Activation of CH4 by Th(+) as studied by guided ion beam mass spectrometry and quantum chemistry

The reaction of atomic thorium cations with CH4 (CD4) and the collision-induced dissociation (CID) of ThCH4(+) with Xe are studied using guided ion beam tandem mass spectrometry. In the methane reactions at low energies, ThCH2(+) (ThCD2(+)) is the only product; however, the energy dependence of the cross-section is inconsistent with a barrierless exothermic reaction as previously assumed on the basis of ion cyclotron resonance mass spectrometry results. The dominant product at higher energies is ThH(+) (ThD(+)), with ThCH3(+) (ThCD3(+)) having a similar threshold energy. The latter product subsequently decomposes at still higher energies to ThCH(+) (ThCD(+)). CID of ThCH4(+) yields atomic Th(+) as the exclusive product. The cross-sections of all product ions are modeled to provide 0 K bond dissociation energies (in eV) of D0(Th(+)-H) ≥ 2.25 ± 0.18, D0(Th(+)-CH) = 6.19 ± 0.16, D0(Th(+)-CH2) ≥ 4.54 ± 0.09, D0(Th(+)-CH3) = 2.60 ± 0.30, and D0(Th(+)-CH4) = 0.47 ± 0.05. Quantum chemical calculations at several levels of theory are used to explore the potential energy surfaces for activation of methane by Th(+), and the effects of spin-orbit coupling are carefully considered. When spin-orbit coupling is explicitly considered, a barrier for C-H bond activation that is consistent with the threshold measured for ThCH2(+) formation (0.17 ± 0.02 eV) is found at all levels of theory, whereas this barrier is observed only at the BHLYP and CCSD(T) levels otherwise. The observation that the CID of the ThCH4(+) complex produces Th(+) as the only product with a threshold of 0.47 eV indicates that this species has a Th(+)(CH4) structure, which is also consistent with a barrier for C-H bond activation. This barrier is thought to exist as a result of the mixed ((4)F,(2)D) electronic character of the Th(+) J = (3)/2 ground level combined with extensive spin-orbit effects.