ELECTRONIC-STRUCTURE OF DISCRETE PSEUDOTETRAHEDRAL OXOVANADIUM CENTERS DISPERSED IN A SILICA XEROGEL MATRIX - IMPLICATIONS FOR CATALYSIS AND PHOTOCATALYSIS

The electronic structure of pseudotetrahedral oxovanadium groups (-O3V =O) dispersed in a silica xerogel matrix, is determined on the basis o f a spectroscopic investigation. From this investigation it was found that the highest occupied molecular orbital of this species is a nonbo nding a(2) symmetry orbital localized on the basal plane ligands. The first excited state is assigned to an E symmetry triplet resulting fro m a one-electron promotion from this a(2) nonbonding orbital to an e s ymmetry antibonding orbital of the terminal V=O group. On the basis of this orbital description, the long-lived, vibronically structured emi ssion at 549 nm is assigned to a (3)E --> (1)A(1) transition from the e antibonding orbital back down to the a nonbonding orbital [(a(2))(1) (e)(1)] --> [(a(2))(2)(e*)]. The vibronic progression in the emission band at 977 +/- 10 cm(-1), previously assigned to the terminal V=O st retch, is reassigned to a V-O stretch involving the basal plane oxygen s, consistent with the orbital assignment. Contrary to previous descri ptions, excitations involving pi --> pi type transitions localized on the terminal V=O group Lie at higher energy. The first well-resolved singlet band at 290 nm is of A(1) symmetry and has a resolved vibronic progression which corresponds to the terminal V=O stretch. This band is assigned to a (1)A(1) --> (1)A(1) transition involving a [(e)(4)(a( 2))(2)(e)] --> [(e)(3)(a(2))(2)(e*)(1)] one-electron promotion which can qualitatively be described as a ''pi-pi'' V=O transition. The ele ctronic structure of the pseudotetrahedral oxovanadium group establish ed in this study differs dramatically from the conventionally accepted model which localizes the ground and first excited state on the termi nal V=O group. This new description, however, is completely consistent with observed photochemical processes and, unlike the previous model, provides a coherent explanation of how factors such as the nature of the substrate directly affect the oxovanadium center.