Activation of methane by Ru+: Experimental and theoretical studies of the thermochemistry and mechanism

The reaction of Ru+ with CH4 is studied using a guided-ion beam tandem mass spectrometer. Consistent with previous work, no reactivity is observed at thermal energies. As the collision energy is increased, dehydrogenation to form RuCH2+ + H-2 is observed with a relatively low energy threshold. At higher energies, other products, RuH+, RuC+, RuCH+, and RuCH3+, are observed with RuH+ dominating the product spectrum. Modeling of the endothermic cross sections provides thresholds that agree well with thermochemistry determined previously, except in the case of RuC+ where a revised 0 K bond dissociation energy (BDE) of 5.43 0.08 eV is measured here. The experimental BDEs of all products observed are in reasonable agreement with theoretical calculations at the B3LYP, QCISD(T), and CCSD(T,full) levels of theory using several different basis sets. Theory is also used to explore the potential energy surfaces associated with the reaction of Ru+ with methane, with some distinct differences found compared with previous computational results. These calculations reveal that this reaction proceeds through a H-Ru+-CH3 intermediate and requires crossing from the quartet spin of the reactants to a doublet spin intermediate, and possibly back to quartet in order to dehydrogenate. Further, both RuCH2+ and HRuCH+ geometries are nearly isoenergetic. Details of the various intermediates, transition states, and crossing points between surfaces are provided. Finally, reactions of Ru+ with methane are compared with those of the first and third-row congeners, Fe+ and Os+, and the differences in behavior and mechanism discussed. (C) 2016 Elsevier B.V. All rights reserved.