Density functional theory studies of UO₂²⁺ and NpO₂⁺ complexes with carbamoylmethylphosphine oxide ligands

The UO₂ ²⁺ and NpO₂ ⁺ extraction complexes with n-octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide (CMPO) and diphenyl-N,N-diisobutylcarbamoyl phosphine oxide (Ph₂CMPO) have been investigated by density functional theory (DFT) in conjunction with relativistic small-core pseudopotentials. For these extraction complexes, especially the complexes of 2:1 (ligand/metal) stoichiometry, UO ₂ ²⁺ and NpO₂ ⁺ predominantly coordinate with the phosphoric oxygen atoms. The CMPO and Ph₂CMPO ligands have higher selectivity for UO₂ ²⁺ over NpO ₂ ⁺, and for all of the extraction complexes, the metal-ligand interactions are mainly ionic. In most cases, the complexes with CMPO and Ph₂CMPO ligands have comparable metal-ligand binding energies, that is, the substitution of a phenyl ring for the n-octyl group at the phosphoryl group of CMPO has no obvious influence on the extraction of UO₂ ²⁺ and NpO₂ ⁺. Moreover, hydration energies might play an important role in the extractability of CMPO and Ph ₂CMPO for these actinyl ions.