Polarized x-ray absorption near-edge structure of highly oxidized chromium porphyrins

Abstract

Polarized X-ray absorption near-edge spectra have been measured for Cr/sup IV/(TTP)O and Cr/sup V/(TTP)N (TTP is the dianion of 5,10,15,20-tetra-p-tolylporphyrin). These spectra, which are very similar for the two molecules, show an intense preedge absorption feature polarized perpendicular to the porphyrin plane. The absorption edges are relatively featureless when the polarization is parallel to the porphyrin plane. On the basis of multiple-scattered-wave X..cap alpha.. calculations, the intense preedge feature is interpreted as a bound ..-->.. bound transition with significant metal character in the excited state. This transition, which has been observed in many metal-oxo species, was not observed in the putative ferryl (Fe=o) intermediates of horseradish peroxidase. The reasons for this absence in Fe=O species and its relationship to the reactivity of metal-oxo porphyrin complexes are discussed in light of the present results.