Accurate quantum chemical modelling of the separation of Eu3+ from Am3+/Cm3+ by liquid–liquid extraction with Cyanex272

The experimentally observed extraction complexes of trivalent lanthanide Eu3+ and actinide Am3+/Cm3+ cations with Cyanex272 [bis(2,4,4-trimethylpentyl) phosphinic acid, denoted as HC272] and Cyanex301 [bis(2,4,4-trimethylpentyl) dithiophosphinic acid, denoted as HC301] have been studied by using relativistic energy-consistent 4f- and 5f-in-core pseudopotentials for trivalent f elements, combined with density functional theory and a continuum solvation model. It has been found that, as a result of hydrogen bonding, HC272 exists primarily as a self-associated species, whereas HC301 is preferably a monomer. The calculations show that in case of all three M3+ (M = Eu, Am, Cm) ions for HC272 the extraction complexes M[H(C272)(2)](3) are formed prior to M(C272)(3), whereas for HC301 the extraction complexes M(C301)(3) have priority over M[H(C301)(2)](3). The calculated M-O and M-S bond lengths and the M-P distances of these preferred extraction complexes agree very well with the available experimental data. The obtained changes of the Gibbs free energies in the liquid-liquid extraction reactions (1): Maqu3+ + 3(HC272)(2,org) -> M[H(C272)(2)](3, org) + 3H(aqu) + and (2): M-aqu(3+) + 3HC301(org) -> M(C301)(3,org) + 3(Haqu)(+) agree with the experimentally observed thermodynamical priorities of HC272 and HC301, i.e., HC272 prefers Eu3+ over Am3+/Cm3+ and HC301 prefers Am3+/Cm3+ over Eu3+. The obtained changes of the Gibbs free energies in reaction (2) (Eu, 68.1 kJ mol(-1); Am, 46.5 kJ mol(-1)) agree quite well with the experimental findings (Eu, 63.3 kJ mol(-1); Am, 44.1 kJ mol(-1)).