Density functional theoretical analysis of structure, bonding, interaction and thermodynamic selectivity of hexavalent uranium (UO2 2+) and tetravalent plutonium (Pu4+) ion complexes of tetramethyl diglycolamide (TMDGA)

Abstract

In the well-known PUREX process, tertiary butyl phosphate shows preferential extraction of UO2 2+ over Pu4+, whereas tetraoctyl diglycolamide (TODGA) displays the reverse selectivity. The reversal of selectivity toward diglycolamide has been investigated here theoretically by considering shorter analogue of TODGA, tetramethyl diglycolamide (TMDGA). Structure, bonding, energetic parameter and thermodynamic parameters of UO2 2+ and Pu4+ ions with TMDGA in the gas and solvent phase have been reported in order to understand their complexation and extraction behavior. The calculation has been performed with generalized gradient approximated BP86 density functional and hybrid B3LYP functional employing SVP and TZVP basis set. The calculated structure obtained at BP86/SVP level of optimization is found to be in close agreement with the reported experimental data. The free energy of extraction, ΔG ext, of UO2 2+ and Pu4+ ions from aqueous phase to the dodecane phase has been computed using the Born-Haber thermodynamic cycle in conjunction with COSMO (conductor like screening model) solvation approach. The free energy of extraction is predicted to be exergonic for explicit monomer water model. The free energy of extraction for Pu4+ ion is shown to be higher than that of UO2 2+ as observed in the reported solvent extraction experiment. Further, the suitability of non-polar solvents as organic diluents has been confirmed by the decreased free energy of extraction with increasing dielectric constant of the solvents. Natural population and orbital analysis indicate the electrostatic and small covalent nature of interactions between the metal ions and the tridentate TMDGA chelating ligand. The results presented here might be helpful in designing new ligands for efficient extraction of actinides.