Nb4C4+, one of the smallest cubic crystallite structures (2×2×2) 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 O2in which V8C10 +is produced. In contrast to the reaction of V8C12 +with O2, 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 +-C2) < 10 eV and D(Nb4 +-C2) > 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 H2upon reaction with the second solvent molecule to form (OH)2and (OCH3)2adducts, 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) ≤ D(H-OH) = 119 kcal/mol and D(Nb4C4 +-OCH3) ~ 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 Nb4C4in a slightly distorted cubic structure with Tdsymmetry. Calculations on Nb4C2found one stable structure with essentially C2Vsymmetry 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 +.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry