Composite thermochemistry of gas phase U(VI)-containing molecules

Reaction energies have been calculated for a series of reactions involving UF₆, UO₃, UO₂(OH)₂, and UO₂F₂ using coupled cluster singles and doubles with perturbative triples, CCSD(T), with a series of correlation consistent basis sets, including newly developed pseudopotential (PP)- and all-electron (AE) Douglas-Kroll-Hess-based sets for the U atom. The energies were calculated using a Feller-Peterson-Dixon composite approach in which CCSD(T) complete basis set (CBS) limits were combined with a series of additive contributions for spin-orbit coupling, outer-core correlation, and quantum electrodynamics effects. The calculated reaction enthalpies (both PP and AE) were combined with the accurately known heat of formation of UF₆ to determine the enthalpies of formation of UO₃, UO₂(OH)₂, and UO₂F₂. The contribution to the reaction enthalpies due to correlation of the 5s5p5d electrons of U was observed to be very slowly convergent with basis set and at the CBS limit their impact on the final enthalpies was on the order of 1 kcal/mol or less. For these closed shell molecules, spin-orbit effects contributed about 1 kcal/mol to the final enthalpies. Interestingly, the PP and AE approaches yielded quite different spin-orbit contributions (similar magnitude but opposite in sign), but the total scalar plus spin-orbit results from the two approaches agreed to within ∼1 kcal/mol of each other. The final composite heat of formation for UO₂F₂ was in excellent agreement with experiment, while the two results obtained for UO₃ were just outside the ±2.4 kcal/mol error bars of the currently recommended experimental value. An improved enthalpy of formation (298 K) for UO₂(OH)₂ is predicted from this work to be −288.7 ± 3 kcal/mol, compared to the currently accepted experimental value of −292.7 ± 6 kcal/mol