A B3LYP and MP2(full) theoretical investigation into explosive sensitivity upon the formation of the intermolecular hydrogen-bonding interaction between the nitro group of RNO2 (R=–CH3, –NH2, –OCH3) and HF, HCl or HBr

Strong External Electric Fields Reduce Explosive Sensitivity: A Theoretical Investigation into the Reaction Selectivity in NH2NO2∙∙∙NH3

Controlling the selectivity of a detonation initiation reaction of explosive is essential to reduce sensitivity, and it seems impossible to reduce it by strengthening the external electric field. To verify this, the effects of external electric fields on the initiation reactions in NH2NO2∙∙∙NH3, a model system of the nitroamine explosive with alkaline additive, were investigated at the MP2/6-311++G(2d,p) and CCSD(T)/6-311++G(2d,p) levels. The concerted effect in the intermolecular hydrogen exchange is characterized by an index of the imaginary vibrations. Due to the weakened concerted effects by the electric field along the −x-direction opposite to the “reaction axis”, the dominant reaction changes from the intermolecular hydrogen exchange to 1,3-intramolecular hydrogen transference with the increase in the field strengths. Furthermore, the stronger the field strengths, the higher the barrier heights become, indicating the lower sensitivities. Therefore, by increasing the field strength and adjusting the orientation between the field and “reaction axis”, not only can the reaction selectivity be controlled, but the sensitivity can also be reduced, in particular under a super-strong field. Thus, a traditional concept, in which the explosive is dangerous under the super-strong external electric field, is theoretically broken. Compared to the neutral medium, a low sensitivity of the explosive with alkaline can be achieved under the stronger field. Employing atoms in molecules, reduced density gradient, and surface electrostatic potentials, the origin of the reaction selectivity and sensitivity change is revealed. This work provides a new idea for the technical improvement regarding adding the external electric field into the explosive system.

Theoretical investigation into the solvent effect on the thermal decomposition of RDX in tetrahydrofuran, acetone, toluene, and benzene

In order to clarify the solvent effect on the thermal decomposition of explosive, the N-NO₂ trigger-bond strengths and ring strains of RDX (cyclotrimethylenetrinitramine) in its H-bonded complexes with solvent molecules (i.e., tetrahydrofuran, acetone, toluene, and benzene), and the activation energies of the intermolecular hydrogen exchanges between the solvent molecules and C₃H₈O₂N₄ or CH₄O₂N₂, as the model molecule of RDX, were investigated by the BHandHLYP, B3LYP, MP2(full), and M06-2X methods with the 6-311 +  + G(2df,2p) basis set, accompanied by a comparison with the calculations by the integral equation formalism polarized continuum model. The solvent effects ignore the ring strain while strengthening the N-NO₂ bond, leading to a possible decreased sensitivity, as is opposite to the experimental results. However, the activation energies are in the order of C₃H₈O₂N₄/CH₄O₂N₂∙∙∙acetone < C₃H₈O₂N₄/CH₄O₂N₂∙∙∙THF < C₃H₈O₂N₄/CH₄O₂N₂∙∙∙toluene < C₃H₈O₂N₄/CH₄O₂N₂∙∙∙benzene < C₃H₈O₂N₄/CH₄O₂N₂, suggesting that the order of the critical explosion temperatures might be RDX∙∙∙acetone < RDX∙∙∙THF < RDX∙∙∙toluene < RDX∙∙∙benzene < RDX, as is roughly consistent with the experimental results. Therefore, the intermolecular hydrogen exchange with the HONO elimination is a possible mechanism of the solvent effect on the initial thermal decomposition of RDX. The solvent effect on the sensitivity is analyzed by the surface electrostatic potentials.

