Thermal Stability of Bis-Tetrazole and Bis-Triazole Derivatives with Long Catenated Nitrogen Chains: Quantitative Insights from High-Level Quantum Chemical Calculations

Abstract

Azobis tetrazole and triazole derivatives containing long catenated nitrogen atom chains are of great interest as promising green energetic materials. However, these compounds often exhibit poor thermal stability and high impact sensitivity. Kinetics and mechanism of the primary decomposition reactions are directly related to these issues. In the present work, with the aid of highly accurate CCSD(T)-F12 quantum chemical calculations, we obtained reliable bond dissociation energies and activation barriers of thermolysis reactions for a number of N-rich heterocycles. We studied all existing 1,1′-azobistetrazoles containing an N 10 chain, their counterparts with the 5,5′-bridging pattern, and the species with hydrazo-and azoxy-bridges, which are often present energetic moieties. The N 8 -containing azobistriazole was considered as well. For all compounds studied, the radical decomposition channel was found to be kinetically unfavorable. All species decompose via the ring-opening reaction yielding a transient azide (or diazo) intermediate followed by the N 2 elimination. In the case of azobistetrazole derivatives, the calculated effective activation barriers of decomposition are ∼26−33 kcal mol −1 , which is notably lower than that of tetrazole (∼40 kcal mol −1 ). This fact agrees well with the low thermal stability and high impact sensitivities of the former species. The activation barriers of the N 2 elimination were found to be almost the same for the azobis compounds and the parent tetrazole, and the effective decomposition barrier is determined by the thermodynamics of the tetrazole−azide rearrangement. In comparison with 1,1′-azobistetrazole, the hydrazo-bridged compound is more stable kinetically due to the lack of pi-conjugation in the azide intermediate. In turn, the azoxy-bridged compounds are entirely unstable due to tremendous azide stabilization by the hydrogen bond formation. In general, the 5,5′-bridged species are more thermally stable than their 1,1′-counterparts due to a much higher barrier of the N 2 elimination. Apart from this, the highly accurate gas-phase formation enthalpies were calculated at the W1−F12 level of theory for all species studied.

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

“…Earlier we benchmarked various coupled cluster methods on a series of simple N-rich heterocycles and energetic nitro compounds. 50,54,55 The CCSD(T)-F12 results with VDZ-F12 and VTZ-F12 basis sets differ from each other by less than 0.8 kJ mol −1 , and the VDZ-F12 values deviates from the complete basis set energy by less than 4 kJ mol −1 . Therefore, we used VDZ-F12 basis set in the present work.…”

Nitrene formation is the first step of the thermal and photochemical decomposition reactions of organic azides

“…For example, they are important components in click chemistry [1][2][3] or synthesis of energetic materials. [4][5][6][7][8][9][10][11][12] Most of these applications as well as their exceptional reactivity arise from their ability to generate nitrene intermediates after N 2 extrusion. [13][14][15][16][17] Although organic azides are known for more than a century, 18 the decomposition reaction mechanism at an atomistic level is still under discussion.…”

Synthesis and Characterization of Binary, Highly Endothermic, and Extremely Sensitive 2,2′-Azobis(5-azidotetrazole)

“…Through this method, a sufficient amount of 3 could be gathered over several batches to allow all possible analyses to be performed. In addition, 3 is not stable long-term , and decomposes slowly in solution with evolution of nitrogen.…”