Understanding Am3+/Cm3+ separation with H4TPAEN and its hydrophilic derivatives: a quantum chemical study

Abstract

Am³⁺/Cm³⁺ separation is an extremely hard but important task in nuclear waste treatment. In this study, Am and Cm complexes formed with a back-extraction agent N,N,N′,N′-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylene-diamine (H₄TPAEN) and its two derivatives with hydrophilic substituents (methoxy and morpholine groups) were investigated using the density functional theory (DFT). The optimized geometrical structures indicated that the Am³⁺ cation matched better with the cavities of the three studied ligands than Cm³⁺, and the Am³⁺ cations were located deeper in the cavities of the ligands. The bond order and quantum theory of atoms in molecules (QTAIM) analyses suggested that ionic interactions dominated An–N and An–O (An = Cm and Am) bonds. However, weak and different extents of partial covalency could also be found in the Am–N and Cm–N bonds. The O donor atoms in the carboxylate groups preferably coordinated with Cm³⁺ rather than Am³⁺, whereas the N atoms preferred Am³⁺. Therefore, the Am³⁺/Cm³⁺ selectivity of H₄TPAEN and its two hydrophilic derivatives may be ascribed to the competition between the An–N and An–O interactions and the few dissimilarities in their geometrical structures. Based on our calculations, the methoxy and morpholine groups in the two derivatives can serve as electron-donating groups and enhance the strength of the An–N_PY bonds (N_PY denotes the nitrogen atom of pyridine ring). When compared with the Am-complex, the Cm-complex exhibited significant strength effect, resulting in the relatively lower Am³⁺/Cm³⁺ separation ability of the H₄TPAEN's hydrophilic derivatives.