Experimental and theoretical studies of Nb6C7(0/+)

Nb6C7+ is studied using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer coupled to a compact supersonic source. Its reactions with H2O, NH3, CH3OH, CH3CN, and C6H6 are reported. For each of these ligands, sequential attachment reactions occur leading to the initial truncation products Nb(6)C(7)L(4)(+). Minor amounts of Nb(6)C(7)L(5)(+) and Nb(6)C(7)L(6)(+) are observed for polar molecules, but they grow in slowly. A measurement of the relative rate constants for the sequential addition reactions of CH3OH, for example, demonstrates quantitatively that addition of the fifth CH3OH is very slow compared to the first four additions. Multiple excitation collisional activation (MECA) on Nb6C7+ yields Nb5C6+, and MECA on Nb5C6+ yields Nb4C4+ as predominant products. Nb6C7+ undergoes iodine abstraction from one CH3I molecule to yield the truncation product, Nb6C7I+. Ab initio Hartree-Fock calculations of Nb6C7 suggest a structure consisting of two cubes with one carbon at the exact inversion center of the D-2h symmetry. This structure, in which four of the six niobium atoms are equivalent but different from the other two, is consistent with the experimental results. The structures of Nb4C4 and Nb5C6 are also optimized theoretically.