A Theoretical Study of the Decomposition Mechanisms in Substituted o-Nitrotoluenes

Unimolecular Decomposition Reactions of Picric Acid and Its Methylated Derivatives─A DFT Study

To handle energetic materials safely, it is important to have knowledge about their sensitivity. Density functional theory (DFT) has proven a valuable tool in the study of energetic materials, and in the current work, DFT is employed to study the thermal unimolecular decomposition of 2,4,6-trinitrophenol (picric acid, PA), 3-methyl-2,4,6-trinitrophenol (methyl picric acid, mPA), and 3,5-dimethyl-2,4,6-trinitrophenol (dimethyl picric acid, dmPA). These compounds have similar molecular structures, but according to the literature, mPA is far less sensitive to impact than the other two compounds. Three pathways believed important for the initiation reactions are investigated at 0 and 298.15 K. We compare the computed energetics of the reaction pathways with the objective of rationalizing the unexpected sensitivity behavior. Our results reveal a few if any significant differences in the energetics of the three molecules, and thus do not reflect the sensitivity deviations observed in experiments. These findings point toward the potential importance of crystal structure, crystal morphology, bimolecular reactions, or combinations thereof on the impact sensitivity of nitroaromatics.

Theoretical studies on the initial reaction kinetics and mechanisms of p-, m- and o-nitrotoluene

The potential energy surfaces (PESs) of three nitrotoluene isomers, such as p-nitrotoluene, m-nitrotoluene, and o-nitrotoluene, have been theoretically built at the CCSD(T)/CBS level. The geometries of reactants, transition states (TSs) and products are optimized at the B3LYP/6-311++G(d,p) level. Results show that reactions of -NO2 isomerizing to ONO, and C-NO2 bond dissociation play important roles among all of the initial channels for p-nitrotoluene and m-nitrotoluene, and that the H atom migration and C-NO2 bond dissociation are dominant reactions for o-nitrotoluene. In addition, there exist pathways for three isomer conversions, but with high energy barriers. Rate constant calculations and branching ratio analyses further demonstrate that the isomerization reactions of O transfer are prominent at low to intermediate temperatures, whereas the direct C-NO2 bond dissociation reactions prevail at high temperatures for p-nitrotoluene and m-nitrotoluene, and that H atom migration is a predominant reaction for o-nitrotoluene, while C-NO2 bond dissociation becomes important by increasing the temperature.

Chemoinformatics for the Safety of Energetic and Reactive Materials at Ineris

The characterization of physical hazards of substances is a key information to manage the risks associated to their use, storage and transport. With decades of work in this area, Ineris develops and implements cutting-edge experimental facilities allowing such characterizations at different scales and under various conditions to study all of the dreaded accident scenarios. This review presents the efforts engaged by Ineris more recently in the field of chemoinformatics to develop and use new predictive methods for the anticipation and management of industrials risks associated to energetic and reactive materials as a complement to experiments. An overview of the methods used for the development of Quantitative Structure-Property Relationships for physical hazards are presented and discussed regarding the specificities associated to this class of properties. A review of models developed at Ineris is also provided from the first tentative models on the explosivity of nitro compounds to the successful application to the flammability of organic mixtures. Then, a discussion is proposed on the use of QSPR models. Good practices for robust use for QSPR models are recalled with specific comments related to physical hazards, notably for regulatory purpose. Dissemination and training efforts engaged by Ineris are also presented. The potential offered by these predictive methods in terms of in silico design and for the development of new intrinsically safer technologies in safety-by-design strategies is finally discussed. At last, challenges and perspectives to extend the application of chemoinformatics in the field of safety and in particular for the physical hazards of energetic and reactive substances are proposed.

