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

Abstract

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). Tshaped interactions have been previously identified in high concentration Np(V) solutions containing simple anions (NO 3 À , ClO 4À , Cl À ) 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), CH 3 (acetate), CH 2 CH 3 (propionate)) and aromatic (R = C 6 H 5 (benzoate), C 6 H 4 OH (4-hydroxybenzoate), C 5 H 4 N (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).

Synthesis, characterization, and density functional theory investigation of (CH6N3)2[NpO2Cl3] and Rb[NpO2Cl2(H2O)] chain structures

“…The ν 1 is also sensitive to NCIs, where strong interactions can weaken the NpvO bond and reduce the symmetry, causing a red-shift in the peak position and the appearance of additional features. [14][15][16]60…”

Three-Dimensional Noncovalent Interaction Network within [NpO2Cl4]2– Coordination Compounds: Influence on Thermochemical and Vibrational Properties

“…The interaction between the actinyl oxo and neighboring groups can be classified into various forms, including actinyl–cation (ACIs), 13 , 14 actinyl–hydrogen (AHIs), 15 , 16 actinyl–halogen (AXIs), 10 , 12 and actinyl–actinyl interactions (AAIs). 17 ACIs, AHIs, and AXIs are found in both hexa- and pentavalent species; however, AAIs are more common in pentavalent species. 17 , 18 When compared to the hexavalent cation (AnO 2 2+ ), the pentavalent cation (AnO 2 + ) has greater electron density at the An n+ center, weakening the An=O bond and increasing axial oxygen participation in NCIs.…”

“… 17 ACIs, AHIs, and AXIs are found in both hexa- and pentavalent species; however, AAIs are more common in pentavalent species. 17 , 18 When compared to the hexavalent cation (AnO 2 2+ ), the pentavalent cation (AnO 2 + ) has greater electron density at the An n+ center, weakening the An=O bond and increasing axial oxygen participation in NCIs. 6 , 19 Because of the reasons outlined above, NpO 2 2+ is projected to have a higher tendency to produce NCIs than UO 2 2+ .…”

“…While the structural chemistry of the actinyl cation is similar between U and Np, subtle differences in the electronic structure impact the overall chemistry of the system, particularly for noncovalent interactions (NCIs) that take place in the second coordination sphere environment. ,, Strong An=O bonding reduces the Lewis basicity of the axial oxygen, but the equatorial ligands increase electron density at the An center, weakening the An=O bond , and promoting a range of NCIs. The interaction between the actinyl oxo and neighboring groups can be classified into various forms, including actinyl–cation (ACIs), , actinyl–hydrogen (AHIs), , actinyl–halogen (AXIs), , and actinyl–actinyl interactions (AAIs) . ACIs, AHIs, and AXIs are found in both hexa- and pentavalent species; however, AAIs are more common in pentavalent species. , When compared to the hexavalent cation (AnO 2 2+ ), the pentavalent cation (AnO 2 + ) has greater electron density at the An n+ center, weakening the An=O bond and increasing axial oxygen participation in NCIs. , Because of the reasons outlined above, NpO 2 2+ is projected to have a higher tendency to produce NCIs than UO 2 2+ .…”

“…The interaction between the actinyl oxo and neighboring groups can be classified into various forms, including actinyl–cation (ACIs), , actinyl–hydrogen (AHIs), , actinyl–halogen (AXIs), , and actinyl–actinyl interactions (AAIs) . ACIs, AHIs, and AXIs are found in both hexa- and pentavalent species; however, AAIs are more common in pentavalent species. , When compared to the hexavalent cation (AnO 2 2+ ), the pentavalent cation (AnO 2 + ) has greater electron density at the An n+ center, weakening the An=O bond and increasing axial oxygen participation in NCIs. , Because of the reasons outlined above, NpO 2 2+ is projected to have a higher tendency to produce NCIs than UO 2 2+ . However, only a minor increase in electron density surrounding the oxo ligands has previously been described, implying a comparable ability to make NCIs. , …”