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

Actinyl-actinyl interactions are particularly prevalent for the pentavalent neptunyl cation (Np(V)O₂)⁺ 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 ΔG 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).