Mechanism of Direct Dissolution of PuO ₂ in the Task-Specific Ionic Liquid [Hbet][NTf ₂ ]: An Insight from Density Functional Theory

Although in recent times, the task-specific ionic liquid [Hbet][NTf₂] has been extensively used for direct dissolution of many solid metal oxides like PuO₂, the mechanism of the dissolution is still not clear. In this work, density functional theory is used to investigate the dissolution mechanism of PuO₂, a frequently encountered useful solid in the nuclear industry. By comparison of reaction energies of plausible pathways of dissolution with the cohesive energy of solid PuO₂, four energetically feasible schemes are identified. These routes are based on the complex formation of Pu⁴⁺ with deprotonated [Hbet]⁺, water, and/or [NTf₂]⁻ ligands. Either four or six deprotonated [Hbet]⁺ ligands coordinate directly to Pu⁴⁺, while in situ generated water molecules further stabilize the complexes. Crucially, participation of the anionic [NTf₂]⁻ ligand, in either the first or second solvation shell, is found to be essential for providing the requisite reaction energy to overcome the lattice energy required to dissolve the solid PuO₂. Energy decomposition analysis elucidates the nature of metal-ligand interactions; RDG and NCI analyses unfold the role of noncovalent interactions in complex formation, whereas Mulliken and CHELPG charge analyses reveal the extent of metal-to-ligand charge transfer and coordinate bonding.