Onset of carbon–carbon bonding in the Nb5Cy (y = 0–6) clusters: a threshold photo-ionisation and density functional theory study

Abstract

We have used photo-ionisation efficiency spectroscopy to determine the ionisation potentials (IPs) of the niobium–carbide clusters, Nb₅C_y (y = 0–6). Of these clusters Nb₅C₂ and Nb₅C₃ exhibit the lowest IPs. Complementary density functional theory calculations have been performed to locate the lowest energy isomers for each cluster. By comparing the experimental IPs with those calculated for candidate isomers, the structures of the Nb₅C_y clusters observed in the experiment are inferred. For all these structures, the underlying Nb₅ cluster has either a “prolate” or “oblate” trigonal bipyramid geometry. Both Nb₅C₅ and Nb₅C₆ are shown to contain carbon–carbon bonding in the form of one and two molecular C₂ units, respectively.