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 CrdO bonds were found to have a bond length of 1.60 Å and bridging Cr-O bonds of 1.80 Å. 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(dO) 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 (e5.0 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.

Vanadium oxide deposited as discrete oxovanadium groups, [(-O) 3 VdO], in transparent silica xerogels were investigated by resonance Raman spectroscopy. Spectra were collected at 351 and 257 nm excitation into two distinct absorption bands of the oxovanadium site. Three new bands associated with vibrations of the vanadium oxide site were observed at 496, 568, and 720 cm -1 . From these additional modes and the previously known vibrations at 1064, 1033, and 923 cm -1 an empirical force field was determined from which a normal-mode analysis of the primary stretching vibrations of the vanadium oxo group was carried out. This analysis indicates that for most of the observed bands the interfacial Si-O-V stretches are the primary component, and in fact, only the weak band at 923 cm -1 was dominated by the terminal VdO stretch. Shifts in the band positions with 18 O isotopic enrichment are in general agreement with the normal-mode analysis, moreover, the enrichment indicates that the bridging groups are generally quite labile to substitution.

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The electronic structure of isolated Cr­(VI) sites supported on silica was reinvestigated using multiple, complementary electronic spectroscopies applied to transparent xerogel monoliths. The absorption spectrum exhibits three previously reported peaks, at 22 800, 29 100, and 41 500 cm–1, as well as a previously unresolved band at ca. 36 900 cm–1. The emission is a long-lived red luminescence with λmax = 13 600 cm–1, emanating from the lowest excited state. Assignment of the excited states was facilitated using time-dependent density functional theory (TD-DFT) calculations performed on cluster models. All of the observed electronic transitions and their energies are accounted for by dioxoCr­(VI) sites. The lowest energy observed excitation at 22 800 cm–1 populates a singlet excited state, while the emitting state is the corresponding triplet state, accessed by intersystem crossing from the singlet state. Spectroscopic bands observed at 29 100, 36 900, and 41 500 cm–1 were assigned, based on the TD-DFT calculation, to spin-allowed transitions that are consistent with emission polarization anisotropy measurements. Small variations in site symmetry at Cr result principally in inhomogeneous broadening of the spectral bands, as well as a red-edge effect in the photoemission spectrum. There is no evidence for a significant contribution from five-coordinate mono-oxoCr­(VI) sites.