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

Abstract

AbstractActinyl‐actinyl interactions are particularly prevalent for the pentavalent neptunyl cation (Np(V)O2)+where these interactions appear either as aT‐ orD‐shape (diamond‐shape).T‐shaped interactions have been previously identified in high concentration Np(V) solutions containing simple anions (NO3−, ClO4−, Cl−) whereasD‐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 ofD‐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 ΔGto form hydratedT‐ andD‐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 theD‐shaped interactions, but spectral changes observed over time indicate a dynamic system. Crystallization experiments from the Np(V) carboxylate systems confirmed the presence ofD‐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).