Theoretical Study of the 5-Aminotetrazole Thermal Decomposition

Autocatalytic decomposition of energetic materials: interplay of theory and thermal analysis in the study of 5-amino-3,4-dinitropyrazole thermolysis

A reliable kinetic description of the thermal stability of energetic materials (EM) is very important for safety and storage-related problems. Among other pertinent issues, autocatalysis very often complicates the decomposition kinetics of EM. In the present study, the kinetics and decomposition mechanism of a promising energetic compound, 5-amino-3,4-dinitro-1H-pyrazole (5-ADP) were studied using a set of complementary experimental (e.g., differential scanning calorimetry in the solid state, melt, and solution along with advanced thermokinetic models, accelerating rate calorimetry, and evolved gas analysis) and theoretical techniques (CCSD(T)-F12 and DLPNO-CCSD(T) predictive quantum chemical calculations). The experimental study revealed that the strong acceleration of the decomposition rate of 5-ADP is caused by two factors: the progressive liquefaction of the sample directly observed using in situ optical microscopy, and the autocatalysis by reaction products. For the first time, the processing of the non-isothermal data was performed with a formal Manelis-Dubovitsky kinetic model that accounts for both factors. With the aid of quantum chemical calculations, we have rationalized the autocatalysis present in the formal kinetic models at the molecular level. Theory revealed an unusual primary decomposition channel of 5-ADP, viz., the two subsequent sigmatropic H-shifts in the pyrazole ring followed by the C-NO₂ bond scission yielding a pyrazolyl and nitrogen dioxide radicals as simple primary products. Moreover, we found the secondary reactions of the latter radical with the 5-ADP to be kinetically unimportant. On the contrary, the substituted pyrazolyl radical turned out to undergo a facile addition to 5-ADP, followed by a fast exothermic elimination of another ˙NO₂ species. We believe the latter process to contribute remarkably to the observed autocatalytic behavior of 5-ADP. Most importantly, the calculations provide detailed mechanistic evidence complementing the thermoanalytical experiment and formal kinetic models.

Structure determination and Hirshfeld surface analysis of new cocrystal and salt forms of 5-aminotetrazole with hydroxy- and nitro-substituted carboxylic acids

Two new crystalline solids, namely, 5-aminotetrazole–3,5-dihydroxybenzoic acid–water (1/4/6), CH3N5·4C7H6O4·6H2O (I), and 5-aminotetrazolium 3,5-dinitrosalicylate, CH4N5 +·C7H3N2O7 − (II), have been synthesized and characterized by single-crystal X-ray diffraction and Hirshfeld surface analysis. The crystal packing arrangements of I and II are governed by N—H...O and O—H...O hydrogen-bonding interactions. In cocrystal I, adjacent acid molecules are linked through O—H...O hydrogen bonds, forming a dimer with an R 2 2(8) motif. In salt II, the tetrazolium cation and acid anion are linked through N—H...O hydrogen bonds to also form a dimer with an R 2 2(8) motif. Further N—H...O and O—H...O hydrogen bonds help to stabilize the crystal packing, along with aromatic π–π stacking interactions in I and carbonyl...π interactions in II. The Hirshfeld surface analysis and fingerprint plots reveal that O...H/H...O interactions contribute 34.4% of the total interactions in the crystal packing of cocrystal I and 36.7% in salt II.

Photodegradation of bis(1H-tetrazol-5-yl)amine (H2BTA), a high nitrogen content tetrazole-based energetic compound in water

Tetrazoles have wide industrial applications, notably in the pharmaceutical industry. Tetrazole derivatives such as bis(1H-tetrazol-5-yl)amine (H₂BTA) have recently been considered by the defense industry as high nitrogen composite propellants. Photodegradation studies under solar simulating conditions showed that H₂BTA was partially degraded in water, while it was completely degraded under UV light at 254 nm. When H₂BTA (0.35 mM) was irradiated with simulated sunlight at pH 3.65, there was a 1-day lag phase before the chemical started to degrade, reaching 43.5% degradation after 7 d. However, when pH increased to 5.76, it degraded without lag phase, suggesting that an HBTA^- anion was involved in the initial degradation of the chemical. 5-Aminotetrazole (5-AT) was identified as a final degradation product and N-(1H-tetrazol-5-yl)formamide(T(5 yl)FA) and 1H-tetrazol-5-ylcarbamic acid (T(5 yl)CA) as intermediate products. At λ = 254 nm, H₂BTA disappeared rapidly, resulting in the loss of 94% after 65 min. 5-AT was detected together with several transient products including N-(1H-tetrazol-5-yl)carbamohydrazonic acid (T(5 yl)CHA) and T(5 yl)FA. Kinetic studies and products analysis revealed that H₂BTA photodegraded via two initial routes. One route (a) marked by the initial loss of HN₃ and another (b) marked by the initial loss of N_2. Route a) was characteristics for irradiation with simulated sunlight; however, routes a) and b) proceeded simultaneously under UV light. 5-AT eventually degraded to presumably give N₂ and/or HN₃ under UV light. Understanding the photodegradation pathway of H₂BTA under simulated sunlight can help in providing the basis for natural attenuation assessment of the chemical in contaminated aquatic environments.

