Complexation Behavior and Selective Separation of Am3+/Eu3+ With Four Novel Ligands Based on Skeleton HDBM: A Scalar Relativistic DFT Study

Abstract

An³⁺/Ln³⁺ separation is fundamental to mitigating the environmental impact of spent nuclear fuel. A thorough investigation into the separation mechanisms of An³⁺/Ln³⁺ and coordination interactions with promising extractants is essential for reducing the environmental hazards posed by nuclear waste. In this study, employing scalar relativistic density functional theory (DFT), we designed ligands L_H, L_Et, L_Br, and L_Ca based on a novel skeleton, (5H-cyclopenta[2,1-b:3,4-b′]dipyridine-2,8-diyl)bis((1H-pyrazol-1-yl)methanone) (HDBM), which was modified with different substituent groups (G = -H, -CH₂CH₃, -Br, and -COOH) to systematically assess their complexation behaviors and selective separation of Am³⁺/Eu³⁺. Bonding characterization confirms that the four classes of ligands with N, O donor atoms exhibit a stronger selective coordination tendency toward Am³⁺ than Eu³⁺ with a stronger tendency for selective coordination. For further corroboration, we also performed molecular orbitals (MOs), Independent gradient model based on Hirshfeld (IGMH), frontier molecular orbitals (FMOs), and extended transition state-natural orbitals for chemical valence (ETS-NOCV), and other analyses, which consistently demonstrate that the ligands interact more strongly with Am³⁺ than with Eu³⁺. EDA calculations showed that the attractive interaction between the molecular fragments exceeds the repulsive force, enhancing molecular stability. Thermodynamic calculations indicated that the selectivity coefficients of L_H, L_Et, L_Br, and L_Ca toward Am³⁺/Eu³⁺ in water, n-butanol, and n-octanol were all more than 97%, with the separation factors ranged from 41.58 to 5.71 × 10⁸. The ligand L_H was the most complexable with Am³⁺ in n-octanol and had the greatest selectivity for Am³⁺/Eu³⁺ separation in n-butanol, with the highest separation factor.