while maintaining the same edge energy, and the XES valence to core regions show significant changes in peak position and intensity upon protonation, particularly in the satellite region. The most intense preedge features in the XAS result from transitions into the unoccupied orbitals of local eg character, while the K[beta]2,5 and satellite peaks in the XES arise primarily from ligand 2p to Mn 1s and ligand 2s to Mn 1s transitions, respectively. The extended x-ray absorption fine structure (EXAFS), and the corresponding first principle calculated EXAFS, were also examined for another series of seven Mn monomers and dimers. The first shell distances are generally well predicted, although there are larger variations in the multiple scattering contributions. The largest deviations between calculated and experimental EXAFS are in the Debye-Waller (DW) factors, although slightly better agreement is obtained by modeling DW factors using the Dynamical Matrix method, as opposed to the more traditional Correlated Debye method. These findings have important implications for applications to manganese active sites in biological and chemical catalysis.