Experimental and theoretical studies of the activation of methane by Ta

A guided-ion-beam tandem mass spectrometer is used to study atomic tantalum cations reacting with CH4 and CD4. Like other third-row transition metal ions, W+, Os+, It(+), and Pt+, the dehydrogenation reaction to form TaCH2(+) + H-2 is exothermic. At higher energies, other products, TaH+, TaCH+, and TaCH3+, 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+-CH3) = 2.69 +/- 0.14 eV. We also examined the reverse reaction, TaCH2+ + H-2 -> Ta+ + CH4, and its isotopic equivalent, TaCH2+ + 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+-CH2) = 4.81 +/- 0.03 eV is obtained. The Ta (+)-H and Ta (+)-CH3 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+-CH2 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 (+)-CH3 intermediate and provide details of the various intermediates and transition states.