V. G. Prokudin

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.

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.

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.

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.

Retardation of unimolecular reactions in the solid phase

The retardation coefficient, KR = kl/ks, for a homogeneous reaction in the bulk of a crystal was calculated within the framework of the free-volume model of unimolecular reactions in the solid phase in the approximation of an isotropic continuum. The key parameters entering into the working formula, more specifically, the additional activation volume for the solid phase and the compressibility coefficient, were estimated using semiempirical methods. The calculated values of KR range from several units to several thousand units, an interval that encompasses all the experimental values of this quantity. Low values of KR are indicative of reactions in the bulk of the crystal lattice. In addition, reactions on defects can occur, being predominant at KR > 100. In all cases, the calculated values of KR give an upper estimate of the degree of retardation, above which no experimental values have been obtained.