Investigation of enhanced Am selectivity for Eu in solvent extraction using a BTPhen ligand substituted with halogen.

The effect of bromine (Br) on the separation of Am/Eu using BTPhen was investigated using DFT calculations. The simulated results agreed well with the reported experimental result of Br substitution. It is concluded that the contribution of the d orbital becomes less important by increasing the number of Br atoms, that is, the relative contribution of the f orbital becomes important. The Am f orbital contributed to both bonding and antibonding interactions with the ligand, whereas the Eu f orbital contributed to the antibonding interaction with the ligand. To study the halogen effect systematically, we introduced a series of new halogen atoms (chlorine, fluorine, iodine) into BTPhen. When the electronegativity of the halogen atom increases, the Δ G for complex formation shifts to the positive direction, and the ΔΔ G which shows the difference in Δ G between Am and Eu becomes a large negative value, suggesting that the Am selectivity is larger. This is due to the increased Δ ρ BCP (= ρ BCP (Am) - ρ BCP (Eu)) between the metal and the ligating nitrogen atom with an increase of electronegativity of the halogen atom.