The efficient extraction of uranyl from spent nuclear fuel wastewater for subsequent reprocessing and reuse in the nuclear cycle is an essential effort towards minimization of long-lived radioactive waste. N-substituted amides and Schiff base ligands are propitious candidates, where extraction occurs via complexation with the uranyl moiety. In this study, we utilized the local vibra- tional modes theory, paired with QTAIM and NBO analyses, to explore chemical bonding in uranyl nitrate, two uranyl picolinamide, one uranyl Schiff base, and three uranyl diamide complexes. The major focus was on (i) the assessment of the equatorial uranyl-ligand O-U and N-U bonding, includ- ing the question of chelation, and (ii) how the strength of the axial uranium oxygen U=O bonds of the uranyl moiety change upon complexation. Our results reveal that the strength of the equatorial uranium-ligand interactions correlates with their covalent character and with charge donation from the O and N lone pairs into the vacant uranium orbitals. We also found an inverse relationship between the covalent character of the equatorial ligand bonds and the strength of the axial uranium-oxygen bond. In summary, our study provides valuable data for a strategic modulation of N-substituted amide and Schiff base ligands towards maximization of uranyl extraction.
Methods: Quantum chemistry calculations were performed with the PBE0 level of theory, where relativistic effects were accounted for via the NESCau Hamiltonian, implemented in Cologne2020, interfaced with Gaussian16. We use a cc-pwCVTZ-X2C basis set for uranium and Dunning’s cc-pVTZ for the remaining atoms. Uranium-ligand bonding was analyzed with LModeA (local vibrational mode analysis), AIMALL (QTAIM analysis), and NBO 7.0 f(NBO analysis.