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 N10 chain, their counterparts with the 5,5'-bridging pattern, and the species with hydrazo- and azoxy-bridges, which are often present energetic moieties. The N8-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 N2 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 N2 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 N2 elimination. Apart from this, the highly accurate gas-phase formation enthalpies were calculated at the W1-F12 level of theory for all species studied.