The heat of formation of gaseous PuO22+ from relativistic density functional calculations

Abstract

Using a set of model reactions, we estimated the heat of formation of gaseous PuO₂²⁺ from quantum-chemical reaction enthalpies and experimental heats of formation of reference species. To this end, we carried out relativistic density functional calculations on the molecules PuO₂²⁺, PuO₂, PuF₆, and PuF₄. We used a revised variant (PBEN) of the Perdew–Burke–Ernzerhof gradient-corrected exchange–correlation functional, and we accounted for spin–orbit interaction in a self-consistent fashion. As open-shell Pu species with two or more unpaired 5f electrons are involved, spin–orbit interaction significantly affects the energies of the model reactions. Our theoretical estimate for the heat of formation Δ_fH^°₀(PuO₂²⁺,g), 418 ± 15 kcal mol⁻¹, evaluated using plutonium fluorides as references, is in good agreement with a recent experimental result, 413 ± 16 kcal mol⁻¹. The theoretical value connected to the experimental heat of formation of PuO₂(g) has a notably higher uncertainty and therefore was not included in the final result.