Analysis and prediction of the Arrhenius parameters of low-temperature thermolysis of nitramines by means of the spectroscopy
Introduction
There is a great interest in the thermal reactivity of nitramines due at least in part to their importance as energetic materials. (e.g. see 1, 2). The homolysis of the N–NO2 bond was proved as a primary step of thermolysis of the secondary nitramines in a condensed state 3, 4, 5. In the case of primary nitramines the homolysis is limiting step in their thermal reactivity in gaseous state, whereas their thermolysis in the condensed state is a bimolecular autoprotolytic reaction [7]. As the valence states of nitro compounds in generally (including ground state) have a large component of biradical character [6]a bimolecular course of the primary thermolysis of the compounds, including some nitramines, cannot be fully excluded from (about the pseudomonomolecular course of their thermolysis, see [8]). All the said types of reactivity of nitramine nitro group depend upon the electron density on its nitrogen atom. The density is predominantly a function of the extend to which the amino nitrogen lone pair is involved in π-bonding with this nitro group, i.e. both the homolysis of N–NO2 bond and hydrogen abstraction by an oxygen of the nitro group should be depended upon this N–N bond strength.
It is a well-known fact that chemical shifts may be taken to indicate the degree of shielding of the atoms which affect the adjacent N–N bond strengths. A relationship corresponding to this has the following general form [9]where Ea is activation energy of non-autocatalyzed thermolysis of nitramines and δN is the chemical shift of nitro group nitrogen. With respect to the kinetic compensation effect in this thermolysis [8]also this relationship was found [9]where A means the Arrhenius preexponent of the above-mentioned decomposition. As the nitro groups play a key role in thermal reactivity of nitro compounds in general a less close correlation results from the application of the chemical shifts δA of amino nitrogens of nitramino groups in the both relationships [9].
The study of thermal reactivity of nitramines is the important starting point for selection and exploitation of these nitro compounds. However, some published conclusions of the study are contradictory, which is due to both unsuitable choice of experimental conditions and a wrong interpretation of results 4, 8, 9. Therefore a method is needed for mutual comparison and evaluation of results obtained in various laboratories. From the point of view of development of the new energetic materials also predicted characteristics of thermal reactivity of nitramines are significant. The relationships 1 and 2 signalize a possibility of their application not only in a prediction but also in analysis of the Arrhenius parameters resulted from the low-temperature thermolysis of nitramines. These problems are discussed in the present paper which extends the findings of the recent study [9].
Section snippets
spectroscopy
Survey of the nitramines studied and their code designation is given in Table 1. The chemical shifts δ of the nitramines were obtained with the help of an AMX-360 Bruker apparatus using the INEPT method. The samples were dissolved in hexadeuteriodimethyl sulfoxide at a concentration of 0.2 mol nitramine per 1 dm3 solution. For some of the substances the values of these shifts were taken from literature 11, 12; those of the substances not yet prepared were predicted. These all δA and δN
Discussion
Fig. 1Fig. 2 are graphic representation of , , respectively. In these figures the dependence A corresponds to N–N bond homolysis in the solid state and the dependence B to the same fission in the liquid state. The Arrhenius parameters have been predicted by means of these dependencies (calibration curves) for nitramines whose thermolysis data have not been experimentally obtained yet (see Table 1). Predicted Ea value for liquid phase thermolysis of DIGEN agrees well with the N–N bond energy
Conclusion
The earlier-described relationship [9]between the Arrhenius parameters of low-temperature thermolysis obtained at the conditions of the Russian manometric method (SMM) and the chemical shifts of nitrogen atoms in nitro groups of their nitramino groups possesses a broader validity. The said relationship makes it possible to:
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predict the Arrhenius parameters for the nitramines whose thermal decomposition has not been studied yet,
- 2.
assess which nitramino group in the molecule is the first to
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2019, Defence TechnologyCitation Excerpt :However, intensive use of quantum chemical methods in the study of EMs in general is head and shoulders above the mentioned chemical approach, as well as in the study of their initiation reactivity [1,27–35] but with a tendency to make a general conclusion for several EMs with different molecular structures. However, Politzer and Murray have shown that the correlations found here [30] are restricted to specific classes(i.e. nitroaromatics, nitroheterocyclics and nitramines) which is perfectly in line with findings from approaches based on physical organic chemistry principles (see Refs. [1,5,12,14,15,17,25]). The already mentioned dipole-dipole interactions are connected with the non-binding inter-atomic distances between oxygen atoms inside and outside all of the nitro groups in these poly-nitro compounds.
Crystal lattice free volume in a study of initiation reactivity of nitramines: Friction sensitivity
2018, Defence TechnologyCitation Excerpt :Its data correlate well with those of DMEDNA and EDNA using the straight line E. Comparison with the nitramines associated with the straight line D clearly shows that the difference between ε−HNIW and its RS-analogue (product with reduced sensitivity [14]) rests in the difference in the intensity and uniformity of intermolecular interactions in their crystals. From this comparison it seems as if in the ε−HNIW crystals only the nitro groups in positions 2,4,6, and 8 had a major part in the intermolecular force in its crystals – in so doing, the most reactive nitramino grouping is in position 2 of this particular nitramine [1,7,20–22]. The above-mentioned disintegration of the nitramine groups in Fig. 1, namely differences between compositions of groups with positive and negative slopes of the corresponding straight lines, on the first look is connected with kind and character of intermolecular interactions in crystals of these compounds.
Characteristics of Thermal Decomposition of Energetic Materials in a Study of Their Initiation Reactivity
2018, Handbook of Thermal Analysis and CalorimetryCitation Excerpt :It is appropriate to include in this list a study by Tsyshevsky et al., according to which the thermal decomposition processes play a major role in defining the sensitivity (initiation reactivity) to detonation initiation of such EMs [14]. The characteristics of these processes are easily available and, when these correspond to a non-autocatalyzed mechanism, it is possible to extrapolate them to the conditions of detonation [1–5,7]. The facts outlined above are a part of this chapter, which also includes certain possibilities for determining the reaction centers in molecules.
Relationship between electric spark sensitivity and activation energy of the thermal decomposition of nitramines for safety measures in industrial processes
2013, Journal of Loss Prevention in the Process IndustriesCitation Excerpt :Therefore, we should know or predict the value of its sensitivity parameters and thermal stability. For thermolysis of nitramines, it was found that the homolysis of the N–NO2 bond is a primary step of the secondary nitramines whereas the homolysis of primary nitramines is a bimolecular autoprotolytic reaction (Zeman, 1999). However, the longest N–N bonds are responsible for homolytic reactivity of nitramines, which may contribute strongly to the intermolecular potential in the crystal state.
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