Ab Initio Molecular Dynamics Simulation Study on the Thermal Decomposition Mechanism of Two F-Containing Compounds: 3,3,7,7-Tetrakis(difluoramino)-octahydro-1,5-dinitro-1,5-diazocine (HNFX) and 1,3,5-Trinitro-2,2-bis(trifluoromethyl)-1,3,5-triazinane (TNBFT).

Abstract

F-containing explosives with additional F atoms compared to traditional CHON ones should contain a more complex decomposition mechanism. Nevertheless, understanding the mechanism and effect of F-containing groups on stability remains limited. This study investigates the thermal decomposition mechanisms of two typical F-containing compounds, 3,3,7,7-tetrakis(difluoramino)-octahydro-1,5-dinitro-1,5-diazocine (HNFX) and 1,3,5-trinitro-2,2-bis(trifluoromethyl)-1,3,5-triazinane (TNBFT), using ground-state ab initio molecular dynamics simulations and compares them with RDX. The results show that HNFX decomposes from the partition of -NF₂ to form NF₃ and HF at 1500 K and above 2000 K, respectively, while TNBFT undergoes the concerted H transfer and HONO elimination at 1500 K and the N-NO₂ bond cleavage at relatively high temperatures. Additionally, HF and fluorocarbons, as the primary fluorinated products, lower the yields of H₂O and CO₂ compared to RDX, but they can act as oxidizers in combustion with Al particles. Moreover, it is found that F-containing groups significantly weaken the bonds nearby and the total molecular stability. Based on our bond strength analysis and simulation results, the reported experimental confirmation of the thermal stability of HNFX may be questionable. This insight is expected to deepen the thermal decomposition mechanisms of F-containing explosives and guide the design of high-performance composites thereof.