Characterization of Bonding Motifs in Actinide Fluoride Compounds with the Actinide (Ac–U) in the Maximum Oxidation State Using Electronic Structure Theory

Density functional theory (B3LYP) and correlated molecular orbital theory (CCSD(T)) calculations were used to predict the properties of novel actinide-fluoride complexes formed by the addition of a fluorine to AnFn compounds, with actinides in their normal maximum oxidation state (n), leading to the formation of an ion-pair complex AnFn-1+F2- for the earlier actinides (Ac, Th, and Pa). UF7 prefers a structure with a weakly associated fluorine, forming a UF6F• complex, likely due to steric hindrance. Ionization of AnFn-1+F2- forms a weakly bound AnFn-1+F2 complex. Ionization of AnFn leads to the formation of AnFn-22+F2- complex so that the electron is not removed from the actinide and maintains the An oxidation state. The ionization energies, An-F bond dissociation energies, and enthalpy for the loss of F20/- of these complexes are in agreement with the available experimental data. The fluoride affinities of AnFn and the electron affinities of AnFn+1 were calculated. The fluoride affinities are large and comparable to those of AsF5 and SbF5 so they are strong Lewis acids. The electron affinities are sizable, indicating that AnFn+1- will be powerful oxidizing agents and should be considered in models of molten salt reactors when fluoride is present.