Density Functional Theory Guided Investigation of Ligand‐Induced Neptunyl‐Neptunyl Interactions

Actinyl-actinyl interactions are particularly prevalent for the pentavalent neptunyl cation (Np(V)O-2)(+) where these interactions appear either as a T- or D-shape (diamond-shape). T-shaped interactions have been previously identified in high concentration Np(V) solutions containing simple anions (NO3-, ClO4-, Cl-) whereas D-shaped have only been isolated in the solid-state in the presence of carboxylate ligands. In this study, Density Functional Theory (DFT) calculations were paired with Raman spectroscopy to evaluate the formation of D-shaped interactions in the presence of aliphatic (R=H (formate), CH3 (acetate), CH2CH3 (propionate)) and aromatic (R=C6H5 (benzoate), C6H4OH (4-hydroxybenzoate), C5H4N (isonicotinate)) carboxylate ligands. DFT studies indicate that the Delta G to form hydrated T- and D-shaped forms are not spontaneous but become so with the addition of the carboxylate ligands. Raman spectra of the Np(V) carboxylate solutions contained vibrational modes associated with the D-shaped interactions, but spectral changes observed over time indicate a dynamic system. Crystallization experiments from the Np(V) carboxylate systems confirmed the presence of D-shaped dimers for the aromatic carboxylates, suggesting that the choice of the anion in solution favors actinyl-actinyl interactions even at low concentrations (<= 20 mM) of Np(V).