Polarized x-ray absorption spectroscopic studies of iron porphyrins and metal hexammines.

摘要

X-ray absorption near edge structure (XANES) contains important electronic and geometric information, complementing the information available from extended X-ray absorption fine structure (EXAFS). Polarized and powder K-edge absorption spectra have been measured for a series of iron porphyrins and cobalt and chromium hexammines.; These studies explored experimentally the angular dependence of the 1s {dollar}to{dollar} 3d transitions. The results show that for centrosymmetric compounds, where dipole transitions are forbidden, quadrupole coupling is the main source of intensity for these transitions and the angular dependence of these transitions can be used to determine the d electron configuration of the metal. In cases where a center of symmetry is absent, dipole transitions are dominant. For a series of Fe porphyrin dimers, the pre-edge transitions were successfully interpreted using the polarized XANES combined with molecular orbital calculations.; The polarization dependence of the edge features for cobalt and chromium hexammine indicates that the shoulder structures on the edge contain scattering contributions from remote atoms. Based on this result, the previous assignments of these features as 1s {dollar}to{dollar} 4s or 1s {dollar}to{dollar} 4p transitions are not appropriate in this case. These results support the proposal that in many cases, XANES can be treated in the same way as EXAFS.; The polarized XANES study also shows that edge energy is a function of several factors such as formal oxidation state, bond length, and ligand type. This implies that, while it is possible to monitor oxidation state change, caution is needed when attempting to determine oxidation state directly from edge energy.; A study of multiple scattering in the EXAFS spectra for Fe porphyrin dimers shows that the scattering amplitude enhancement due to the focusing effect is strongly dependent on the bond strength. This makes angle determination using the focusing effect considerably more problematic than previously thought. However, the temperature dependence of the focusing effect provides a method to obtain dynamic information about the bridging atom.