Theoretical Study Of THz Spectroscopy Of CL-20/HMX Energetic Cocrystal

Currently,the improvement of physicochemical properties of explosives by the way of cocrystallization has become a research hotspot in the field of energetic materials.However,in the absence of single-crystal X-ray diffraction data,rigorously characterizing the results of cocrystallization attempts can be difficult.Specifically,differentiating between physical mixtures and true cocrystalline material is challenging due to the propensity for energetic molecules to form solvates and/or polymorphs.Therefore,finding effective characterizing means of cocrystal explosive will be the key technology issue needed to be solved for the development and further application of cocrystal explosives.The formation of cocrystal explosive is mostly driven by the intermolecular interactions,such as hydrogen bonds,π-π stacking,van der Waalsforces,and so on.The THz spectroscopy is used for assignment of these intermolecular interactions directly and explicitly since their energy level transitions are located in the detection range of THz spectroscopy.So,CL-20/HMX cocrystal is chosen as model system in this thesis.The THz spectra of HMX,CL-20 and cocrystal CL-20/HMX have been studied based on the molecular dynamics simulation with quantum mechanics calculation.Also,we has analysised the corresponding vibration modes.At the same time,we discuss the effects of vacancy defects on the THz spectrum of CL-20/HMX cocrystal and(010)face in CL-20/HMX cocrystal.The research results in this thesis will provide the theoretical basis and technology support for the application of THz spectrum in cocrystal explosives,wide the application range and enrich the study directions of THz spectrum.The specific contents and main conclusions are as follows:(1)The theoretical study of THz spectroscopy of CL-20 and HMX cocrystal coformers: theoretical THz spectra were simulated and the vibration modes were assigned about CL-20 and HMX crystals.Results indicate the simulation THz spectra are in good agreement with the experimental values of HMX and CL-20 crystals.The THz spectra of CL-20 with different polymorphs are different due to different lattice packing models and intermolecular interactions.(2)The theoretical study of THz spectroscopy of CL-20/HMX energetic cocrystal: theoretical THz spectra were simulated and the vibration modes were assigned for CL-20/HMX cocrystal.The results indicate that there are five novel distinctive THz peaks for CL-20/HMX cocrystal,located respectively at 0.23,0.49,1.47,1.73 and 2.27 THz compared to the coformers,CL-20 and HMX.Thereinto,1.1,1.47,and 1.73 THz absorption peaks are caused by C-H···O hydrogen-bonding interactions and 1.73 THz feature is assigned to the heterogeneous intermolecular hydrogen bonds.(3)THz spectrum analysis of the CL-20/HMX cocrystal models with different vacancy concentrations: constructing a series of models with different vacancy concentrations,we have investigated the effects of vacancy defects on the THz spectrum of CL-20/HMX cocrystal and(010)face by the theoretical simulations.The results show the vacancy defects result in the loosing of the crystal structure,thus reducing the crystalline field effects.Besides,the molecules on the surface or in the vicinity of voids become confused.Through the THz spectrum analysis of the CL-20/HMX cocrystal and(010)face with different vacancy concentrations,it can be found that the THz spectra are completely different from that of the perfect single crystal.With the increase of vacancy defect,the absorption spectra have broaden and the intensities of THz peak in low frequency range have strengthen.The main reasons may attribute to destruction of C-H … O hydrogen bonding net and other intermolecular interactions and descrease of the crystalline field effects in cocrystal due to the vacancy defects.