Probing the Electronic Structure of Early Transition-Metal Oxide Clusters:  Polyhedral Cages of (V₂O₅)_n^- (n = 2−4) and (M₂O₅)₂^- (M = Nb, Ta)

Vanadium oxide clusters, (V₂O₅)_n, have been predicted to possess interesting polyhedral cage structures, which may serve as ideal molecular models for oxide surfaces and catalysts. Here we examine the electronic properties of these oxide clusters via anion photoelectron spectroscopy for (V₂O₅)_n^- (n = 2−4), as well as for the 4d/5d species, Nb₄O₁₀^- and Ta₄O₁₀^-. Well-resolved photoelectron spectra have been obtained at 193 and 157 nm and used to compare with density functional calculations. Very high electron affinities and large HOMO−LUMO gaps are observed for all the (V₂O₅)_n clusters. The HOMO−LUMO gaps of (V₂O₅)_n, all exceeding that of the band gap of the bulk oxide, are found to increase with cluster size from n = 2−4. For the M₄O₁₀ clusters, we find that the Nb/Ta species yield similar spectra, both possessing lower electron affinities and larger HOMO−LUMO gaps relative to V₄O₁₀. The structures of the anionic and neutral clusters are optimized; the calculated electron binding energies and excitation spectra for the global minimum cage structures are in good agreement with the experiment. Evidence is also observed for the predicted trend of electron delocalization versus localization in the (V₂O₅)_n^- clusters. Further insights are provided pertaining to the potential chemical reactivities of the oxide clusters and properties of the bulk oxides.