Theoretical Study on Coordination Chemistry of Two Water‐Soluble Ligands for Stripping Separation of Transplutonium Ions

Purification and mutual separation of trivalent actinides (An³⁺: Am, Cm, Bk, Cf) present significant challenges in nuclear waste treatment. Elucidating complexation mechanisms with stripping ligands is crucial for ligand development. Given the radiotoxicity, quantum chemical methods are essential. This DFT study investigates chelation of An³⁺ ions with water-soluble ligands H₂L²ᵖʸ and H₂L²ᵖᶻ. Probable back-extraction complexes ([AnLⁱ(H₂O)ⱼ]⁺, AnLⁱ(NO₃)(H₂O)^k) exhibit near-identical geometries with minor An─O/N bond length variations. Analyses reveal enhanced chelating ability for diamine N atoms adjacent to carboxylate O versus N-heterocyclic donors (pyridine/pyrazine). Crucially, interaction intensity and partial covalency between An³⁺ and donor atoms (N_AM, O_CA, N_PY/PZ) display a distinct bimodal variation with Cm as the turning point. This trend likely originates from closer orbital energy matching between the 5f orbitals of Am/Bk/Cf and ligand 2p orbitals compared to Cm. Thermodynamic analyses demonstrate superior back-extraction ability for H₂L²ᵖʸ, while H₂L²ᵖᶻ excels in Am/Eu separation. The downward-upward trend in reaction free energies aligns with covalency variations, suggesting potential intra-group separation capability for all four ions. Estimated separation factors SF_Am/Cm, SF_Bk/Cm, and SF_Cf/Cm are projected within 2.45-3.45.