Probing the electronic and structural properties of the niobium trimer cluster and its mono- and dioxides: Nb3On- and Nb3On (n = 0-2).

We report a photoelectron spectroscopy and density functional theory (DFT) study on the electronic and structural properties of Nb(3)(-), Nb(3)O(-), Nb(3)O(2)(-), and the corresponding neutrals. Well-resolved photoelectron spectra are obtained for the anion clusters at different photon energies and are compared with DFT calculations to elucidate their structures and chemical bonding. We find that Nb(3)(-) possesses a C(2v) ((3)A(2)) structure, and Nb(3) is a scalene C(s) ((2)A'') triangle. Both Nb(3)O(-) and Nb(3)O are found to have C(2v) structures, in which the O atom bridges two Nb atoms in a Nb(3) triangle. The ground-state of Nb(3)O(2)(-) is found surprisingly to be a low symmetry C(1) ((1)A) structure, which contains a bridging and a terminal O atom. Molecular orbital analyses are carried out to understand the structures and bonding of the three clusters and provide insights into the sequential oxidation from Nb(3)(-) to Nb(3)O(2)(-). The terminal NbO unit is common in niobia catalysts, and the Nb(3)O(2)(-) cluster with a NbO unit may be viewed as a molecular model for the catalytic sites or the initial oxidation of a Nb surface.