Coordination environment and vibrational spectroscopy of Cr(VI) sites supported on amorphous silica

Chromium(VI) sites homogeneously dispersed in a transparent silica xerogel matrix have been investigated to determine the coordination environment and rationalize the Raman spectra. X-ray absorption fine structure (EXAFS) analysis gives a structure that is consistent with Cr containing two terminal oxygens and is bound to the silica by two Cr-O-Si linkages. The structure was refined to an R factor of 1.28%. The terminal Cr=O bonds were found to have a bond length of 1.60 angstrom and bridging Cr-O bonds of 1.80 angstrom. The Raman spectrum, collected with 785 nm excitation above the absorption edge of the chromium, shows a strong band at 986 cm(-1) and a resolved shoulder at 1001 cm(-1). Isotopic labeling and polarization studies of low concentrations of Cr (0.5 mol %) indicate that the strong 986 cm(-1) band is the totally symmetric Cr(=O)(2) mode; however, the isotopic shift and strong polarization of the 1001 cm(-1) mode preclude it from being the antisymmetric component of the terminal dioxo stretch. At higher concentrations (<= 5.0 mol %) the high-energy shoulder becomes a resolved peak at 1004 cm(-1). While isotopic labeling shifts the peak to a position predicted for the antisymmetric stretch, the polarization ratio increases but does not reach a value that is unambiguous for an antisymmetric mode.