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

摘要

Nb4C4+, one of the smallest cubic crystallite structures (2x2x2) of the 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 to 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 reaction of V8C12+ with O-2, however, no higher order reactions are observed to proceed by elimination of one or two CO molecules. Taken together 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 ion is also found to react with both water and methanol through two competitive pathways which are dependent on the background Ar cooling gas. The main pathway for the cooled ions is attachment of the first solvent molecule followed by the elimination of H-2 upon reaction with the second solvent molecule to form (OH)(2) and (OCH3)(2) adducts, respectively. A maximum of two additional solvent molecules then attach to the adduct ions. The reactions with water and methanol proceed with efficiencies 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 calculations are reported on Nb4C4 in a slightly distorted cubic structure with Td symmetry. Calculations on Nb4C2 found one stable structure with essentially C-2v symmetry corresponding to a bicapped tetrahedron. The electronic structure of the ground and low-lying excited states is discussed as well as the observed reactivity of Nb4C4+. [References: 48]