Do Mono-oxo Sites Exist in Silica-Supported Cr(VI) Materials? Reassessment of the Resonance Raman Spectra

The monomeric, single-atom oxochromium species present on the surface of silica-supported Cr(VI) catalysts was characterized in detail using resonance Raman (RR) spectroscopy over a range of excitation wavelengths corresponding to the primary electronic transitions of Cr(VI)/SiO2. The findings resolve a long-standing controversy regarding the possible contribution of mono-oxoCr(VI) sites, (SiO)4Cr═O, postulated to coexist with the well-established dioxoCr(VI) sites, (SiO)2Cr(═O)2. Density functional theory (DFT) calculations and a normal coordinate analysis conducted using a chromasiloxane model cluster confirm prior assignments of bands in the nonresonant Raman spectrum at 986 and 1001 cm–1 to the symmetric and antisymmetric stretching modes, respectively, of the dioxoCr(VI) sites. For all excitation energies, the symmetric stretch shows apparent resonant enhancement. Since all of the electronic transitions are strongly allowed, this finding is consistent with A-term enhancement. UV excitation at 257 nm ...