SONG Xue-yan1, LI Yan1, XIA Qi-ying2*, JU Xue-hai1*
1. School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2. School of Chemistry and Chemical Engineering, Linyi University, Linyi 276005, China
Abstract：Dihydroxylammonium 5,5’-bistetrazole-1,1’-diolate (TKX-50) is a newly synthesized high energetic explosives. Density functional theory calculations were carried out to study the terahertz (THz) spectra of TKX-50 and its pressure and isotope effects. The results show that TKX-50 has four peaks in low frequencies of 1.17, 1.19, 2.50 and 2.95 THz, and six absorption peaks at high frequencies of 3.87, 4.79, 5.33, 5.75, 6.65 and 7.85 THz. Among them, four peaks at 1.17, 3.87, 5.33 and 6.65 THz have large absorption intensities. Vibrational analyses of normal modes show that the absorption peak at 1.17 THz belongs to the N—H…O stretching; and those at 3.87, 5.33 and 6.65 THz are attributed to the NH3 rocking. In the 0～4.0 THz, the absorption peaks of 1.17, 1.91, 2.50, 2.95 and 3.87 THz for TKX-50 correspond to the peaks of 1.13, 1.85, 2.44, 2.91 and 3.63 THz for the deuterated TKX-50, respectively. The deuteration of TKX-50 brings about the red shifts of THz spectra along with the absorption intensities decreasing at the same time. In addition, the peaks at 4.79 THz and 5.33 THz for TKX-50, which are assigned to the rocking of NH3, become overlapped and redshifted to 4.66 THz due to deuteration. As the pressure changes, the variations of molecular distances are larger than those of bond lengths. The intermolecular O…H distance generally decreases with increasing pressure, especially the NO…HN distance. However, the lengths of the O—H and N—H bonds irregularly vary with pressures. When the pressure increases from 1 atm to 30 GPa, the peaks at 4.79, 5.33 and 5.75 THz will red shift and overlap, accompanied by a larger change of absorption intensity. Then at 30 GPa, the absorption peaks overlap to a weak peak at 4.54 THz. With increasing pressure, the absorption at 6.65 THz is slightly blue shifted, while the peak at 7.85 THz is red shifted firstly but then gradually blue shifted. This is because the atomic equilibrium distance decreases with increasing pressure, leading to an increase in bond strength; in addition, when the crystal is severely compressed, the steric hindrance for the NH3 rocking becomes large, and large potential energy needs to overcome for the rocking, resulting in a blue shift of THz absorption. The low frequency peaks irregularly vary with pressures, mainly because the length of the N—H bond varies irregularly with pressures. Since the vibrational movements of the molecule are constrained by the steric hindrance at high pressures, the THz absorption intensities of TKX-50 decrease with increasing pressure. The above studies not only reveal the THz spectral characteristics of TKX-50 and the effects of pressure and isotope but also provide the theoretical basis for further study of the optical properties in THz region and their application in rapid penetrable detections for similar energetic materials.
Key words：TKX-50; Terahertz spectroscopy; Density functional theory; Pressure; Deuteration
宋雪燕，李 燕，夏其英，居学海. TKX-50太赫兹光谱的理论研究[J]. 光谱学与光谱分析, 2020, 40(10): 3056-3060.
SONG Xue-yan, LI Yan, XIA Qi-ying, JU Xue-hai. Theoretical Study on Terahertz Spectra of TKX-50. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2020, 40(10): 3056-3060.
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