A Density Functional Theory Study on the Separation of Strontium from Lanthanides and Actinides by TODGA

This study presents a systematic theoretical investigation into the coordination and separation mechanisms of the diglycolamide-based extractant N,N,N′,N′-tetraoctyldiglycolamide (TODGA) with Sr(II), Am(III), and Eu(III) cations. Using TMDGA as a representative model, the structural and electronic properties of the 1:3 metal–ligand complexes, together with the thermodynamics of the corresponding extraction pathways, were investigated using density functional theory (DFT). Geometrical analysis and Mayer bond orders reveal that the carbonyl oxygen atoms possess a significantly higher coordination affinity for the metal centers than the central ether oxygen. Furthermore, the metal–oxygen (M–O) interactions in the trivalent Am(III) and Eu(III) complexes exhibit more pronounced covalent character relative to the divalent Sr(II) analogue. The fundamental bonding mechanisms were elucidated through Natural Adaptive Orbital (NAdO) and charge distribution analyses, which established a clear relationship between valence orbital participation─specifically the (n–1) d and (n–2) f manifolds─and the covalency. Complementary Quantum Theory of Atoms in Molecules (QTAIM) analysis confirms that the metal–ligand interactions are predominantly closed-shell in nature, albeit with varying degrees of localized covalency. Phase-specific thermodynamic calculations further reveal that the extraction energetics are strongly dependent on the chemical form and solvation environment of the metal species. For the hydrated nitrate-containing extraction pathway, the calculated thermodynamic favorability follows Eu(III) > Sr(II) > Am(III), with the reaction free energies following the solvent order n-hexane < n-dodecane < cyclohexanone. These results demonstrate that the overall extraction thermodynamics cannot be inferred from intrinsic metal–ligand bonding alone, but instead arise from the interplay of metal hydration, nitrate participation, and phase-specific solvation.