Energetics and Mechanisms of C-H Bond Activation by a Doubly Charged Metal Ion: Guided Ion Beam and Theoretical Studies of Ta2+ + CH4

A guided ion beam tandem mass spectrometer is used to study the kinetic-energy dependence of doubly charged atomic tantalum cations (Ta2+) reacting with CH4 and CD4. As for the analogous singly charged system, the dehydrogenation reaction to form TaCH22+ + H-2 is exothermic. The charge-transfer reaction to form Ta+ + CH4+ and the charge-separation reaction to form TaH+ + CH3+ are also observed at low energies in exothermic processes, as is a secondary reaction of TaCH22+ to form TaCH3+ + CH3+. At higher energies, other doubly charged products, TaC2+ and TaCH32+, are observed, although no formation of TaH2+ was observed. Modeling of the endothermic cross sections provides 0 K bond dissociation energies (in electronvolts) of D-0(Ta2+-C) = 5.42 +/- 0.19 and D-0(Ta2+-CH3) = 3.40 +/- 0.16. These experimental bond energies are in poor agreement with density functional calculations at the B3LYP/HW+/6-311++G(3df,3p) level of theory. However, the Ta2+-C bond energy is in good agreement with calculations at the QCISD(T) level of theory, and the Ta2+-CH3 bond energy is in good agreement with density functional calculations at the BHLYP level of theory. Theoretical calculations reveal the geometric and electronic structures of all product ions and are used to map the potential energy surface, which describes the mechanism of the reaction and key intermediates. Both experimental and theoretical results suggest that TaH+, TaCH22+, and TaCH32+ are formed through a H-Ta2+-CH3 intermediate.