Fox-7 Thermal Decomposition Mechanism And Crystal Properties Of Theoretical Research

This dissertation is devoted to study the thermal decomposition mechanism and crystal properties of the new explosive FOX-7 by quantum chemistry method.In this paper,the fully optimized geometries and properties have been calculated with DFT method.The result shows that in gas phase FOX-7 is the most stable judged by molecular geometry,bond level, NBO charge,second-order stablilized energy,total energy and the frontier orbital energy gaps. The cis-isomer is the most unstable and may be not suitable as explosive.The properties of trans-isomer are obviously better than that of cis-isomer.The decomposition mechanism is not clear based on experiment results at present.So the ab initio molecular dynamics(AIMD)was used to simulate thermal decomposition behavior in gas-phase for FOX-7.The energy barriers were calculated with ab initio molecular orbital (MO)methods at higher level of theory to make sure that which was the dominant thermal decomposition channel.The rate constants were evaluated based on the energies from G3MP2B3 level of theory for the C-NO₂ cleavage and nitro-to-nitrite rearrangement reactions. Ab initio molecular dynamics simulation and molecular orbital methods calculation results show that the initial decomposition mechanism can be divided into three kinds:C-NO₂ cleavage,hydrogen migration and nitro-to-nitrite.C-NO₂ cleavage energy is a little higher than that of nitro-to-nitrite rearrangement.The energy barrier of hydrogen migration is the lowest but its backward energy barrier is far lower than that of the forward barrier,so it is not important in the initial thermal decomposition.The rate constant calculated results show that C-NO₂ cleavage is the dominant initial thermal decomposition mechanism at high temperature and nitro-to-nitrite is more important at low temperature.This conclusion is in good agreement with molecular dynamics simulation at high temperature,and can explain the existed facts in literature.As one product of FOX-7 at high temperature,so we investigate the effect of NO₂ on decomposition of FOX-7.The calculation result indicates that the maximal decomposition energy barrier of FOX-7 descends when it reacts with NO₂.So the decomposition of FOX-7 can be accelerated when NO₂ exists.Calculated results with Ab initio molecular dynamics simulation and molecular orbital methods indicate that the dominant initial decomposition for trans-isomer of FOX-7 is C-NO₂ cleavage.Concerted cleavage of nitro group and hydrogen is also an important initial decomposition channel. However,decomposition energy for trans form is lower than that of FOX-7.So it is less stable.The molecular geometry and vibration properties of FOX-7 molecule and crystal were investigated by means of quantum mechanics which can make us understand intermolecular interaction and the special stability of FOX-7 better.The geometry of crystal molecule agrees well with the experiment result.The vibration frequencies of FOX-7 molecule and crystal were calculated and the internal modes were analyzed.Hydrogen bond can affect vibration mode and frequency shifting correlates with geometry of gas and crystal molecule.By means of density functional theory,within the local density approximation and generalized gradient approximation,we have studied the structural properties of crystal FOX-7 under hydrostatic pressure up to 7.5GPa.The geometry of crystal molecule agrees well with experiment result.A series of physical properties were obtained through theoretical calculation.