Zhao Yonghong, Li Zhi, Liu Jianjun, Hu Cong, Zhang Huo, Qin Binyi, Wu Yifang
School of Mechano-Electronic Engineering, Xidian University, Xi'an, Shanxi 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, Shanxi 710126,China; Guilin University Of Aerospace Technology, Guilin, Guangxi 541004, China.
Spectrochim Acta A Mol Biomol Spectrosc. 2018 Jan 15;189:528-534. doi: 10.1016/j.saa.2017.08.041. Epub 2017 Aug 25.
The characteristic absorption spectra of crystalline urea in 0.6-1.8 THz region have been measured by terahertz time-domain spectroscopy at room temperature experimentally. Five broad absorption peaks were observed at 0.69, 1.08, 1.27, 1.47 and 1.64 THz respectively. Moreover, density functional theory (DFT) calculation has been performed for the isolated urea molecule, and there is no infrared intensity in the region below 1.8 THz. This means that single molecule calculations are failure to predict the experimental spectra of urea crystals. To simulate these spectra, calculations on a cluster of seven urea molecules using M06-2X and B3LYP-D3 are performed, and we found that M06-2X perform better. The observed THz vibrational modes are assigned to bending and torsional modes related to the intermolecular H-bond interactions with the help of potential energy distribution (PED) method. Using the reduced-density-gradient (RDG) analysis, the positions and types of intermolecular H-bond interactions in urea crystals are visualized. Therefore, we can confirm that terahertz spectroscopy can be used as an effective means to detect intermolecular H-bond interactions in molecular crystals.
室温下，通过太赫兹时域光谱实验测量了结晶尿素在0.6 - 1.8太赫兹区域的特征吸收光谱。分别在0.69、1.08、1.27、1.47和1.64太赫兹处观察到五个宽吸收峰。此外，对孤立的尿素分子进行了密度泛函理论（DFT）计算，在1.8太赫兹以下区域没有红外强度。这意味着单分子计算无法预测尿素晶体的实验光谱。为了模拟这些光谱，使用M06 - 2X和B3LYP - D3对七个尿素分子的簇进行了计算，我们发现M06 - 2X表现更好。借助势能分布（PED）方法，将观察到的太赫兹振动模式归因于与分子间氢键相互作用相关的弯曲和扭转模式。使用密度降低梯度（RDG）分析，可视化了尿素晶体中分子间氢键相互作用的位置和类型。因此，我们可以确认太赫兹光谱可作为检测分子晶体中分子间氢键相互作用的有效手段。
