Modified Evans–Polanyi–Semenov relationship in the study of chemical micromechanism governing detonation initiation of individual energetic materials
Introduction
The influence of shock on energetic materials results in adiabatic compression of the molecular layer struck. According to Klimenko and Dremin [1], [2], [3], [4], the kinetic energy of the shock in this compression is accumulated, through translational–vibrational relaxation processes, by translational and vibrational modes of molecular crystals of the material within 10−13 to 10−12 s. This causes a considerable quasi-overheating (20 000–40 000 K [3], [4]) especially of vibrational modes. A nonequilibrium state is established with concomitant primary fission of the energetic material into ions and radicals [2], [3], [4]. Chemical reactions of these active particles causes the shock front to spread and evoke a second equilibrium stage of detonation behind the front. This or similar ideas of transformation of low-frequency vibrations of crystal lattice (acoustic phonons) into high-frequency vibrations (vibrons), with subsequent spontaneous localization of vibrational energy in the explosophore groupings [10], [11], have been applied by a number of authors in their studies of shock reactivity of energetic materials (for representative papers see [5], [6], [7], [8], [9], [10], [11]). Conclusions of this type also correspond to the older simplified idea, which was formulated by Bernard [12], [13] on the basis of the kinetic theory of detonation, that only explosophore groups are compressed ahead of the shock front, as a result of the activation of explosive molecules.
Since the middle 1970s, the studies of impact and shock reactivities and the chemical micromechanism of the initiation of organic polynitro compounds have also been adopting quantum chemistry methods (see, e.g. [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]). From the findings thus obtained it follows that the principal reactive mojety is the nitro group or, more specifically, CNO2, NNO2 or ONO2 bonds [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. There exist direct experimental [28], [29], [30] as well as indirect semi-empirical [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50] pieces of evidence for the above-mentioned facts.
It is well known that the above-mentioned groupings are carriers also thermal reactivity of corresponding energetic materials (i.e. polynitro compounds, see [18], [19], [20], [21], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63]). The mechanisms of primary fragmentation in thermal decomposition of organic polynitro and polynitroso compounds can be divided into [64] the following:
- •
homolysis of CNO2, NNO2, ONO2 [18], [19], [20], [21], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [42], [46], [47], [48], [62], [63] and NNO [35], [65] bonds;
- •
homolysis via a five- or six-membered transition state or aci-form [38], [49], [50], [51], [52], [53];
- •
in a few cases homolytic fragmentation without a primary participation of a nitro group [66], [67], [68], [69].
From what has been said so far it follows that the primary fragmentations in both the detonation and thermal decomposition of explosive molecules are similar. The similarity or identity between the primary mechanism of low-temperature and detonation reactions is a topic of numerous papers [32], [33], [34], [35], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [69], [70]. The identity is also confirmed by some striking pieces of experimental evidence. First of all they include the evidence (obtained with the help of Raman spectroscopy and XPS) of primary fission of NNO2 bond in 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) exposed to shock wave [28], [29]. On the basis of deuterium kinetic isotope effect (DKIE) it was proved [54], [71] that the rate-limiting step for the thermal decomposition of 2,4,6-trinitrotoluene (TNT) in the condensed state and that for the initiation of its detonation are identical. The presence of furoxanes and furazanes in the XPS spectrum of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) exposed to shock [30], [72], [73] provides the further evidence—the pyrolysis of ortho-nitroanilines is a method of synthesis of benzofurazane [74] and in the case of 1,3-diamino-2,4,6-trinitrobenzene (DATB) this reaction leads to 4-amino-5,7-dinitrobenzofurazane [75]. The identity is also reflected in a relationship between the kinetics of the low-temperature thermal decomposition of the energetic materials and reaction rates in the reaction zone of their detonation [76], [77].
The homolytic character of primary fission in both the detonation and low-temperature thermal decompositions of energetic materials was a motive for Zeman et al. [34] to use the Evans–Polanyi–Semenov (E–P–S) equation [78], [79] to study the chemical micromechanism governing initiation of energetic materials [34], [35], [40], [41], [42], [44], [45], [46], [69], [70], [80]. The original E–P–S equation describes a relationship between activation energies E of most substitution reactions of free radicals and corresponding heats of reaction DH:where B is a constant. Eq. (1) is valid for closely related molecules and indicates that the strength of bond being split is a decisive factor in the given reaction [78], [79]. Substitution of ΔH by real heat of explosion Qreal and E by activation energy Ea of the low-temperature thermal decomposition has led to first version of modified E–P–S equation [34], [35], [40], [69], [70] in the general shape (here C is a constant):which is applicable for the detonation of energetic materials. The present paper re-examines some aspects of Eq. (2) from the point of view of new findings and reviewed applications of this equation to the study of the chemical micromechanism of initiation of individual energetic materials.
Section snippets
Basic methods of following the thermal decomposition (thermal reactivity)
The course of the primary fission processes in thermal decomposition of the energetic materials is generally followed by studying secondary effects of them, i.e. the type and quantity of gaseous products, and/or thermal effects, and/or the mass decrease accompanying the reaction. The thermal decomposition of individual energetic materials thus can be divided into two limiting types [61]:
- 1.
Fission the products of which have low molecular weight; its mechanism does not involve too many intermediate
Polynitro arenes and polynitro heteroarenes
The set of polynitro arenes and polynitro heteroarenes in Table 1 is divided—in the sense of Eq. (2)—into several groups documented by Fig. 1, Fig. 2, Fig. 3. The substances given in Fig. 1 are characterized by the presence of a hydrogen atom at γ-position with respect to the nitro group, i.e. they are polynitro compounds exhibiting the so-called trinitrotoluene mechanism of thermal decomposition [38], [50], [51], [52]. Exceptional is polychlorinated derivatives of TNB (i.e. CTB, DCTB, and
Conclusion
The homolytic fragmentations or reactions of the CNO2, NNO2, NNO, and ONO2 groupings, or other bearers of explosibility [14], [69], [70] (i.e. explosophores), are common primary fission processes of energetic materials under thermal [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], impact [17], [18], [29], [30], [40], [48], shock [14], [15], [16], [17], [25], [29], [40], [46], and electric spark [42], [43], [44], [45], [47] stimuli.
A mechanism of the thermal decomposition
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