G. M. Nazin

Thermal stability of high-energy compounds

The influence of molecular structure on the stability of high-energy compounds is considered. The kinetic parameters of the decomposition of various energy-rich groups in monofunctional compounds are established. Data on decomposition of compounds with mixed functional groups are described. The sites of primary breakdown are determined and the mutual influence of functional groups on the stability is considered.

THERMAL DECOMPOSITION OF NITROMETHANE

1. n nA study was made of the decomposition of nitromethane in the presence of various inhibitors (NO, toluene, cis-2-butene), and at various ratios of the surface area and volume of the reaction vessel. n n n n n2. n nThe characteristics of the homogeneous monomolecular reaction were determined.

Kinetics and mechanism of thermal decomposition of nitropyrazoles

The decomposition of mono-, di-, and trinitropyrazole derivatives in the condensed state was studied by the manometric method. The reaction rate depends on the number and position of nitro groups in the pyrazole cycle and on the polarity of the medium and aggregate state of the substance. The activation energy of the initial non-catalytic stage of decomposition Е1 decreases on going from monoto trinitropyrazoles from 142 to 132 kJ mol−1, and the preexponential factor is 109±0.5 s−1. In a diphenyl solution the decomposition rate is lower than that in the melt, and this difference decreases with increasing in the number of nitro groups in the molecule. For the decomposition of trinitropyrazole in the solid state, Е1 decreases by 10 kJ mol−1. All these facts are explained in terms of the mechanism, according to which the reaction occurs as the oxidation of the adjacent carbon atom by the nitro group and proceeds via a strongly polar cyclic transition state.

Relation between the N-NO2 bond length and stability of the secondary nitramines

The fact of the constancy of activation entropy of N-NO2 bond homolysis in a series of secondary nitramines was utilized for correction of the experimental values of activation energy E of this process proceeding from the reliable data for the rate constants of the nitramines decomposition in solutions. When comparing the refined values of E (kJ mol−1) with the N-N bond length dN-N (Å) the following correlations were obtained: for cyclic and framework nitramines E = 663 − 356dN-N, and for the aromatic nitramines E = 1810 − 1227dN-N. A linear relationship between E and d is observed in the series of similar compounds. It depends on the electronic and steric effects of substituents.

Kinetics and mechanism of the thermal decomposition of keto-RDX

The thermal decomposition of keto-RDX occurs through the homolysis of the N-NO2 bond in one of the nitramide groups. This bond is substantially longer (1.438 Å) than the analogous bond in the RDX molecule (1.382 Å). The activation parameters of the decomposition, E (kcal/mol) and logA [s−1] were found to be 36.2 and 14.8 in benzene and 35.4 and 12.84 in the solid phase. In the latter case, the reaction proceeds through the dislocation mechanism. The crushing of large crystals produces no effect on the decomposition rate, which, however, depends on the regime of crystallization. The rearrangement into unstable diazoxy esters, a process typical of linear nitramides, does not occur in keto-RDX because of steric hindrances.

Thermal stability of 3,4,5-trinitropyrazole and its ammonium salt

The thermal decomposition of trinitropyrazole (I) and its ammonium salt proceeds with a very strong self-acceleration, caused mainly by the catalytic action of the condensed products. The first-order rate constant for the initial stage k1 describes the decomposition to a depth of conversion of 0.5% and is characterized by the following kinetic parameters E (kJ/mol) and log(A, s−1): 131.8 and 9.60 for the liquid phase and 116.0 and 8.57 for the solid state. The rate constant k1 is smaller if the reaction occurs in nonpolar solvents and if I is methylated at position 1. All these data are interpreted in the framework of a mechanism according to which the reaction involves the oxidation by a nitro group of a neighboring carbon atom and proceeds through a highly polar cyclic transition state. Evaluation of the thermal stability of I is conducted using the method of a reference series composed of well-known regular HEs, which for the first time was implemented in terms of k1. In the temperature range 20–80°C, the stability of trinitropyrazole is close to that of nitroglycerin. Trinitropyrazole ammonium salt is severalfold more stable than trinitropyrazole itself.

Chain and monomolecular decomposition of hexogen in solutions

The decomposition of hexogen dissolved in alkylaromatic hydrocarbons, alcohols, ketones, ethers, chloroform, and some other solvents occurs via the chain mechanism. This mechanism is supported by slowing down of the reaction when inhibitors are added, the solvent deuterium kinetic isotope effect, and the dependence of the rate on the reactivity of the C—H bond in solvents. The chain reaction propagates through the transfer of a free valence from the primary N-radicals formed by N—NO2 bond dissociation to the C-centered radicals of the solvent. The solvents are inert when the N—H bond dissociation energy is >380 or <200 kJ mol–1, and hexogen decomposition in such solvents is monomolecular.

Crystal defects and stability of RDX

NQR studies demonstrated that structure imperfection of RDX crystals is affected by impurities, the particle size, and the modes of crystallization and pressing of the sample. The initial rate of RDX decomposition depends on the same factors. In pure samples, the rate varies 1.5—2 times and is proportional to imperfection. In spite of crystal destruction, the rate of thermal decomposition in pressed samples decreases due, apparently, to strengthening of the cell effect.

Kinetics and Mechanism of the Thermal Decomposition of the Isoxazoline Compounds

The decomposition of the isoxazole ring at 160–280°C formed by the addition of ethylene to methyland ethyl–substituted benzonitrile oxides is studied. The reaction consists in the cleavage of the isoxazole ring into acetaldehyde and the corresponding aromatic nitrile. Its rate in the melt and in inert solvents, such as chlorobenzene, only weakly depends on the structure of the aromatic substituent, with the kinetic parameters having low values: E = (104 ± 8) kJ/mol and log(A, s–1) = 7.2 ± 0.8. Only if the benzene ring has three ethyl substituents, the decomposition of the compound in the melt proceeds faster than in solution. The rate of the decomposition of the ring decreases with increasing viscosity of the medium, being higher in solvents with a lower strength of the C–H bond. For compounds with the fully deuterated isoxazole ring, the inverse kinetic H/D isotope effect is observed. The results are explained in terms of the biradical mechanism of the opening of the isoxazole ring that involves an effective recombination of the biradical and its loss through a synchronous rearrangement with acetaldehyde release.

Cage effect in the thermal decomposition of solid-state azobisisobutyronitrile

The method of cross-recombination of isotopically labeled NC(CH3)2C and NC(CD3)2C radicals with equal free-valence reactivity is applied to measure the cage effect, F = krec/(krec + kdif), in the thermal decomposition of azobis(isobutironitrile) in isomorphic crystals consisting by half of fully deuterated molecules of the initial substance. It is shown that the decomposition occurs at crystal lattice defects. The products are analyzed by chromatography-mass spectrometry at the degree of conversion of 1% at 70°C. The CE effect and diffusion coefficient at 70°C have been found to be F = 0.87 and D = 7.7 · 10−7 cm2/s. These results are indicative of a high mobility of molecules on the inner surface of the crystal.