FOX-7转晶行为的太赫兹光谱及理论计算研究

1, 1 -二氨基- 2, 2 -二硝基乙烯(FOX-7)是一款新型高能钝感炸药, 为了研究温度变化对其分子结构特性的影响, 利用太赫兹时域光谱技术对持续升温(298K→393 K)过程中FOX-7 在0.2—2.5 THz 频率范围内的吸收光谱进行了在线探测, 结果发现随着样品温度的升高, FOX-7 的吸收谱带发生变化, 于384 K 时出现一个新的吸收特征峰, 且该吸收峰的峰强逐渐升高. 基于密度泛函理论(DFT), 对样品在298 K 和393 K 温度下的晶体结构进行了小于2.5 THz 范围内吸收频谱的模拟计算, 完成了对FOX-7 两种晶型实验吸收特征峰的指认. 分析表明FOX-7 的分子结构会受温度的影响而发生改变, 互为异构晶型的基团表现出的振动模式不同, 温度384 K 时FOX-7 开始发生α→β 晶型转变, 且该晶型转变过程是可逆的, 新出现的1.12 THz 特征峰在393 K 时的振动是由—NO₂ 和—NH₂ 的摆动及各自的扭动所致.

Terahertz spectrum and simulation of the phase transformation of FOX-7

1, 1-diamino-2, 2-dintroethylene (FOX-7) is a novel explosive of high energy and low sensibility. In order to study the effect of temperature changes on the molecular structural characteristics of the explosive, its absorption spectra in the frequency range of 0.2–2.5 THz at a constant rate of heating from 298 K to 393 K are detected by terahertz time-domain spectroscopy (THz-TDS). Results show that a number of characteristic absorption peaks with different intensities appear at 1.59–2.13 THz when the temperature is 298 K, while the absorption spectra change with the increase of temperature of the explosive sample; a new characteristic absorption peak located at 1.12 THz appears at 384 K, and its absorption peak intensity gradually increases, but disappears when the temperature drops to 298 K. The absorption spectra of FOX-7 molecular crystal at 298 and 393 K within the 0.2–2.5 THz region based on density functional theory (DFT) are also simulated by using Materials Studio 6.0 software in this article, and the simulated results agree well with the experimental data. In addition, the vibrational modes of the characteristic peaks of two kinds of crystalline in the experimental absorption spectra are analyzed and identified, showing that the formation of the characteristic absorption peaks is closely related to the molecular vibration, and the molecular structure may change under the influence of temperature, and the tautomeric polymorphism of the crystalline has different vibrational modes. This article indicates that the process of phase transformation of FOX-7 starts from 384 K, and this process is reversible; the characteristic absorption peak at 1.12 THz is composed of two kinds of vibrations (the swinging and torsional vibrations of the nitro and amido groups).