Theoretical prediction of the trigger linkages, surface electrostatic potentials, and explosive sensitivities of 1,4-dinitroimidazole-N-oxide in the external electric fields

In order to introduce effectively the external electric fields into the explosive systems, the change trends of the strengths of trigger linkages, nitro group charges, and explosive sensitivities of 1,4-dinitroimidazole-N-oxide (1,4-DNIO) were investigated in the external electric fields at the B3LYP/6-311++G(2d,p) and M06-2X/aug-cc-pVTZ levels. The formulas for calculating the impact sensitivity by the surface electrostatic potentials were discussed. The results show that the N-NO₂ bond is always the most likely trigger linkage, followed by N → O. This is the very valuable information for the researchers engaged in the molecular design or synthesis of the energetic explosives: The influences of the weak N → O coordination bond attached to the aromatic ring on the explosive sensitivity can be ignored when the N-NO₂ bond exists. In the external electric fields along the positive directions of the N → O and C-NO₂ bond axes as well as the negative direction of the N-NO₂ bond axis, the dissociation energies (BDEs) of the N-NO₂ bond and h₅₀ values are increased, leading to the decreased impact sensitivities. The changes of the bond lengths, AIM electron density values, nitro group charges, BDEs of the trigger linkages, and impact sensitivities correlate well with the external electric field strengths, respectively. The effects of the fields on the electric spark sensitivities and shock initiation pressures are not obvious. The essence of the low BDEs of the N-NO₂ bond was revealed by the resonance theory of the aromatic ring. Graphical abstract Changes of the impact sensitivities versus field strengths.

Reversible intramolecular hydrogen transfer: a completely new mechanism for low impact sensitivity of energetic materials

The intramolecular H transfer of energetic NO₂-compounds has been recognized as a possible primary step in triggering molecular decomposition for a long time.

Diverting Reactive Intermediates Toward Unusual Chemistry: Unexpected Anthranil Products from Davis-Beirut Reaction

The discovery of a new variation on the Davis-Beirut reaction is described in which an atypical heterocyclic framework (the anthranil or benzo[c]isoxazole framework) is formed as the result of diversion of a key reactive intermediate away from its expected reactivity-a potentially general approach to reaction design and development. Experimental and computational support for the proposed mechanism and origins of altered reactivity are described.

Reply to the comment on “Decomposition mechanisms of trinitroalkyl compounds: a theoretical study from aliphatic to aromatic nitro compounds” by G. Fayet, P. Rotureau, B. Minisini, Phys. Chem. Chem. Phys., 2014, 16, 6614

Decomposition reactions of trinitrobutane.

Decomposition mechanisms of trinitroalkyl compounds: a theoretical study from aliphatic to aromatic nitro compounds

The chemical mechanisms involved in the decomposition of trinitroethyl compounds were studied for both aliphatic and aromatic derivatives using density functional theory calculations. At first, in the case of 1,1,1-trinitrobutane, used as a reference molecule, two primary channels were highlighted among the five investigated ones: the breaking of the C-N bond and the HONO elimination. Then, the influence of various structural parameters was studied for these two reactions by changing the length of the carbon chain, adding substituents or double bonds along the carbon chain. If some slight changes in activation energies were observed for most of these features, no modification of the competition between the two investigated reactions was highlighted and the breaking of the C-N bond remained the favoured mechanism. At last, the reactions involving the trinitroalkyl fragments were highlighted to be more competitive than reactions involving nitro groups linked to aromatic cycles in two aromatic systems (4-(1,1,1-trinitrobutyl)-nitrobenzene and 2-(1,1,1-trinitrobutyl)-nitrobenzene). This showed that aromatic nitro compounds with trinitroalkyl derivatives decompose from their alkyl part and may be considered more likely as aliphatic than as aromatic regarding the initiation of their decomposition process.

QSPR modeling of thermal stability of nitroaromatic compounds: DFT vs. AM1 calculated descriptors

The quantitative structure-property relationship (QSPR) methodology was applied to predict the decomposition enthalpies of 22 nitroaromatic compounds, used as indicators of thermal stability. An extended series of descriptors (constitutional, topological, geometrical charge related and quantum chemical) was calculated at two different levels of theory: density functional theory (DFT) and semi-empirical AM1 approaches. Reliable models have been developed for each level, leading to similar correlations between calculated and experimental data (R(2) > 0.98). Hence, both of them can be employed as screening tools for the prediction of thermal stability of nitroaromatic compounds. If using the AM1 model presents the advantage to be less time consuming, DFT allows the calculation of more accurate molecular quantum properties, e.g., conceptual DFT descriptors. In this study, our best QSPR model is based on such descriptors, providing more chemical comprehensive relationships with decomposition reactivity, a particularly complex property for the specific class of nitroaromatic compounds.