Modified Evans–Polanyi–Semenov relationship in the study of chemical micromechanism governing detonation initiation of individual energetic materials

Abstract

The paper deals with basic problems of the study of the thermal reactivity of individual energetic materials with special attention on the compatibility of results obtained using different methods. It is shown that the activation energies for thermal decomposition of these materials, obtained under comparable conditions, correlate with the respective detonation heats through a modified Evans–Polanyi–Semenov equation. These correlation relationships apply within groups of molecules with closely related structures and underline the importance of the bond that is primarily split in the molecule for the detonation of a given material. They also indicate that the primary fragmentation processes during low-temperature thermal decomposition are the same as those controlling the detonation reaction. It is shown that the correlation relationships found are also valid in the case of electric spark initiation.

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, CNO₂, NNO₂ or ONO₂ 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 CNO₂, NNO₂, ONO₂ [18], [19], [20], [21], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [42], [46], [47], [48], [62], [63] and NNO [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 NNO₂ 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:

$$\text{E=B+α′}\text{ΔH}$$

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 Q_real and E by activation energy E_a 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):

$$\text{E}{}_{\mathrm{<mtext>a</mtext>}}\text{=C+αQ}{}_{\mathrm{<mtext>real</mtext>}}$$

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.