The heat of formation of the uranyl dication: Theoretical evaluation based on relativistic density functional calculations

By using a set of model reactions, we estimated the heal of formation of gaseous UO₂²⁺ from quantum-chemical reaction enthalpies and experimental heats of formation of reference species. For this purpose, we performed relativistic density functional calculations for the molecules UO ₂²⁺, UO₂, UF₆, and UF₅. We used two gradient-corrected exchange-correlation functionals (revised Perdew-Burke-Ernzerhof (PBEN) and Becke-Perdew (BP)) and we accounted for spin-orbit inter action in a self-consistent fashion. Indeed, spin-orbit interaction notably affects the energies of the model reactions, especially if compounds of U^IV are involved. Our resulting theoretical estimates for Δ_fH₀²(UO₂²⁺), 365 ± 10 kcal mol ¹ (PBEN) and 370 ± 12 kcal mol ¹ (BP), are in quantitative agreement with a recent experimental result, 364 ± 15 kcal mol ^-1. Agreement between the results of the two different exchange-correlation functionals PBEN and BP supports the reliability of our approach. The procedure applied offers a general means to derive unknown enthalpies of formation of actinide species based on the available well-established data for other compounds of the element in question.