Complexation Behavior of the Tri‑<i>n</i>‑butyl Phosphate Ligand with Pu(IV) and Zr(IV): A Computational Study

Tri-<i>n</i>-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior extraction abilities for Pu­(IV) over a large number of fission products including Zr­(IV). We have applied density functional theory (DFT) calculations to explain this selectivity by investigating differences in electronic structures of Pu­(NO<sub>3</sub>)<sub>4</sub>·2TBP and Zr­(NO<sub>3</sub>)<sub>4</sub>·2TBP complexes. On the basis of our quantum chemical calculations, we have established the lowest energy electronic states for both complexes; the quintet is the ground state for the former, whereas the latter exists in the singlet spin state. The calculated structural parameters for the optimized geometry of the plutonium complex are in agreement with the experimental results. Atoms in Molecules analysis revealed a considerable amount of ionic character to M–O­{TBP} and M–O­{NO<sub>3</sub>} bonds. Additionally, we have also investigated the extraction behavior of TBP for metal nitrates and have estimated the extraction energies to be −73.1 and −57.6 kcal/mol for Pu­(IV) and Zr­(IV), respectively. The large extraction energy of Pu­(IV) system is in agreement with the observed selectivity in the extraction of Pu.