Molecular Design, High-pressure Behaviors, And Thermal Decomposition Mechanisms Of Energetic Nitrogen-rich Compounds

Energetic nitrogen-rich compounds as promising and important candidates of high energy density compounds (HEDCs) may be applied in many fields of energetic materials. It is thus very important to perform in-depth and systematic studies on their structures and properties. In the present thesis, many molecular design strategies were used to construct many energetic nitrogen-rich compounds (like azole, azine, furozan, furoxan, and cage deriatives) and theoretical and computational chemistry methods were employed to evaluate their structure and propoties. Among the compounds, potential candidaes of HEDCs, high-energy oxidizers, and insensitive high-energy explosives were selected. Periodic DFT was used to study the high-pressue behaviors of ANBDF (7-amino-6-nitrobenzodifuroxan), furoxan, and NTO (5-nitro-2,4-dihydro-1,2,4-triazole-3-one). Conventional DFT-based molecular dynamics (DFT-MD) and dispersion-corrected DFT-based MD (DFT-D-MD) were employed to comparatively study the crystal, molecular, and electronic structure of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under room temperature and pressure of 0-100 GPa. Finally, the initial decomposition mechanisms and full decomposition processes of model compounds DiAT (3,6-di(azido)-1,2,4,5-tetrazine) and furoxan crystals were simulated using ab initio MD (AIMD). The contents of the dissertation are divided into four parts mainly:1. Different molecular design methods:oxygen balance around zero, oxygen balance equal to zero, double substitutions of nitro and a trinitromethyl groups, cage hexaprismane as a basic skeleton, a combination of oxygen balance equal to zero, double substitutions of nitro and amino groups, and N-oxide in one molecule, and introducing N-oxide groups into cage alkanes were used to construct different types and many series of energetic nitrogen-rich compounds. DFT method was employed to predict their detonation properties, thermal stability, and sensitivity. Compared with known explosives such as RDX (1,3,5-trinitro-1,3,5-triazinane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane), ONC (octanitrocubane), CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), TNT (1-methyl-2,4,6-trinitrobenzene) and so on, potential candidates of HEDCs, high-energy oxidizers, and insensitive high-energy explosives were selected.2. The crystal, molecular, and electronic structure and some properties crystalline ANBDF, furoxan, and NTO under high pressure were studied by periodic DFT calculations. There are three structural transformations in ANBDF crystal occurred at the pressure of 35,70, and 100 GPa, respectively. Furoxan undergoes three structural transformations at 114-115, 124, and 136 GPa, respectively, to form three new structures. The calculated elastic constants show that the three new structures are mechanically stable. NTO crystal decomposes by the N-O bond cleveage of nitro group and polymerizes by forming a new N-H covalent bond between one nitrogen atom in the ring and one hydrogen atom linked to the ring in another molecule. As the pressure increases, the optical absorption activity of the three crystals strengthens gradually.3. DFT-MD and DFT-D-MD methods were used to comparatively study the crystal, molecular, and electronic structures of crystalline TATB in the pressure of 0-100 GPa under room temperature. The results indicate that the lattice parameters and P-V isotherm by DFT-D-MD agree well with the experimental results, whereas DFT-MD results either underestimate or overestimate the experiemnts obviously. The DFT-D-MD results show that TATB is chemical stable in the whole investigated pressure range, in agreement with the experiments, but the DFT-MD simulations misestimate that TATB decomposes at 50 GPa and decomposes and polymerizes at 100 GPa. The band structures obtained by the PBEO with dispersion correction are in agreement with available experiment results.4. AIMD simulations were performed to study the initiation decomposition mechanisms and full decomposition processes crystalline DiAT as a model compound for energetic organic azide derivatives and furoxan as a model compound for energetic furoxan derivatives at high temperatures. Three different initiation mechanisms were observed in unimolecular decomposition of DiAT:N-N2 cleavage, ring opening, and isomerization. Among them, the preferential initial decomposition step is the homolysis of the N-N2 bond in azido group. In the multimolecular decomposition of DiAT crystal, the release mechanisms of nitrogen gas are found to be very different in early and later decomposition stages. The initial decomposition step of Furoxan was the N1-O3 bon cleveage. The hydrogen radicals play a catalytic role on the following decomposition. They not only catalyze the opening of furoxan rings and breaking of N-O bonds, but also capture and transport the oxygen atoms from nitrogen atoms to carbon atoms and promote the release of some small products.