Coupled Cluster Study of the Heats of Formation of UF 6 and the Uranium Oxyhalides, UO 2 X 2 (X = F, Cl, Br, I, and At).

The atomization enthalpies of the U(VI) species UF ₆ and the uranium oxyhalides UO ₂ X ₂ (X = F, Cl, Br, I, and At) were calculated using a composite relativistic Feller-Peterson-Dixon (FPD) approach based on scalar relativistic DKH3-CCSD(T) with extrapolations to the CBS limit. The inherent multideterminant nature of the U atom was mitigated by utilizing the singly charged atomic cation in all calculations with correction back to the neutral asymptote via the accurate ionization energy of the U atom. The effects of SO coupling were recovered using full 4-component CCSD(T) with contributions due to the Gaunt Hamiltonian calculated using Dirac-Hartree-Fock. The final atomization enthalpy for UF ₆ (752.2 kcal/mol) was within 2.5 kcal/mol of the experimental value, but unfortunately the latter carries a ±2.4 kcal/mol uncertainty that is predominantly due to the experimental uncertainty in the formation enthalpy of the U atom. The analogous value for UO ₂ F ₂ (607.6 kcal/mol) was in nearly exact agreement with the experiment, but the latter has a stated experimental uncertainty of ±4.3 kcal/mol. The FPD atomization enthalpy for UO ₂ Cl ₂ (540.4 kcal/mol) was within the experimental error limit of ±5.5 kcal/mol. FPD atomization energies for the non-U-containing molecules (used for reaction enthalpies) H ₂ O and HX (X = F, Cl, Br, I, and At) were within at most 0.3 kcal/mol of their experimental values where available. The FPD atomization enthalpies, together with FPD reaction enthalpies for two different reactions, were used to determine heats of formation for all species of this work, with estimated uncertainties of ±4 kcal/mol. The calculated heat of formation for UF ₆ (-511.0 kcal/mol) is within 2.5 kcal/mol of the accurately known (±0.45 kcal/mol) experimental value.