Thermochemistry of gaseous UF5 and UF4

Thermochemical data for gaseous UF5 and UF4 were obtained from equilibrium and electron impact measurements made by high temperature mass spectrometry. Equilibrium measurements yielded for the gaseous reaction Ag+UF5=AgF+UF4 the enthalpy change ΔH298=16.9±1.5 kcal by second law analysis. Additionally, the sublimation of UF4(s) was studied by the torsion–effusion method, leading to ΔH298(sub) =76.2±0.5 kcal/mol. These results were combined with auxiliary thermochemical data to derive the standard heats of formation ΔHf °298(UF5, g) =−460.6±3.4 kcal/mol and ΔHf °298(UF4, g) = −377.8±0.6 kcal/mol, and the bond dissociation energies D0(F5U–F) =68.3±3.5 kcal and D0(F4U–F) =101.0±3.4 kcal. Threshold energies for formation of UF5 + and UF4 + by dissociative ionization of UF6 were determined along with the ionization potentials of UF6(14.00 eV), UF5 (11.29 eV), and UF4 (9.96 eV), leading to the BDE values D0(F5U–F) =68.0±3.2 kcal and D0(F4U–F) =102.4±3.2 kcal, in agreement with the equilibrium data. Uncertainties associated with the measurements and the interpretation of the data are discussed. The entropy of UF5(g) inferred from the equilibrium measurements was used in a third law treatment of published UF5 sublimation data, giving thermochemical results in good agreement with the above. The UF4 sublimation measurements lead to a vapor entropy that seems more compatible with a molecular configuration of lower symmetry than the obvious tetrahedral structure.