Improved selectivity of trivalent Am over Cm by modulating donor centres of aza-macrocyclic ligand with soft donors: A theoretical comparison.

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. The structure–activity relationships between the extractants and their separation performances are expected to provide practical approaches for designing proficient ones for actinide separation. The rigidity in the ligand backbone as well as the coordination environment created by the side groups play an important role in such separations. Macrocyclic ligands such as crown ethers with hydrophilic groups are also used for the separation of transplutonium actinides. 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 f7 configuration shows resistance in participating in bonding as was observed from NPA, QTAIM, NBO and DOS analyses. Whereas, the Am(III) ions having f6 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. The use of soft donor atoms viz. S in a crown ether like environment along with hard carboxylate Os for chelation of trivalent Am and Cm is established to bring about promising selectivity for the former, along with enhanced extractability. Designing of new ligands with similar features in the future with modulated softness of donor centres can be anticipated to bring about further improvedselectivity between these two nearly identical minor actinide ions. [Display omitted] [ABSTRACT FROM AUTHOR]