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 methane by the third‐row transition‐metal cation Os+ is studied experimentally by examining the kinetic energy dependence of reactions of Os+ with CH4 and CD4 using guided‐ion‐beam tandem mass spectrometry. A flow tube ion source produces Os+ in its electronic ground state and primarily in the ground spin–orbit level. Dehydrogenation to form [Os,C,2 H]++H2 is exothermic, efficient, and the only process observed at low energies for reaction of Os+ with methane, whereas OsH+ dominates the product spectrum at higher energies. The kinetic energy dependences of the cross sections for several endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Os+C)=6.20±0.21, D0(Os+CH)=6.77±0.15, and D0(Os+CH3)=3.00±0.17. Because it is formed exothermically, D0(Os+CH2) must be greater than 4.71 eV, and a speculative interpretation suggests the exothermicity exceeds 0.6 eV. Quantum chemical calculations at the B3LYP/def2‐TZVPP level show reasonable agreement with the experimental bond energies and with previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surfaces. Notably, the structure of the dehydrogenation product is predicted to be HOsCH+, rather than OsCH2+, in contrast to previous work.

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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.