Synthesis And Properties Of Asymmetrically Substituted And Fused-Ring Tetrazine Energetic Compounds

Due to the high-nitrogen-content and low sensitivity,tetrazine compounds have a good application prospect in the new generation of high energy insensitive explosives.Among the tetrazine energetic compounds,asymmetrically substituted tetrazines show comparable energetic property with symmetrically substituted tetrazines,while they are seldom reported and their stability can be improved.Fused-ring compounds are newly emerging energetic compounds with good stability in recent years.Furthermore,synthesizing energetic salts is always the powerful method to regulate the properties of energetic materials(EMs).Based on the above discussion and for the purpose of searching for high erergy insensitive EMs of asymmetrically substituted tetrazines with excellent properties,1,2,4,5-tetrazine are selected as the skeleton to construct tetrazine explosives.Firstly,three kinds of asymmetrically substituted tetrazines and eight kinds of their energetic salts were obtained.Then,fused-ring structure was incorporated and six kinds of fuse-ring tetrazine compouds and nineteen kinds of their energetic salts were synthesized.IR,EA and NMR were used to fully characterize the structure of the synthesized compounds.Meanwhile,the crystals of fifteen compounds were cultivated and the crystal structures were obtained.The results show that most of the compounds show good molecular planarity and the homogenization of the bond lengths is observed.Beyond that,hydrogen-bonding interactions and ?-? stacking are also found.DSC and TG-DTG methods were performed to study the thermal decomposition behaviors of the obtained asymmetrically substituted and fused-ring tetrazine(ASFRTz)EMs.Non-isothermal kinetic methods were used to calculate the dynamic parameters thus the pre-exponential factors and activation energies were obtained.Then,the thermal safety parameters were calculated and the rules affecting the thermal stability of ASFRTz compounds were summarized.The results revealed that many of the synthesized compounds have similar activation energy during the initial and violent decomposition stage.Most of the compounds decompose from their solid phase.Perchlorate,ammonium and guanidinium salt can improve the thermal stability while the diamino-triazolium salt show comparable thermal stability with the neutral compound.Incorporating polycyclic skeleton and the azo moiety can enhance the thermal safety.However,the nitrification of the fused ring will weaken the thermal safety and the synthesis of energetic salts will improve it.The impact sensitivity and density of all the compounds were measured and their enthalpies of formation were calculated by designing isodesmic reactions.Based on the above obtained data,the detonation properties of all the compounds were calculated.For hydroxylammonium 3-hydrazino-6-(1H-1,2,3,4-tetrazol-5-ylimino)-1,2,4,5-tetrazine(3),ammonium(17),hydroxylammonium(18),and hydrazinium(19)3-nitramino-6-(1H-tetrazol-5-yl)-[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine,the detonation velocities exceed 9000 m·s-1,the detonation pressures are greater than 30 GPa and the impact sensitivities are lower than RDX.In addition,the detonation velocities of many other compounds are higher than or close to RDX and their sensitivities are lower than the latter,which show good application prospects.The results also show that,although azo moiety can greatly increase the enthalpy of formation of energetic compounds,but its contribution to the final detonation velocity and detonation pressure of fused-ring tetrazines is limited,and it will bring high impact sensitivity.Comparing with this,the polycyclic skeleton can decrease the impact sensitivity and improve the enthalpy of formation.Among all the synthesized compounds,the nitrate,perchlorate,hydroxylammonium,hydrazinium and ammonium salt show superior energetic performance than others.Moreover,high-density insensitive EMs can be constructed by strong intermolecular hydrogen bonds.The intermolecular interactions of four kinds of solvent-free crystals were studied by Hirshfeld method and RDG method,and the relationship between their density,sensitivity and structure was analyzed.It was found that the large conjugated structure and the strong intermolecular hydrogen-bonding interactions contribute to the formation of stable and high-density explosives,which can be used to guide the construction of high-energy insensitive EMs in the future.