Toward reliable characterization of energetic materials: interplay of theory and thermal analysis in the study of the thermal stability of tetranitroacetimidic acid (TNAA)

The thermal stability of energetic materials, being of the utmost importance for safety issues, is often considered in terms of kinetics, e.g., the Arrhenius parameters of the decomposition rate constant. The latter, in turn, are commonly determined using conventional thermoanalytical procedures with the use of simple Kissinger or Ozawa methods for kinetic data processing. However, thermal decomposition of energetic materials typically occurs via numerous exo- and endothermal processes including fast parallel reactions, phase transitions, autocatalysis, etc. This leads to numerous drawbacks of simple approaches. In this paper, we proposed a new methodology for characterization of the thermochemistry and thermal stability of melt-cast energetic materials, which is comprised of a complementary set of experimental and theoretical techniques in conjunction with a suitable kinetic model. With the aid of the proposed methodology, we studied in detail a novel green oxidizer, tetranitroacetimidic acid (TNAA). The experimental mass loss kinetics in the melt was perfectly fitted with a model comprised of zero-order reaction (sublimation or evaporation) and first-order thermal decomposition of TNAA with the effective Arrhenius parameters Ea = 41.0 ± 0.2 kcal mol-1 and log(A/s-1) = 20.2 ± 0.1. We rationalized the experimental findings on the basis of highly accurate CCSD(T)-F12 quantum chemical calculations. Computations predict that thermolysis of TNAA involves an intricate interplay of multiple decomposition channels of the three tautomers, which are equilibrated via either monomolecular reactions or concerted double hydrogen atom transfer in the H-bonded dimers; the calculated Arrhenius parameters of the effective rate constant coincide well with experiment. Most importantly, calculations provide detailed mechanistic evidence missing in the thermoanalytical experiment and explain formation of the experimentally observed primary products N2O and NO2. Along with the kinetics and mechanism of decomposition, the proposed approach yields accurate thermochemistry and phase change data of TNAA.

Proposed Proton-Transfer Mechanism for the Initial Decomposition Steps of BTATz

The first steps in the gas-phase decomposition mechanism of N3,N6-bis (1 H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine, BTATz, anions and the kinetic isotope effects in these processes were studied using combined multistage mass spectrometry (MS/MS) and computational techniques. Two major fragmentation processes, the exergonic loss of nitrogen molecules and the endergonic loss of hydrazoic acid, were identified. The observation of a primary isotope effect supported by calculations suggests that the loss of a nitrogen molecule from the tetrazole ring involves proton migration, either to or within the terazole ring, as a rate-determining step. The fragmentation of a hydrazoic acid occurs through an asymmetrical retro-pericyclic reaction. Calculations show the relevance of these mechanisms to neutral BTATz. Our findings may contribute to the understanding of decomposition routes in these nitrogen-rich energetic materials and allow tailoring their reactivity and decomposition pathways for better control of performance.

Effects of nitroguanidine on the thermal behavior and burning characteristics of 5-amino-1H-tetrazole-based propellants

The 5AT/Sr(NO₃)₂mixture has used in novel fire-suppression applications. NQ is proved to be an efficient ingredient to improve the combustion characteristics of 5AT/Sr(NO₃)₂mixture.

Theoretical studies on the propulsive and explosive performance of strained polycyclic cage compounds

Carbon-based cage compounds could be better candidates as propellant ingredients than the currently used ones as per molecular modellingab initiocalculations.

Pursuing reliable thermal analysis techniques for energetic materials: decomposition kinetics and thermal stability of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50)

Reliable kinetics of thermolysis for a novel explosive dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) was obtained using a variety of thermoanalytical and kinetic methods and verified by modeling of adiabatic self-heating.