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

Abstract

A guided ion beam tandem mass spectrometer is used to study the kinetic-energy dependence of doubly charged atomic tantalum cations (Ta(2+)) reacting with CH4 and CD4. As for the analogous singly charged system, the dehydrogenation reaction to form TaCH2(2+) + H2 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 TaCH2(2+) to form TaCH3(+) + CH3(+). At higher energies, other doubly charged products, TaC(2+) and TaCH3(2+), are observed, although no formation of TaH(2+) was observed. Modeling of the endothermic cross sections provides 0 K bond dissociation energies (in electronvolts) of D0(Ta(2+)-C) = 5.42 +/- 0.19 and D0(Ta(2+)-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 Ta(2+)-C bond energy is in good agreement with calculations at the QCISD(T) level of theory, and the Ta(2+)-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(+), TaCH2(2+), and TaCH3(2+) are formed through a H-Ta(2+)-CH3 intermediate.

Dynamics of chemical reactions of multiply-charged cations: Information from beam scattering experiments

“…The scattering investigation of the analogous reaction leading to NO + + N + + O [48] showed very similar results and led to the conclusion that Reaction (20) proceeded by a similar mechanism as Reaction (19), namely,…”

“…LaFe 2+ YFe 2+ ) and alkanes were described, too [16]. Guided-beam and theoretical studies of reactions between Ta 2+ and methane [19] provided information on bond energies of the species involved, mapped the potential energy surfaces and intermediates and suggested that the products were formed through a H-Ta 2+ -CH 3 intermediate.…”

Activation of Methane by Gaseous Metal Ions

“…[10] Most of these reactions are driven by the radicaloid nature of the oxide cations with the spin mostly located on oxygen, which leads to a high preference for hydrogen-atom abstraction from methane to generate methyl radicals. This situation is in contrast to the dehydrogenation of methane by various 5d elements to give metal carbenes MCH 2 + , [6,11] or MCH 2 2+ , [6,12] and to the C À C coupling reactions with methane observed for medium-sized hydrocarbon dications, [13] in which the dehydrogenation takes place with the "CH 2 " fragment remaining at the ionic species.…”

Gas-Phase Reaction Studies of Dipositive Hafnium and Hafnium Oxide Ions: Generation of the Peroxide HfO22+

“…Other dipositive metal ions that have been shown to similarly activate methane include Zr 2+ , Nb 2+ , Ta 2+ , 34,39 and Th 2+ . 40 It should be noted that thermal Hf + ions are unreactive with CH 4 and the dehydrogenation reaction to form HfCH 2 + is endothermic, this in contrast to most third-row transition metal ions, as discussed by Armentrout and co-workers.…”