Experimental and Theoretical Studies of the Activation of Methane by Ta⁺

Abstract

A guided-ion-beam tandem mass spectrometer is used to study atomic tantalum cations reacting with CH 4 and CD 4 . Like other third-row transition metal ions, W + , Os + , Ir + , and Pt + , the dehydrogenation reaction to form TaCH 2 + + H 2 is exothermic. At higher energies, other products, TaH + , TaCH + , and TaCH 3 + , are observed with TaH + dominating the product spectrum. Modeling of the endothermic cross sections provides the 0 K bond dissociation energies (in eV) of D 0 (Ta + -CH) ) 5.82 ( 0.16 and D 0 (Ta + -CH 3 ) ) 2.69 ( 0.14 eV. We also examined the reverse reaction, TaCH 2 + + H 2 f Ta + + CH 4 , and its isotopic equivalent, TaCH 2 + + D 2 . By combining the cross sections for the forward and reverse processes, an equilibrium constant for this reaction is derived, from which D 0 (Ta + -CH 2 ) ) 4.81 ( 0.03 eV is obtained. The Ta + -H and Ta + -CH 3 experimental bond energies are in reasonable agreement with density functional calculations at the BHLYP/ HW+/6-311++G(3df,3p) level of theory, whereas the Ta + -CH and Ta + -CH 2 bond energies are predicted well by B3LYP/HW+/6-311++G(3df,3p) calculations. Theoretical calculations at this latter level of theory reveal that these reactions proceed through a H-Ta + -CH 3 intermediate and provide details of the various intermediates and transition states. † Part of the special issue "Richard E. Smalley Memorial Issue".

Activation of CH4 by Th+ as Studied by Guided Ion Beam Mass Spectrometry and Quantum Chemistry

“…This agostic structure has been found to be characteristic of the early transition metals. 25,28,30,85 Geometrical parameters found here are similar to the structures reported by Di Santo et al 41 and de Almeida and Duarte, 42 who also found a 2 A′ ground state. The CH 2 wagging motion passes through the symmetric 2 A 1 state (a transition state having C 2v symmetry) lying only 0.05 eV higher in energy.…”

“…22,23,25,27,28,30,53,69,70,86 In all cases, the difference is attributed to competition with the formation of the metal carbene because of a shared, common intermediate. Because reactions of M + with H 2 produce only one product, no competition exists that may delay the onset of reaction.…”

Gas-phase activation of methane by ligated transition-metal cations

“…More recently, the details of these reactions, including those metals that cannot activate methane under thermal conditions, were analyzed by combined experimental and theoretical approaches, e.g., for M ϭ Fe (8), Ni (9), Zr (10), Hf (11), Mo (12), Ta (13), W (14), Re (15), Author contributions: D.S. and H.S.…”

“…In fact, most of the above-mentioned examples of methane activation lead to species that do not react further with methane; exceptions are found among 5d metals (2,3,13,14,19,20). In addition, bare transition-metal ions correspond to the oxidation state (ϩI), which is unusual in condensed-phase chemistry.…”

Chemie mit Methan: Studieren geht über Probieren!

“…[11b] W, [14] Ir, [15] und Ta [16] ) durchgeführt. Hçchst interessant war dabei der Befund, dass ein durch Spin-Bahn-Kopplung vermittelter Wechsel von einem High-spin-zu einem Low-spin-Zustand im Zuge der Dehydrierung und vor allem bei der Bildung des zentralen Insertionsintermediates H-M + -CH 3 von entscheidender Bedeutung ist.…”