A theoretical study on the strength of the C-NO2 bond and ring strain upon the formation of the intermolecular H-bonding interaction between HF and nitro group in nitrocyclopropane, nitrocyclobutane, nitrocyclopentane or nitrocyclohexane.

As a follow-up to our investigation into the influence of H-bond on the C-NO2 trigger bond, a comparison of the effect of the H-bond on the ring strain energy with the C-NO2 bond dissociation energy was carried out in the HF complex with nitrocyclopropane, nitrocyclobutane, nitrocyclopentane, and nitrocyclohexane by using the DFT-B3LYP and MP2 (full) methods with the 6-311++G(2df,2p) and aug-cc-pVTZ basis sets. The C-NO2 bond length decreases with strengthening of trigger-bond while the ring perimeter increases companied by weakening of ring strain upon the complex formation. The H-bonding energy correlates well with the increment of ring perimeter and the change of ring bond angle. For nitrocyclopropane∙∙∙HF, the effect of H-bond on the ring strain energy is notable, while for the other complex, it is negligible. Therefore, for nitrocyclopropane∙∙∙HF, the origin of the change of explosive sensitivity might be due to the increment of the C-NO2 bond dissociation energy and decrease of the ring strain energy, while for the other complex, it might be only due to the strengthening of C-NO2 bond. The analysis of electron density shifts shows that the C-C bond in ring loses density while the C-NO2 bond gains density, leading to the weakened ring strain and strengthened C-NO2 bond, and thus the possibly reduced explosive sensitivity.