Activation of CH4, C2H6, and C3H8 by gas-phase Nb+ and the thermochemistryof Nb-ligand complexes

The kinetic energy dependence of the reactions of Nb+ (D-5) With methane, e thane, and propane have been studied using guided ion beam mass spectrometr y. It is found that dehydrogenation is efficient and the dominant process a t low energies in all three reaction systems. At high energies, products re sulting from both C-H and C-C cleavage processes are appreciable. The obser vation of dihydride and hydride-methyl niobium cation products provides ins ight into the reaction mechanisms operating in these processes. The results for Nb+ are compared with those for the first-row transition metal congene r V+ and the differences in behavior and mechanism discussed. Modeling of t he endothermic reaction cross sections yields the 0 K bond dissociation ene rgies (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+-CH3) = 2.06 +/- 0.11, D-0[Nb+-(H)(CH3)] = 4.78 +/- 0.11, D-0(Nb+-C2H) = 4.34 +/- 0.19, D-0(Nb+-C2H2) = 2.90 +/- 0.06, D-0( Nb+-C2H3) = 3.43 +/- 0.21, D-0(Nb+-C2H4) = 2.8 +/- 0.3, D-0(Nb+-C2H5) = 2.4 5 +/- 0.12, D-0(Nb+-C3H2) = 5.25 +/- 0.19, and lower limits for D-0(Nb+- C3 H3) greater than or equal to 3.76 +/- 0.23 and D-0(Nb+-C3H5) greater than o r equal to 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. (C) 2000 Elsevier Science B.V.