D-,L-和DL-奥硝唑随温度变化的太赫兹光谱

Temperature-dependent terahertz spectroscopy of D-,L-and DL-ornidazole

利用太赫兹时域光谱(terahertz time domain spectroscopy,THz-TDS)技术,在6 K到298 K之间,测量了D-,L-和DL-奥硝唑随温度变化的太赫兹(THz)光谱.实验结果表明,在0.3到2.5 THz波段,在常温时D-和L-奥硝唑的吸收峰几乎相同,但与DL-奥硝唑的吸收峰存在差异,到低温时这种差异变得更加明显,为鉴别奥硝唑旋光异构体与其消旋体提供了新的方法;低温时观察到了在常温时很难分辨的吸收峰,为振动模式指认提供更多的信息;随着温度的升高,吸收峰中心频率朝着低频的方向单调偏移,通过对实验数据的拟合,发现振动模式随温度变化符合Bose-Einstein统计规律.最后对奥硝唑旋光异构体及其消旋体分子进行量子化学计算,并模拟得到0.2—2.5 THz的低频振动光谱,根据光谱实验结果,从分子水平上对其特征吸收信号进行了理论分析.

Terahertz time-domain spectroscopy （THz-TDS） has been used to measure the vibrational spectra of polycrystalline D-, L- and DL-ornidazole over the temperature range 6-298 K. The large differences of the spectra are observed in the frequency range 0.3 THz-2.5 THz, which indicates that THz-TDS is an effective tool to distinguish between enantiomers （D-, L-ornidazole） and its racemic compound （ DL-ornidazole）. A number of absorption peaks which could not be observed at room temperature are well resolved in the low temperature range. This provides more experimental details for mode assignment. The evolution of these absorption features between 6 K and 298 K was continuously mapped. The absorption frequencies shift to lower values as temperature increases. The measured temperature dependence can be well fitted if we consider the phonon modes follow a Bose- Einstein distribution. Finally, the quantum chemical calculations were carried out to simulate the experimental THz spectra of the three samples in the range of 0.2--2.5 THz. Compared with the calculated results, the observed characteristic absorptions are explained at molecular level.