Enhancement in selectivity of trivalent Am over Cm by modulating donor centres of aza- macrocyclic ligands: An implication of their differences in bonding

Abstract Efficient management of the high-level liquid waste (HLW) generated during the reprocessing of nuclear fuel involves mutual separation and transmutation of the minor actinides like Am and Cm, which are chemically very similar. In the present work, an in-silico analysis of the bonding of Am(III) and Cm(III) ions with donor center modulated azacrown based ligand, i.e., a ligand formed by replacing the four ‘O’s of the azacrown macrocycle in N,N’-bis[(6-carboxy-2-pyridyl)-methyl]-1,10-diaza-18-crown-6 (L1) with four ‘S’s (L2) is carried out using relativistic density functional theory to develop a deeper understanding of the impact of both, soft donors as well as the cavity effects on the separation of these metal ions. The subtle differences in the bonding is brought about by the covalent interaction of the metal and the aza-macrocyclic O and S donors. The electron donation from the ligand is mostly accepted in the metal 6d orbitals and minimally to the 5f orbitals. But the f-orbital participation plays the crucial role in bringing about the subtle difference in the bonding between Am(III) and Cm(III) ions with the ligands. The trivalent Cm ion with half-filled initial f 7 configuration shows resistance in participating in bonding as was observed from NPA, QTAIM, NBO and DOS analyses. Whereas, the Am(III) ions having f 6 configuration are eager to accept f-electrons to achieve the stable half-filled configuration. The energetics elucidated that both L1 and L2 ligands are Am selective. But the extraction capacity and selectivity is higher for the L2 ligand. The Am selectivity can be attributed to the significant covalent interaction of S donors which have better electron donating capacity (than O) to the electron deficient Am(III) ion. This study is anticipated to throw some light on the implication of the bonding features on their separation.