Transition metal oxide clusters with character of oxygen-centered radical: a DFT study

Density functional theory (DFT) calculations are applied to study the structure and bonding properties of groups 3–7 transition metal oxide clusters M _x=1–3O _y ^q and Sc _x=4–6O _y ^q with 2y ? nx + q = 1, in which n is the number of metal valence electrons and q is the charge number. These clusters include MO₂, M ₂O₃ ⁺, M ₂O₄ ^?, and M ₃O₅ (M = Sc, Y, La); MO₂ ⁺, MO₃ ^?, M ₂O₄ ⁺, M ₂O₅ ^?, M ₃O₆ ⁺, and M ₃O₇ ^? (M = Ti, Zr, Hf), and so on. The obtained lowest energy structures of most of these clusters are with character of oxygen-centered radical (O^·). That is, the clusters contain oxygen atom(s) with the unpaired electron being localized on the 2p orbital(s). Chromium and manganese oxide clusters (except CrO₄ ^?) do not contain O^· with the adopted DFT methods. The binding energies of the radical oxygen with the clusters are also calculated. The DFT results are supported by available experimental investigations and predict that a lot of other transition metal oxide clusters including those with mixed-metals (such as TiVO₅ and CrVO₆) may have high oxidative reactivity that has not been experimentally identified. The chemical structures of radical oxygen over V₂O₅/SiO₂ and MoO₃/SiO₂ catalysts are suggested and the balance between high reactivity and low concentration of the radical oxygen in condensed phase catalysis is discussed.