选取了在化工、 医药生产中常见的中间体——二甲基苯甲酸作为研究对象, 利用太赫兹时域光谱（THz-TDS）系统研究了室温条件下, 二甲基苯甲酸的六种同分异构体在太赫兹波段的吸收光谱。 实验结果表明, 在0.2～2.2 THz波段范围内, 六种物质的特征频谱响应有非常明显的区别, 而红外光谱仪的测量结果则显示, 在1 450～1 700 cm-1波段范围内不同的异构体表现出相似的吸收特征。 利用密度泛函理论(DFT)对六种物质的吸收频谱进行计算, 并对它们的吸收峰产生的原因进行分析, 可知在1 450～1 700 cm-1范围内六种物质的吸收峰均是由苯环的骨架伸缩振动和羧基上CO的伸缩振动引起的, 而在0.2～2.2 THz波段, 由于六种物质分子中整个苯环与相对位置不同的三个支链之间存在面外相对摆动, 造成了集体振动模式的差异, 从而表现出不同的吸收特征。 通过分析发现, 六种样品在两个频率范围分别表现出的差异和相似的吸收特征, 源于其分子结构间的差异性和相似性, 而利用太赫兹波段吸收特征的差异, 可以实现对二甲基苯甲酸六种同分异构体的鉴别。 上述研究结果表明, 利用太赫兹和红外光谱技术研究同分异构体光谱特性的差异性和相似性是可行的, 也为快速鉴别二甲基苯甲酸异构体种类和检测该化工中间体的纯度提供了一条有效的途径。

In this paper, the absorption spectra of 6 isomers of dimethylbenzoic acid, which were widely used in chemical and pharmaceutical production as intermediate substance, were measured by using the terahertz time-domain spectroscopy (THz-TDS) system in the range 0.2～2.2 THz at room temperature. The experimental results show that the six measured isomers present apparent different spectral response. However, the results of using infrared spectroscopy indicates that different isomers show high similarity in absorption spectra in the range 1 450～1 700 cm-1. The vibrational frequencies are calculated by using the density functional theory (DFT), and identification of vibrational modes are given. It is clear that the absorption peaks of the 6 isomers in the range 1 450～1 700 cm-1 come from the stretching vibration of benzene ring and CO, while the absorption peaks in the terahertz range are caused by the relative wagging of benzene ring and all the chains out of plane, which lead to the different absorption characteristics of the 6 isomers in the range 0.2～2.2 THz. The results suggest that the difference and similarity of the absorption spectra observed in the two different frequency range are resulted from the difference and similarity of the molecular structures of the six isomers. By using the different absorption characteristics, we can identify the six isomers of dimethylbenzoic acid effectively. Our study indicates that it is feasible to distinguish the isomers by using terahertz and infrared spectroscopy technique. It provides an effective way to identify different isomers and test the purity of the intermediate substance in the process of production quickly and accurately.