Theoretical study on the bonding properties and separation performance of ligands with symmetric and asymmetric side rings and ortho-phenanthroline skeletons for Am(III)/Eu(III).

contextSelective separation of trivalent actinides from chemically similar lanthanides remains a central challenge in advanced nuclear fuel reprocessing. To identify structural and electronic factors that govern Am(III)/Eu(III) selectivity, we investigated six 1,10-phenanthroline-based N-donor ligands bearing symmetric or asymmetric side rings. Calculations indicate metal-ligand interactions are predominantly ionic with measurable covalent contributions; Am(III) complexes exhibit shorter bonds, higher Wiberg bond indices, and larger charge transfer than Eu(III) analogues. Projected density of states and charge-decomposition analyses show stronger 5f participation in Am(III) bonding, rationalizing preferential Am binding. Electrostatic potential and thermodynamic results further indicate that symmetric pyrrole side rings yield lower surface potentials and stronger binding, and computed free-energy differences confirm complexation with Am(III) is thermodynamically more favorable. These results delineate how electrostatics and f-orbital covalency jointly determine Am/Eu discrimination and provide practical guidance for designing more effective N-donor extractants.

methodsDensity functional theory (PBE0) was used with relativistic effective core potentials (Am: ECP60MWB-SEG; Eu: ECP28MWB-SEG). Solvent effects (n-dodecane, cyclohexanone, 1-octanol) were modeled with CPCM. Bonding analyses included Wiberg bond indices, QTAIM, NBO/CDA, EDA, and PDOS. Gibbs free energies were derived from 298.15 K thermal corrections, and all optimized structures were verified as minima by frequency analysis; spin-orbit coupling was not included.