Activation of CH4 by gas-phase Mo+, and the thermochemistry of Mo-ligand complexes

The kinetic-energy dependence of the reactions of Mo+ (S-6) with methane has been studied using guided ion beam mass spectrometry. No exothermic reactions are observed in this system, as also found previously, but efficient dehydrogenation occurs at slightly elevated energies. At higher energies, MoH+ dominates the product spectrum and MoC+, MoCH+, and MoCH3+ are also observed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in eV) of D-0(Mo+-C)) 4.55 (0.19, D0(Mo+-CH)) 5.32 +/- 0.14, D-0(Mo+-CH2) = 3.57 +/- 0.10, and D-0(Mo+- CH3)) 1.57 +/- 0.09. The results for Mo+ are compared with those for the first- and third-row transition-metal congeners, Cr+ and W+, and the differences in behavior and mechanism are discussed. Theoretical results are used to elucidate the geometric and electronic structures of all product ions as well as the complete potential-energy surface for reaction. The efficiency of the coupling between the sextet and quartet spin surfaces is also quantified.