V. V. Nedel’ko

Phase transformations of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane: the role played by water, dislocations, and density

A comparative study of the kinetics and mechanisms of the α → γ and ɛ → γ polymorphic transitions in polycrystalline haxanitrohexaazaisowurtzitane was performed using electron and optical microscopy, calorimetry, IR spectroscopy, and quantitative X-ray phase analysis. The kinetics of both processes is complex because of the morphology of the crystals, their defect structure, and impurities. As distinct from the ɛ → γ process, which always occurs as a single crystal-polycrystal transition (through nucleation by the dislocation mechanism with subsequent movement of the phase separation front), the α → γ process can also follow the quasi-homogeneous mechanism and occur as a single crystal-single crystal transition.

The thermal decomposition of hexamethylenetetraammonium dodecahydro-closo-dodecaborate

The kinetics and products of the thermal decomposition of hexamethylenetetraammonium dodecahydro-closo-dodecaborate in air, argon, and a vacuum were studied using thermogravimetry, volumetry, mass spectrometry, and IR spectroscopy. According to the nonisothermal kinetic data, noticeable rates of the formation of volatile products were observed at temperatures higher than 150°C. The thermal decomposition of the salt occurred in stages. At 160–200°C, the thermal decomposition of hexamethylenetetraammonium dodecahydro-closo-dodecaborate could not be described by simple kinetic equations. The dependence of the initial reaction rates on inverse temperature (lnV0−1/K) was linear, which showed that the thermal decomposition of the salt obeyed the Arrhenius equation V0 = 109.4 ± 0.6exp[(−20500 ± 1800)/RT], %/min. The obtained temperature dependences of the kinetic parameters of thermolysis were used to predict the time of salt storage and the conditions of work with it. A comparison of the kinetics of the thermolysis of hexamethylenetetraammonium dodecahydro-closo-dodecaborate and free hexamethylenetetraamine in open and closed reaction systems showed that the thermolysis of hexamethylenetetraammonium dodecahydro-closo-dodecaborate was not accompanied by salt dissociation to hexamethylenetetraamine and dodecahydro-closo-dodecaborate acid. The products of its thermolysis volatile under normal conditions were trimethylamine with a small admixture of nitrogen. The solid residue after thermolysis was a high-porosity insoluble product, whose volume was 6–8 times larger than the volume of the initial sample. An analysis of the IR spectra of the solid thermolysis product showed that it had a well-defined salt character. The special features of the IR spectra of initial hexamethylenetetraammonium dodecahydro-closo-dodecaborate and the product of its thermolysis led us to suggest that an acid-base equilibrium of the type [R3N-H+] + A ↔ [R3N… H+…A] occurred in it and, probably, in the initial salt. Here, R3N is the tertiary amino group, and A is the borohydride acid residue. Indications of amorphization allowed us to suggest that polymer structures were formed as a result of intramolecular interaction between the borohydride anion and onium cation.

Dehydration-induced structural transformations of the microporous zirconosilicate elpidite

The structural transformations accompanying the thermal dehydration of natural elpidite, Na2ZrSi6O15 · 3H2O, have been studied by X-ray powder diffraction and IR spectroscopy. The crystal structures of both elpidite (a = 7.1136(1), b = 14.6764(2), c = 14.5977(2) Å; sp. Gr. Pbcm) and the dehydration product Na2ZrSi6O15 (a = 14.0899(1), b = 14.4983(1), c = 14.3490(1)Å; sp. gr. Cmce = Cmca) are based on a heteropolyhedral Si-Zr-O framework. The Na cations and (in hydrous elpidite) H2O molecules reside in extra-framework sites. The dehydration-induced distortion of the framework leads to a doubling of the a cell parameter, and the water loss is accompanied by a considerable decrease in molar volume.

Thermal decomposition of [1,2,5]Oxadiazolo[3,4-e][1,2,3,4]-Tetrazine-4,6-di-N-Oxide

The thermal decomposition of [1,2,5]oxadiazolo[3,4-e][1,2,3,4]-tetrazine-4,6-di-N-oxide (furazano tetrazine dioxide, FTDO) in the solid state, melt, and dinonyl phthalate solution is studied. The kinetic and thermodynamic parameters of the process are determined. These results enable to estimate the thermal stability of FTDO. The composition of the gaseous reaction products and the elemental composition of the condensed product are determined. On the basis of kinetic, analytical, and spectral data, the mechanism of the process, including the formation of the N-nitrosofurazanoaziridine intermediate, is discussed.

The thermal decomposition of azidopyridines

The thermal decomposition of new heteroaromatic polyazides 2,6-diazido-3,5-dicyanopyridine, 2,4,6-triazido-3,5-dicyanopyridine, and 2,3,4,5-tetraazido-6-cyanopyridine was studied by thermogravimetry, volumetry, mass-spectrometry, and IR spectroscopy. Reaction kinetic parameters were determined. The only gaseous product of the thermal decomposition of all the azides studied was nitrogen, its degree of purity was 99.0–99.8 vol %. 2,6-Diazido-3,5-dicyanopyridine and 2,4,6-triazido-3,5-dicyanopyridine had thermal stability and thermal decomposition parameters close to those of the majority of aromatic azides. The mechanism of thermal decomposition of these azides includes the splitting off of the nitrogen molecule at the initial limiting process stage. Subsequent intermolecular reactions with the participation of nitrenes result in the formation of an amorphous substance containing polyconjugated fragments with sp2 hybridization, which form planar two-dimensional networks. 2,3,4,5-Tetraazido-6-cyanopyridine has very low thermal stability; the rate of nitrogen release during its decomposition is almost 1000 times higher than with 2,6-diazido-3,5-dicyanopyridine and 2,4,6-triazido-3,5-dicyanopyridine at comparable temperatures. This was explained by the presence of the ortho azido group (there is no ortho arrangement of azido groups in 2,6-diazido-3,5-dicyanopyridine and 2,4,6-triazido-3,5-dicyanopyridine).

