Study On The Structure And Properties Of Four Typical Heat-resistant Energetic Compounds

2,2’,4,4’,6,6’-hexanitrobiphenyl（HNBP）,2,2’,4,4’,6,6’-hexanitrostilbene（HNS）,2,5-dipicryl-1,3,4-oxadiazole（DPO）and 5,5’-bis（2,4,6-trinitrophenyl）-2,2’-bis（1,3,4-oxadiazole）（TKX-55）are four typical heat-resistant energy-containing compounds,with advantages of high thermal decomposition temperature,low mechanical sensitivity,and high energy level.Moreover,the molecular structure of these four heat-resistant energy-containing compounds is composed of two trinitrobenzenes in different bridging ways,which has high similarity.Therefore,we conducted a comparative analysis of these four compounds.The effect of bridge connection structure on the heat resistance of energetic compounds was investigated.In this paper,density functional theory（DFT）and reactive molecular dynamics（Reax FF MD）methods were used to investigate the structure and properties of four heat-resistant energetic compounds,namely HNBP,HNS,DPO,and TKX-55.Firstly,the electronic structures of four energetic compounds were studied using density functional theory（DFT）methods.The effects of the introduction of the intermediate bridge linking groups on the initiation bond dissociation energy,charge population on molecules,frontier molecular orbitals,and molecular surface electrostatic potential of energetic compounds were studied from a molecular perspective.The results showed that the introduction of intermediate groups increased the initiation bond energy of energetic compounds,effectively enhancing the thermal stability of energetic compounds.The charge population analysis showed that the addition of intermediate groups has a certain impact on the charge population of all carbon atoms on the aromatic rings on both sides of the four energetic compound molecules,which increases the charge carried by the two carbon atoms connecting the intermediate groups,enhances the bonding strength with the surrounding atoms,and makes the molecular structure of the compound more stable.The analysis of frontier molecular orbitals showed that the larger the conjugated system of organic molecules,the more stable the structure,and the smaller theΔE_{（LUMO-HOMO）}difference.The molecular surface electrostatic potential（ESP）parameters of four energetic compounds showed that the introduction of intermediate groups can effectively reduce the sensitivity of energetic compounds.The stretching vibration absorption peak of the bridge connecting group was determined by infrared vibration spectroscopy,which was in good agreement with experimental values.Then,the intramolecular and intermolecular interactions in the crystal cells of four heat-resistant energetic compounds were studied from a crystal perspective,and the fundamental reasons for the excellent heat resistance of energetic compounds were discussed.The results showed that there are intramolecular interactions in the four energetic compounds,which were dominated by van der Waals interactions,supplemented by weak hydrogen bond interactions and steric hindrance interactions.The analysis of Hirshfeld’s surface,fingerprints,and density of states showed that intermolecular hydrogen bond interactions and N-O···πinteractions play an important role in the thermal stability of energetic compounds.Finally,using the Reactive Molecular Dynamics（Reax FF MD）simulation method,the thermal decomposition reactions of four heat-resistant energetic compounds were simulated.The potential energy evolution,the molecular number evolution of the reactant,and the evolution of the final stable small molecule product during the thermal decomposition process of the four energetic compounds were compared and analyzed,and their reaction kinetic parameters were calculated.The results showed that energetic compounds with better heat resistance need to absorb more heat and have higher heat release during thermal decomposition,which can be used as an indicator to predict the thermal stability of energetic compounds.During the thermal decomposition process,the loss rate of four reactant molecules increases with the increase in temperature,indicating that the increase in temperature will promote the thermal decomposition of energetic compounds.During the thermal decomposition process at2500 K,the existence time of four reactant molecules in the system was consistent with their stability order,and energetic compounds with stronger thermal stability exist longer in the system.By analyzing the evolution of intermediate products and ultimately stable small molecule products,it was speculated that the initial thermal decomposition reaction of the four energetic compounds was the formation of NO₂ through the cleavage of nitro groups on the aromatic ring,followed by ring opening reactions.Finally,the activation energies and pre-exponential factors of the four energetic compounds at the initial endothermic reaction stage and the intermediate exothermic stage were calculated.It was concluded that in the endothermic reaction stage,HNBP had the lowest activation energy,and was the most prone to decomposition among the four energetic compounds.The activation energies of HNS and DPO did not differ significantly,while TKX-55 had the highest activation energy and the strongest thermal stability.