Study On The Absorption Spectrum Of α / β-HMX Terahertz

HMX, also called Octogen, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, of the formula as C4H8N8O8, is a powerful and relatively insensitive nitroamine high explosive with good explosive performance, commonly used in the conventional/sophisticated weapons and Civil Engineering applications, including as the high-energy polymer-bonded explosives forming, the solid propellant, the industrial explosive materials, the oil exploration and also as the satellite launching.HMX have four different crystal structure forms as α, β, γ and8, and usually, the P form, which not only has the most stable crystal structure, but also has the highest energy density and the best physical/chemical performance of all four, is most widely used in a variety kind of applications. However, large numbers of studies based on different experiments indicate that the polymorphs of HMX have themselves transformed from one to another between each four crystal forms in certain conditions, which would have bad effects on the performance of the explosive. The crystal transform is in essence a changing process of the molecular structures, and the terahertz wave spectroscopy, as a new research technique of spectroscopy, has huge advantages in the molecular structure representation, and also with a sensitive reaction in the structural characterization of polymorphs explosives.Based on the terahertz time-domain spectroscopy, the terahertz spectrum of samples with different content of HMX explosives and different granularity were studied. Also, the α/β-HMX terahertz spectrum were obtained, and the characteristic peaks of a and β HMX in the frequency range of0.2THz-2.0THz were located. Thereinto, a-HMX had three absorption peaks at0.82THz,1.51THz and1.93THz, in which, the strongest absorption appearance at the frequency of1.51THz while the weakest at0.82THz, in another hand, there was only one absorption peak obtained at1.80THz frequency of P-HMX sample.Based on the density functional theory, the vibration frequency in a molecule and crystal system were calculated by different kinds of functional using Gaussian09and DMol3software, and the experimental spectrum and the computed results were compared. Based on the visualization module of software, the terahertz absorption peaks was identified and the formation mechanism of α/β-HMX terahertz absorption peak was discussed. The results showed that the1.80THz absorption peak of the P-HMX is formed from the swing of two nitro groups, and to the a-HMX crystals, the0.82THz absorption peak is formed from the symmetrical swing of a pair of nitro groups, the1.51THz absorption peak is formed from a symmetrical swing of the other pair of nitro groups, and the formation of1.93THz absorption peak is more complicated, which containing a swing of nitro groups, a twist of nitro groups and a twist of the C-H group. The formation mechanism of terahertz absorption peak was came out by the energy level transition of the molecular structure movements from the ground state to the excited state, in which, the THz protons were absorbed at the specific frequency and the absorption peaks formed in the terahertz spectrum.