Reactions of Th⁺ + H₂, D₂, and HD Studied by Guided Ion Beam Tandem Mass Spectrometry and Quantum Chemical Calculations

Kinetic energy dependent reactions of Th(+) with H2, D2, and HD were studied using a guided ion beam tandem mass spectrometer. Formation of ThH(+) and ThD(+) is endothermic in all cases with similar thresholds. Branching ratio results for the reaction with HD indicate that Th(+) reacts via a statistical mechanism, similar to Hf(+). The kinetic energy dependent cross sections for formation of ThH(+) and ThD(+) were evaluated to determine a 0 K bond dissociation energy (BDE) of D0(Th(+)-H) = 2.45 ± 0.07 eV. This value is in good agreement with a previous result obtained from analysis of the Th(+) + CH4 reaction. D0(Th(+)-H) is observed to be larger than its transition metal congeners, TiH(+), ZrH(+), and HfH(+), believed to be a result of lanthanide contraction. The reactions with H2 were also explored using quantum chemical calculations that include a semiempirical estimation and explicit calculation of spin-orbit contributions. These calculations agree nicely and indicate that ThH(+) most likely has a (3)Δ1 ground level with a low-lying (1)Σ(+) excited state. Theory also provides the reaction potential energy surfaces and BDEs that are in reasonable agreement with experiment.