Activation of CH_4, C_2H_6, and C_3H_8 by gas-phase Nb~+ and the thermochemistry of Nb-ligand complexes

摘要

The kinetic energy dependence of the reactions of Nb~+ (~5D) with methane, ethane,and propane have been studied using guided ion beam mass spectrometry. It is found that dehydrogenation is efficient and the dominant process at low energies in all three reaction systems. At high energies, products resulting from both C-H and C-C cleavage processes are appreciable. The observation of dihydride and hydrido-methyl niobium cation products provides insight into the reaction mechanisms operating in these processes. The results for Nb~+ are compared with those for the first-row transition metal congener V~+ and the differences in behavior and mechanism discussed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in electron volts) of D_0(Nb-H) > 2.3 ± 0.1, D_0(Nb~+ -2H) = 4.64 ± 0.06, D_0(Nb~+ -C) = 5.28 ± 0.15, D_0(Nb~+ -CH) = 6.02 ± 0.20, D_0(Nb~+ -CH_2) = 4.44 ± 0.09, D_0(Nb~+ -CH_3) = 2.06 ± 0.11, D_0[Nb~+ -(H)(CH_3)] = 4.78 ± 0.11, D_0(Nb~+ -C_2H) = 4.34 ± 0.19, D_0(Nb~+ -C_2H_2) = 2.90 ± 0.06, D_0(Nb~+ -C_2H_3) = 3.43 ± 0.21, D_0(Nb~+ -C_2H_4) = 2.8 ± 0.3, D_0(Nb~+ -C_2H_5) = 2.45 ± 0.12, D_0(Nb~+ -C_3H_2) = 5.25 ± 0.19, and lower limits for D_0(Nb~+ -C_3H_3) ≥ 3.76 ± 0.23, and D_0(Nb~+ -C_3H_5) ≥ 1.4 ± 0.1. The observation of exothermic processes sets lower limits for the bond energies of Nb~+ to propyne and propene of 2.84 and 1.22 eV, respectively.