Guided Ion Beam and Quantum Chemical Investigation of the Thermochemistry of Thorium Dioxide Cations: Thermodynamic Evidence for Participation of f Orbitals in Bonding

Kinetic energy dependent reactions of ThO+ with O-2 are studied using a guided ion beam tandem mass spectrometer. The formation of ThO2+ in the reaction of ThO+ with O-2 is observed to be slightly endothermic and also exhibits two obvious features in the cross section. These kinetic energy dependent cross sections were modeled to determine a 0 K bond dissociation energy of D-0 (OTh+-O) = 4.94 +/- 0.06 eV. This value is slightly larger but within experimental uncertainty of less precise previously reported experimental values. The higher energy feature in the ThO2+ cross section was also analyzed and suggests formation of an excited state of the product ion lying 3.1 +/- 0.2 eV above the ground state. Additionally, the thermochemistry of ThO2+ was explored by quantum chemical calculations, including a full Feller-Peterson-Dixon (FPD) composite approach with correlation contributions up to CCSDT(Q) and four-component spin-orbit corrections, as well as more approximate CCSD(T) calculations including semiempirical estimates of spin-orbit energy contributions. The FPD approach predicts D-0(OTh+-O) = 4.87 +/- 0.04 eV, in good agreement with the experimental value. Analogous FPD results for ThO+, ThO, and ThO2 are also presented, including ionization energies for both ThO and ThO2. The ThO2+ bond energy is larger than those of its transition metal congeners, TiO2+ and ZrO2+, which can be attributed partially to an actinide contraction, but also to contributions from the participation off orbitals on thorium that are unavailable to the transition metal systems.