Zhao Yonghong, Li Zhi, Liu Jianjun, Chen Tao, Zhang Huo, Qin Binyi, Wu Yifang
School of Mechano-Electronic Engineering, Xidian University, Xi'an, Shaanxi 710126, China; Guangxi Key Laboratory of Automatic Detecting Technology and Instruments, School of Electronic Engineering and Automation, Guilin University of Electronic Technology, Guilin, Guangxi 541004, China.
School of Mechano-Electronic Engineering, Xidian University, Xi'an, Shaanxi 710126, China; Guilin University of Aerospace Technology, Guilin, Guangxi 541004, China.
Spectrochim Acta A Mol Biomol Spectrosc. 2018 Mar 5;192:336-342. doi: 10.1016/j.saa.2017.11.040. Epub 2017 Nov 22.
The characteristic absorption spectra of two structural isomers of dimethylurea(DMU) in 0.6-1.8 THz region have been measured using terahertz time-domain spectroscopy (THZ-TDS) at room temperature. Significant differences have been found between their terahertz spectra and implied that the THZ-TDS is an effective means of identifying structural isomers. To simulate their spectra, calculations on single molecule and cluster of 1,1-DMU and 1,3-DMU were performed, and we found that the cluster calculations using DFT-D3 method are better to predict the experimental spectra. Using the normal mode as displacements in redundant internal coordinates and the GaussView program, most observed THz vibrational modes are assigned to bending and rocking modes related to the intermolecular hydrogen bonding interactions, and twisting mode of ethyl groups. The different spectral features of two isomers mainly arise from different intermolecular hydrogen bonds resulting from different atom arrangements in molecules and different molecule arrangements in crystals. Using the reduced-density-gradient (RDG) analysis, the positions and types of intermolecular hydrogen bonding interactions in 1,1-DMU and 1,3-DMU crystals are visualized. Therefore, we can confirm that THz-TDS can be used as an effective means for the recognition of structural isomers and detection of intermolecular hydrogen bonding interactions in these crystals.
在室温下，利用太赫兹时域光谱（THZ - TDS）测量了二甲基脲（DMU）两种结构异构体在0.6 - 1.8太赫兹区域的特征吸收光谱。发现它们的太赫兹光谱之间存在显著差异，这表明THZ - TDS是识别结构异构体的有效手段。为了模拟它们的光谱，对1,1 - DMU和1,3 - DMU的单分子和簇进行了计算，我们发现使用DFT - D3方法的簇计算能更好地预测实验光谱。利用冗余内坐标中的简正模式作为位移并结合GaussView程序，大多数观测到的太赫兹振动模式被归属于与分子间氢键相互作用相关的弯曲和摇摆模式以及乙基的扭转模式。两种异构体不同的光谱特征主要源于分子中不同的原子排列和晶体中不同的分子排列所导致的不同分子间氢键。使用密度降低梯度（RDG）分析，可视化了1,1 - DMU和1,3 - DMU晶体中分子间氢键相互作用的位置和类型。因此，我们可以确认THZ - TDS可作为识别这些晶体中结构异构体和检测分子间氢键相互作用的有效手段。
