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 D 0(Th+–H) ≥ 2.25 ± 0.18, D 0(Th+–CH) = 6.19 ± 0.16, D 0(Th+–CH2) ≥ 4.54 ± 0.09, D 0(Th+–CH3) = 2.60 ± 0.30, and D 0(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 (4F,2D) electronic character of the Th+ J = 3/2 ground level combined with extensive spin–orbit effects.