Kinetics and mechanism of a polymorphous transition in polycrystalline ε-hexanitrohexaazaisowurtzitane

Optical and electron microscopy, calorimetry, IR spectroscopy, and XRD analysis were used to study the ε → γ polymorphous transition in polycrystalline hexanitrohexaazaisowurtzitane. The kinetics of the process is extremely complex due to the intrinsic morphological features and defects in individual crystals in the polycrystalline sample. A limiting phenomenon, which manifests itself in a sharp slowdown of the process at a critical temperature, was revealed. The polymorphous transformation is triggered by dehydration, the removal of the residual water from the crystal. It was demonstrated that dislocations play an important role in the process under study.

Thermal decomposition of 2,4,6-triazido-1,3,5-triazine

The thermal decomposition of 2,4,6-triazido-1,3,5-triazine in the melt and a dinonyl phthalate solution is studied by thermogravimetry, manometry, mass spectrometry, and IR spectroscopy. The kinetic and activation parameters of the process are determined. The only gaseous product of the reaction is nitrogen. This fact, along with the structure of the condensed residue formed during the thermal decomposition of 2,4,6-triazido-1,3,5-triazine in the melt, are indicative of the abstraction of a nitrogen molecule from an azide group at the initial stage and of the subsequent reactions leading to the formation of a planar network of polyconjugated bonds between C and N atoms. For the thermal decomposition of 2,4,6-triazido-1,3,5-triazine in solution the preexponential factor and activation energy are found to be 1012.8 s–1 and 34100 cal/mol, respectively, which are characteristic of the thermal decomposition of most azides. To explain why these parameters are substantially higher for the reaction in the melt (1017.4 s–1 and 42300 cal/mol), it is assumed that, in this case, the process proceeds by the polymerization (polycondensation) mechanism to form twodimensional networks, with the apparent kinetic parameters being effective quantities. Based on these data, it is concluded that the high sensitivity of 2,4,6-triazido-1,3,5-triazine to external influences is of kinetic nature.

Dehydration kinetics of the microporous zirconosilicate elpidite

The dehydration kinetics of natural elpidite, Na2ZrSi6O15 · 3H2O, have been studied by isothermal (110–254°C) and nonisothermal (20–600°C) thermogravimetry. The process comprises two steps. In the first step, 50% of the water is released according to a first-order rate law with a rate constant k1 = 108.9 ± 0.2 exp[(−98740 ± 3100)/RT] s−1 (R = 8.314 J/(mol K)). The second dehydration step cannot be described by a simple kinetic law. The effective activation energy for this step (76.5 kJ/mol) has been evaluated from the temperature dependence of its initial rate. The results on the dehydration kinetics of elpidite are discussed in relation to the structural transformations of the framework and the hydrogen bonds formed by the water molecules in the zeolite channels.

Thermal decomposition of 2,4,6-triazidopyridine

The thermal decomposition of 2,4,6-triazidopyridine in the melt is studied using thermogravimetry, manometry, mass spectrometry, and IR spectroscopy. In the temperature range of 120–160°C, the process obeys the first-order kinetic law, being described by the Arrhenius equation k [s–1] = 1012.8 ± 0.4exp[–(31200 ± 1500)]/RT with values of the parameters typical of the thermal decomposition of aromatic and heterocyclic azides. The reaction produces nitrogen, as the only gaseous product. Unlike the other heterocyclic azides, the decomposition of which is characterized by anomalously high values of the Arrhenius parameters, the thermal decomposition of 2,4,6-triazidopyridine yields a condensed product having a system of polyconjugated bonds with higher force characteristics. It is concluded that the decomposition of 2,4,6-triazidopyridine proceeds by a mechanism in which the rate-limiting step is the dissociation of the nitrogen molecule from the azide group to form a nitrene.

Thermal decomposition of 2,4,6-triazidopyrimidine in the melt

The kinetics and the products of thermal decomposition of 2,4,6-triazidopyrimidine in the melt were investigated by thermogravimetry, manometry, mass-spectrometry, and IR spectroscopy. The kinetic and activation parameters of the processes were found. Nitrogen was the only gaseous product of the reaction. The structure of the solid reaction product was determined. A mechanism of thermal decomposition of 2,4,6-triazidopyrimidine, including elimination of a nitrogen molecule to give nitrene in the initial step, was proposed. The unusually high pre-exponential factor in the Arrhenius equation (1016.7±0.7 s–1) was attributed to a significant contribution of polymerization (polycondensation) to the overall process, resulting in the formation of carbon—nitrogen two-dimensional networks.