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

Abstract

Noncovalent interactions (NCIs) can influence the stability and chemical properties of pentavalent and hexavalent actinyl (AnO2 +/2+) compounds. In this work, the impact of NCIs (actinyl–hydrogen and actinyl–cation interactions) on the enthalpy of formation (ΔH f) and vibrational features was evaluated using Np­(VI) tetrachloro compounds as the model system. We calculated the ΔH f values of these solid-state compounds through density functional theory+ thermodynamics (DFT+ T) and validated the results against experimental ΔH f values obtained through isothermal acid calorimetry. Three structural descriptors were evaluated to develop predictors for ΔH f, finding a strong link between ΔH f and hydrogen bond energy (E H total) for neptunyl–hydrogen interactions and total electrostatic attraction energy (E electrostatic total) for neptunyl–cation interactions. Finally, we used Raman spectroscopy together with bond order analysis to probe Np=O bond perturbation due to NCIs. Our results showed a strong correlation between the degree of NCIs by axial oxygen and red-shifting of Np=O symmetrical stretch (ν1) wavenumbers and quantitatively demonstrated that NCIs can weaken the Np=O bond. These properties were then compared to those of related U­(VI) and Np­(V) phases to evaluate the effects of subtle differences in the NCIs and overall properties. In general, the outcomes of our study demonstrated the role of NCIs in stabilizing actinyl solid materials, which consequently governs their thermochemical behaviors and vibrational signatures.

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

“…Our previous work with uranyl 11 and neptunyl 57 solid state compounds has highlighted the importance of Non-Covalent Interactions (NCIs) in driving crystallization and the overall formation energies. To analyze the NCIs of Np-Gua and Np-Rb , we adopted the methodology developed in our prior work.…”

“…Ligated water molecules present in the Np-Rb chains form strong hydrogen bonding (BO = 0.109) with the axial oxygens on the neighboring neptunyl( v ) chains. The BO “yl” sum of Np-Cs was reported as 0.374, 57 which allows us to rank the BO “yl” sum of complexes as Np-Cs > Np-Rb > Np-Gua (Table 2). This trend in BO “yl” sum and interaction energies can be rationalized as being related to the overall molecular charges as it goes from −3 in the isolated Np-Cs units to −2 and −1 for Np-Gua and Np-Rb , respectively.…”

Structural chemistry of penta- and hexanitrato thorium(iv) complexes isolated using N–H donors

“…For metal ions with no f-electrons, such as Th, vibrational spectroscopy has proven a valuable tool for further examining actinide-ligand bonding as well as the relative strength of noncovalent interactions. 10,[64][65][66][67] As such, compounds 1-15 were examined using Raman and IR spectroscopy; the spectra for 14, [3-ClPyH] 2 [Th(NO 3 ) 6 ], are presented in Fig. 17 as a representative example and the spectra for the other compounds are provided as ESI † (Fig.…”

“…[1][2][3][4] Indeed, such recognition as well as the application of these elements to areas ranging from biomedicine 5,6 to quantum information science 7 has motivated research efforts centered on actinide structural chemistry, bonding, and reactivity. [8][9][10][11][12][13][14] In aqueous systems, actinide speciation is principally governed by oxidation state. [2][3][4] Whereas thorium exists near exclusively as a tetravalent metal ion in water, uranium, neptunium, and plutonium have multiple accessible oxidation states, with the higher-valent ions forming 'yl' complexes and the lower-valent ions adopting more spherical coordination geometries.…”

“…1–4 Indeed, such recognition as well as the application of these elements to areas ranging from biomedicine 5,6 to quantum information science 7 has motivated research efforts centered on actinide structural chemistry, bonding, and reactivity. 8–14…”