On the electronic and structural properties of tri-niobium oxide clusters Nb3O(n)- (n = 3-8): photoelectron spectroscopy and density functional calculations.

The electronic and structural properties of a series of triniobium oxide clusters, Nb(3)O(n)(-) and Nb(3)O(n) (n = 3-8), are investigated using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. PES spectra are obtained for Nb(3)O(n)(-) (n = 3-8) at various photon energies and are used to compare with the DFT calculations. A trend of sequential oxidation is observed as a function of O content until Nb(3)O(8)(-), reaching the highest oxidation state of Nb. DFT calculations are performed to search for the lowest energy structures for both the anionic and neutral clusters. For Nb(3)O(3)(-), the three O atoms are shown to prefer the bridging sites of a triangular Nb(3), leading to two nearly degenerate cyclic structures of C(s) symmetry. The next three O atoms from n = 4-6 each occupy a terminal site directly bonded to Nb, resulting in a symmetric Nb(3)O(6)(-) with C(3v) symmetry and a low-lying isomer of C(s) symmetry. The seventh O atom is bonded to two Nb atoms forming a double bridge, whereas the eighth O atom is bonded to a terminal site so that in Nb(3)O(8)(-) each Nb atom reaches its maximum oxidation state of +5. The structures and electronic states for the triniobium oxide clusters are significantly different from the corresponding tritantalum oxide clusters, in particular, for Nb(3)O(3)(-), Nb(3)O(5)(-), and Nb(3)O(7)(-). Molecular orbital analyses are performed to elucidate the chemical bonding and the electronic and structural evolution in these triniobium oxide clusters.