Molecular design of long intra‐annular nitrogen chains: 3H‐tetrazolo[1,5‐d]tetrazole‐based high‐energy‐density materials

Abstract

Energetic compounds containing long nitrogen chain, have been a research hotspot.Fused heterocycles are stable due to their aromatic systems. Compounds obtained by combining long nitrogen chain and fused ring can not only retain good energetic property, but also ensure better stability. In this work, eight fused heterocycle-based energetic compounds, 3H-tetrazolo [1,5-d]tetrazole (1) and its derivatives (2-8), were designed, containing a nitrogen chain with seven nitrogen atoms. The HOF, thermal stability, and energetic properties of these compounds were studied using the DFT method. The results show that the introduction of -NO 2 , -N 3 , -NF 2 , -ONO 2 , and -NHNO 2 groups greatly increased the density, HOF, detonation velocity, and detonation pressure. The densities of 3, 5, 7, and 8 fall within the range designated for high-energy-density materials. The calculated detonation velocities of the compounds 3 and 8 are as high as 9.86 and 9.78 km s À1 , which are superior to that of CL-20. Kinetic study of the thermal decomposition mechanism indicates that the N-R bonds may not be the weakest bonds in these compounds. The tetrazole ring opening of the heterocycle-based energetic compounds, followed by N 2 elimination, is predicted to be the primary decomposition channel, regardless of whether substituent groups are present.energetic compounds, fused heterocycles, nitrogen chain, thermal stability 1 | INTRODUCTION Energetic materials have been widely used in explosives, propellants, and pyrotechnic agents. The development of high-performance, high-density, high-stability and environmentally friendly energetic materials has been a focus in recent years. Nitrogen-rich heterocyclic compounds are rich in high-energy chemical bonds such as N=N, N-N, C-N, and N-O and have large ring tension, and therefore, they have high heat of formation. Their high-nitrogen, low-carbon structure makes it easier for them to achieve oxygen balance. Furthermore, most of the explosion products of nitrogen-rich heterocyclic compounds are N 2 , which is friendly to the environment. So they are one of the most promising high-energy-density materials (HEDMs) [1][2][3].The heat of formation of nitrogen-rich heterocyclic compounds is related to the nitrogen content and the connection mode of the nitrogen atoms. The heat of formation increases with increasing nitrogen content and the number of directly connected nitrogen atoms [4][5][6][7]. The commonly used method to obtain long nitrogen chain is to oxidize the N-NH 2 moiety of a nitrogen heterocycle to form a tetrazene structure (N=N-N=N). Such growth of catenated nitrogen atom chains mainly occurs between two heterocycles (inter-annular nitrogen chains) [8-10].