Review of the molecular and crystal correlations on sensitivities of energetic materials

Highly efficient design on the levels of molecule and crystal, as well as formulation, is highly desired for accelerating the development of energetic materials (EMs). Sensitivity is one of the most important characteristics of EMs and should be compulsorily considered in the design. However, owing to multiple factors responsible for the sensitivity, it usually undergoes a low predictability. Thus, it becomes urgent to clarify which factors govern the sensitivity and what is the importance of these factors. The present article focuses upon the progress of the molecular and crystal correlations on the sensitivity, and the molecule-based numerical models for sensitivity prediction in the past decades. On the molecular level, composition, geometric structure, electronic structure, energy and reactivity can be correlated with the sensitivity; while the sensitivity can be also related with molecular packing pattern, intermolecular interaction, crystal morphology, crystal size and distribution, crystal surface/interface and crystal defect on the crystal level. And most of these factors, in particle on the crystal level, have been employed as variables in numerical models for predicting sensitivity of categorized EMs. Besides, we stress that more attention should be paid to the sensitivity correlations on the inherent structures of EMs, molecule and crystal packing, because they can be readily dealt by molecular simulations nowadays, facilitating to reveal the physical nature of sensitivity.

A theoretical investigation into the strength of N-NO2 bonds, ring strain and electrostatic potential upon formation of intermolecular H-bonds between HF and the nitro group in nitrogen heterocyclic rings C n H2n N-NO2 (n = 2-5), RDX and HMX

Changes in N-NO2 bond strength, ring strain energy and electrostatic potential upon formation of intermolecular H-bonds between HF and the nitro group in nitrogen heterocyclic rings C n H2n N-NO2 (n = 2-5), RDX and HMX were investigated using DFT-B3LYP and MP2(full) methods with the 6-311++G(2df,2p) and aug-cc-pVTZ basis sets. Analysis of electron density shifts was also carried out. The results indicate that H-bonding energy correlates well with the increment of ring strain energy. Upon complex formation, the strength of the N-NO2 trigger-bond is enhanced, suggesting reduced sensitivity, while judged by the increased ring strain energy, sensitivity is increased. However, some features of the molecular surface electrostatic potential, such as a local maximum above the N-NO2 bond and ring, σ + (2) and electrostatic balance parameter ν, remain essentially unchanged upon complex formation, and only a small change in the impact sensitivity h 50 is suggested. It is not sufficient to determine sensitivity solely on the basis of trigger bond or ring strain; as a global feature of a molecule, the molecular surface electrostatic potential is available to help judge the change of sensitivity in H-bonded complexes. Graphical Abstract The strengthened N-NO2 bond suggests reduced sensitivity, while it is reverse by theincreased ring strain energy upon the complex formation. However, the molecular surfaceelectrostatic potential (V S) shows the little change of h 50. The V S should be taken into accountin the analysis of explosive sensitivity in the H-bonded complex.

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.

A B3LYP AND MP2(FULL) THEORETICAL INVESTIGATION INTO THE C–NO2 BOND STRENGTH UPON THE FORMATION OF HF OR Na+ COMPLEX INVOLVING THE NITRO GROUP OF NITRO-1,2,4-TRIAZOLE

The change of bond dissociation energy (ΔBDE) in the C–NO2 bond upon the formation of the intermolecular hydrogen-bonding or molecule-cation interaction between the nitro group of seven kinds of nitro-1,2,4-triazoles and HF or Na+ was investigated using the B3LYP and MP2(full) methods with the 6-311++G**, 6-311++G(2df, 2p) and aug-cc-pVTZ basis sets. The C–NO2 bond strength was enhanced and the charge of nitro group turned more negative in complex in comparison with those in isolated nitro-1,2,4-triazole molecule. The increment of the C–NO2 BDEs correlated well with the H-bonding interaction energy or molecule-cation interaction energy. The analysis of AIM, NBO and electron density shifts showed that the electron density shifted toward the C–NO2 bond upon complex formation, leading to the strengthened C–NO2 bond and the possibly reduced explosive sensitivity. The ΔBDE of the C–NO2 bond in the Na+ complex is far larger greater than that in the corresponding HF system. Thus, introducing cation into the structure of the nitrotriazole might be more efficacious to reduce explosive sensitivity than the formation of the intermolecular hydrogen-bonded complex.