Resonance Raman spectroscopy of discrete silica-supported vanadium oxide

Vanadium oxide deposited as discrete oxovanadium groups, [(-O)(3) V=O, 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 V=O stretch. Shifts in the band positions with O-18 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.