Molecular Properties of Thorium Hydrides: Electron Affinities and Thermochemistry

High-level electronic structure calculations of the ground and low-lying energy electronic states for ThH and ThH^- for x = 2-5 are reported and compared to available anion photoelectron detachment experiments. The adiabatic electron affinities (EAs) are predicted to be 0.82, 0.88, 0.51, and 2.36 eV for x = 2 to 5, respectively, at the Feller-Peterson-Dixon (FPD) level. The vertical detachment energies (VDEs) are predicted to be 0.84, 0.88, 0.81, and 4.38 eV for x = 2-5, respectively. The corresponding experimental VDEs are 0.871 eV for x = 2, 0.88 eV for x = 3, and 4.09 eV for x = 5. As for ThH, there is a significant spin-orbit (SO) correction for the EA of ThH₂, and this correction decreases substantially for x > 2. The observed ThH₂^- photoelectron spectrum has many transitions as predicted at the CASPT2-SO level. The FPD bond dissociation energies (BDEs) increase from 67 to 75 kcal/mol for x = 2 to x = 4 at the FPD level. The BDE for ThH₅ is much lower as it is a complex of H₂ with ThH₃. The hydride affinities for x = 2 to 4 are all comparable and near 70 kcal/mol. A natural bond orbital analysis is consistent with a significant Th⁺-H^- ionic contribution to the Th-H bonds. There is very little participation of the 5f orbitals in the bonding and the valence electrons on the Th are dominated by 7s and 6d for the neutrals and anions except for ThH₂^- where there is a significant contribution from the 7p.