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.