The Theoretical Study Of THz Spectroscopy Of CL-20/TNT 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. The formation of cocrystal explosive is drived by the weak intermolecular interactions, such as hydrogen bonds, ?-? stacking, van der Waals forces, 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, based on the unique advantages of THz spectrum on the detections of weak intermolecular interactions, in this thesis, taking CL-20/TNT cocrystal as a prototype, we investigated its THz spectra and analyzed the corresponding vibrational modes using theoretical simulations. Also, we discussed the effects of temperature and vacancy defect on the THz spectrum of CL-20/TNT cocrystal. The specific contents and main conclusions are as follows:(1) Theoretical THz spectra were simulated and the vibration modes were assigned about ?-CL-20, TNT crystals and CL-20/TNT cocrystal. The results indicate that there are five novel distinctive THz peaks for CL-20/TNT cocrystal, located respectively at 0.25, 0.38, 0.87, 1.60 and 1.85 THz compared to the coformers, ?-CL-20 and TNT. Hereinto, 0.25, 0.38, and 0.87 THz absorption peaks originate from the lattice vibrations; meanwhile, 0.87 THz is also caused by C-H…O hydrogen-bonding bending vibrations; 1.60 and 1.85 THz features is assigned to the C-H…O hydrogen-bond stretching vibrations.(2) Crystal anisotropy of CL-20/TNT cocrystal along with different crystal faces presents three distinctive molecular arrangements and intermolecular interactions, such as, the C-H…O hydrogen bond along the(010) direction, ?-? stacking following(120) direction, and CL-20-CL-20 intermolecular electrostatic interactions in the(010) direction, respectively. In order to further validate the response of weak intermolecular interacti ons on the THz spectra, the THz spectra of different orientations of CL-20/TNT cocrystal are calculated. It concludes that 0.07 THz peak is a symbol of CL-20-TNT ?-? stacking interaction. Also, the C-H … O hydrogen bondings in CL-20/TNT cocrystal are further verified by means of the THz spectral features of(0 10) crystal faces. Addition, the THz spectral comparison of ?-and ?-CL-20 crystals as well as(001) crystal face indicates that the CL-20 molecule conformation in CL-20/TNT cocrystal is the same as that in ?-CL-20 other than in ?-CL-20.(3) The THz spectra of CL-20/TNT cocrystal at the different temperatures(5 K-296 K) were investigated, as well as the different orientations. The low-frequency absorption peaks(below 1.5 THz) disappear. Meanwhile, the two new absorption peaks at 4.58 and 5.99 THz are discovered at the lower temperatures, which are attributed to the phonon vibrations or intermolecular vibration modes. Compared to the THz spectra at 296 K, five THz peaks get blue-shifted, but two red-shifted with the temperature decreasing. The frequencyshifts reveal that the crystal structure and intermolecular interactions are changed as the temperature.In addition, the distinct THz peaks at 0.79, 1.53, 3.84 and 5.45 THz could be used as the intramolecular natural vibration peaks of(001) face in CL-20/TNT cocrystal. The absorption peak at 0.07 THz of(120) face further verifies the origin of ?-? stacking because there is no change with temperature.(4) The defect concentration leads to sagging of crystal structure and chaos of molecules. When defect concentrations are larger than 5%, the conformation of partial CL-20 molecules in the vicinity of void transforms from ?- to ?-CL-20. When defect concentration increases to 20%, the crystal structures have an obvious tendency to collapse.The THz spectra of CL-20/TNT cocrystal with different vacancy defect concentrations were calculated. The results show that the intensities of THz peaks below 0.5 THz increase with added defect concentration. Simultaneously, the intensities of absorption peaks at 0.73, 0.87, 1.60 and 1.85 THz decease, even this peaks disappear. When defect come to 20%, the absorption band s at 0.5-1 THz broaden and the THz spectrum is completely different from th ose of CL-20/TNT cocrystal.