| The nitrogen-rich energetic compounds, well known as the fourth generation of high energy density materials (HEDM), have been widely used in almost all fields of energetic materials, such as insensitive explosives, propellants, and gas generating agents. Therefore, it is necessary to perform systematic and in-depth studies on their structure and properties. In this thesis, we used several theoretical methods to study the geometric structure, electronic structure, detonation performance, crystal structure, and other properties of nitrogen-rich energetic materials, including aza-heterocyclic compound, aza-cage compounds, all-nitrogen compounds, and triazine guanidinium salts. The contents of the dissertation can be divided mainly into three parts:The first part concentrates on molecular design for HEDM. By molecular modification, the CH groups in the ring of typical nitramine HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), bicyclic-HMX (cis-2,4,6,8-tetranitro-1H,5H-2,4,6,8-tetraazabicyclo[3.3.0]octane), and TNAD (trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin) are replaced by nitrogen atoms, and more nitro groups are introduced. As a result, the increment of nitrogen content and oxygen balance lead to higher density and more internal energy. The variation trends of structure, stability, and energy are discussed. Considering the requirements of HEDMs, we find ten promising HEDM candidates with good explosive performances and stability.Based on density functional theory (DFT) and molecular mechanics (MM) methods, the fully optimized structure, heat of formation (HOF), density, crystal structure, detonation velocity, and detonation pressure of cage compound2,4,6,8-tetranitro-1,3,5,7-tetraazacubane (TNTAC) are reported. Pyrolysis mechanism is investigated and ascertained by comparig the bond dissociation energy (BDE) of two possible trigger bonds (C-N in cage and C-NO2on side chain. In comparison with the famous cage explosives CL-20(2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) and ONC (octanitrocubane), TNTAC exhibits better detonation performance and meets the stability requirement. Therefore, TNTAC is expected to be a novel candidate of HEDM with high exploitable values.Computational studies on a novel series of bridged triazines such as4,4’,6,6’-tetra(azido)azo-1,3,5-triazine (TAAT) and4,4’,6,6’-tetra(azido)hydrazo-1,3,5-triazine (TAHT) show that HOF can be increased largely by-N3group, and-NF2,-NO2,-N=N-, and-N=N(O)-groups are effective structural units for improving the detonation performance. p→π conjugation interaction is found between the nitrogen-bridge group and the triazine ring, which makes the bridged triazine molecule stable as a whole. Taking the detonation performance and thermal stability into account, three bridged triazines are recommended as energetic insensitive explosives.DFT calculations have been performed on the geometrical structure, electronic structure, stability, and energetic properties of nine synthesized N5+-containing salts. The results show that (N5)2SnF6, N5PF6, N5BF4, and N5SO3F have medium stability and release much more energy than Pb(N3)2and HMX during thermal decomposition. They can be considered as potential candidates of very energetic explosives. In addition, similar studies are carried out on a series of azidamines N-(N3)n (n=1-6). The results provide some useful information for the research of novel all-nitrogen compounds.The second part focuses on the periodic quantum mechanics studies on TAAT and TAHT. The influence of pressure on the structures and properties has also been investigated.The solid DFT method in the CASTEP program is used to optimize the crystal structure and calculate band structures. The results demonstrate that LDA/CA-PZ is more reliable than GGA/RPBE and GGA/PW91for studying the polyazide crystals. Frontier band structure, band gap, and density of states (DOS) of TAAT and TAHT are reported for the first time. The correlations between them and their sensitivity are discussed in detail. The band gaps decrease generally with the increasing of pressure, and drop to nearly zero at100GPa. This indicates the molecular crystals undergo an electronic phase transition from a semiconductor to metallic systems. The band fluctuation increases with the increment of pressure, and DOS becomes more dispersed and smoother. An analysis of the valence and conduction bands near Fermi level shows that the N atoms of the bridges-N-N-and-NH-NH-act as an active center. What is more, the azide-tetrazole transformation in TAAH and TAHT molecules are observed as the pressure increases, and new structures have formed.The third part centers on the dispersion corrected DFT method (DFT-D) studies of the intramolecular hydrogen bonding interactions and properties of a series of nitroamino[1,3,5]triazine-based guanidinium salts at the B97-D/AUG-cc-PVDZ level. By means of natural bond orbital (NBO), atoms in molecules (AIM), and energy decomposition analysis (EDA), the hydrogen bonds are characterized, and the origin of hydrogen bond and the interaction energies are analyzed. The effect of different substituents and cations (including guanidinium, aminoguanidinium, diaminoguanidnium, triaminoguanidinium, and guanylurea cations) on the hydrogen bonding interactions and other properties are also discussed. LP(N or O)→σ*(N-H) orbital interactions are found between the cations and anions, and they are associated with a seven or eight-membered pseudo ring which enhances the molecular stability. The electrostatic and orbital interactions contribute mainly to the stability, and the dispersion energy has very small contributions. Results show that the guanylurea cation is better for improving the thermal stabilities of the ionic salts than other cations. The contributions of different substituents to the thermal stability increase in the order of-NO2<-NF2<-N3(-ONO2)<-NH2... |
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