Experimental and Theoretical Studies on Nb₄C4^0/+: Reactivity and Structure of the Smallest Cubic Niobium—Carbon Cluster

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 T_dsymmetry. Calculations on Nb₄C₂found one stable structure with essentially C_2Vsymmetry 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 ⁺.