Theoretical Studies On The Molecular Design,cocrystalization,nano Effects,and Decomposition Mechanisms Of Explosives

In this thesis,density functional theory（DFT）,ab initio molecular dynamics（AIMD）,density functional tight-binding MD（DFTB-MD）and Reax FF/lg reactive MD（RMD）methods were used to systematically study the detonation performance and impact sensitivity of bicyclo-1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane（HMX）derivatives,the crystal structure and properties ofα-HMX at low temperatures,the intermolecular interactions,vibrational spectra,and detonation properties of cocrystal explosive CL-20（2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane）/TNT（2,4,6-trinitrotoluene）nanoparticles,the decomposition mechanisms of cocrystal BTF（benzotrioxifoxan）/TNB（1,3,5-trinitrobenzene）nanoparticles at high temperatures,the conformational transformation and initial decomposition mechanisms ofβ-HMX nanoparticles,the secondary reaction mechanisms of the N₂-H₂O-CO₂ mixture of detonation products under high temperaturses and pressures.The main works of this thesis are as follows:1)Molecular design of bicyclo-HMX derivativesBased on HMX ring as a skeleton,five novel bicyclo-HMX derivatives were designed.Then their structures,density,heats of formation,detonation performance,and impact sensitivity were studied by DFT-B3LYP/6-311g（d）and thermodynamic method.The results indicate that all designed bicyclo-HMX derivatives present excellent performance.Among them,one compound has the best performance.Its density,detonation heat,detonation velocity,detonation pressure,and impact sensitivity(h₅₀)are 1.97 g/cm³,2383.30 cal/g,10.61 km/s,52.66 GPa,and 53.94 cm,respectively.Therefore,this compound can be regarded as a very potential candidate of high energy density compound.2)Crystal structure and properties ofα-HMX at low temperaturesThe average structure,electronic structure,optical properties,vibrational spetra,and mechanical properties ofα-HMX crystal under low temperatures（300-425 K）were investigated using DFT and AIMD.The average structure of its crystal is hardly affected by the temperature.Increasing the temperature can accelerate the free rotation of the nitro groups,and moreover,the delocalization of the electrons becomes more and more strong.As the temperature increases,the asymmetrical stretching modes of NO₂ strengthen,and the vibrational modes of lateral chains（NO₂ and CH₂）can be coupled intensely.This will offer an important energy transfer passage for H migration and N-NO₂cleavage.The effects of the temperature on its optical properties are different in different frequency regions.The crystal has good ductility at 375 K.3)Conformational transformation and initial decomposition mechanisms ofβ-HMX nanoparticles under low temperaturesBy the RMD and DFT methods,the structural changes and decomposition reactions of theβ-HMX nanoparticles under low temperatures were studied.The temperature has a small effect on the orientation and positions of the NO₂groups and so makes obvious influence on molecular conformation.The initial temperatures ofβ→δandδ→βconformational transformationare 458and 463 K,respectively.The initial decomposition of its nanoparticles includes two steps:one is the decay of the HMX molecules under the influence of solid phase,the other is the flow phase reactions of the intermediates in the decompositions.4)Study the Intermolecular interactions,vibrational spectra,and detonation performance of CL-20/TNT cocrystalRMD and DFT methods were used to study the structure and properties of CL-20 and TNT crystals and CL-20/TNT cocrystal.An analysis of Hirshfeld surface indicates that there is strong p-πinteractions in the CL-20/TNT cocrystal,making it be in regular crystal packing.Based on an analysis of RDG,it is found that O?H and N?O interactions are main driving force for the formation of the cocrystal and O?O interactions are the utmost important stabilizing force.The vibrational spectra and RDF show that the vibrational coupling of the CL-20 and TNT molecules makes the cocrystal has high absorption peaks in the region of 1200-1750 cm^-1.In all,the regular packing between the TNT and CL-20 and intermolecular hydrogen bonding and p-πpacking interactions have the cocrystal be low impact sensitivity.5)Thermal decomposition mechanisms of cocrystal BTF/TNB nanoparticlesThe initial decomposition of the cocrystal BTF/TNB nanoparticles with the diameters of2.2-3.0 nm under high temperatures of 2400-3000 K were investigated by RMD and DFT methods.Volume expansion of the nanoparticles competed with its decomposition and smaller nanoparticles can promote the diffusion of the molecules and products in the nanoparticles.The nanoparticles have higher decomposition rate at higher temperatures.The smaller the size of the nanoparticle is,the larger its decomposition rate is,and the weaker the interactions between intermediates and lattice are.The number of the clusters during the decomposition of the nanoparticle first increases and then decreases.6)Secondary reaction mechanisms of the N₂-H₂O-CO₂ mixture of detonation productsThe secondary reaction process of the N₂-H₂O-CO₂ mixture of detonation products under high temperatures and pressures was stimulated using RMD.As the temperature increases,the intermolecular interactions in the mixture gradually decrease.At the same pressures,the phase separation of the system may take place at low temperature.The potential energy of the mixture first increases and decreases quickly,resulting in the formation of a lot of carbon clusters.The higher the temperature is,the larger the maximum value of the potential energy is,indicating that the sooner the chemical reaction begins.The same is true of that under the influence of the pressure.As the temperature increases,the decay rate of the molecules in the system continues to increase and the growth rate of the radical H and OH also increases.