EXPERIMENTAL AND THEORETICAL-STUDIES ON NB4C40 + - REACTIVITY AND STRUCTURE OF THE SMALLEST CUBIC NIOBIUM-CARBON CLUSTER/

Nb4C4+, one of the smallest cubic crystallite structures (2x2x2) of th e transition metal-carbon cluster family, is studied with the use of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with a compact supersonic source. Nb4C4+ reacts with oxygen t o form Nb4C2+ and, presumably, 2CO. This reaction is analogous to that of V8C12+ with O-2 in which V8C10+ is produced. In contrast to the re action of V8C12+ with O-2, however, no higher order reactions are obse rved to proceed by elimination of one or two CO molecules. Taken toget her with other experiments, these results suggest that 8 eV < D(Nb4C2-C-2) < 10 eV and D(Nb-4(+)-C-2) > 10 eV. In addition, this cluster io n is also found to react with both water and methanol through two comp etitive pathways which are dependent on the background Ar cooling gas. The main pathway for the cooled ions is attachment of the first solve nt molecule followed by the elimination of H-2 upon reaction with the second solvent molecule to form (OH)(2) and (OCH3)(2) adducts, respect ively. A maximum of two additional solvent molecules then attach to th e adduct ions. The reactions with water and methanol proceed with effi ciencies of 3.9% and 2.7%, respectively. In the absence of cooling gas , abstraction to form Nb4C4OH+ and Nb4C4OCH3+ from water and methanol, respectively, dominate and suggest D(Nb4C4+-OH) less than or equal to D(H-OH) = 119 kcal/mol and D(Nb4C4+-OCH3) similar to D(H-OCH3) = 104 kcal/mol. Both of these product ions are observed to attach a maximum of 3 more solvent molecules at longer reaction times. Ab initio calcul ations are reported on Nb4C4 in a slightly distorted cubic structure w ith Td symmetry. Calculations on Nb4C2 found one stable structure with essentially C-2v symmetry corresponding to a bicapped tetrahedron. Th e electronic structure of the ground and low-lying excited states is d iscussed as well as the observed reactivity of Nb4C4+.