Identification and low-frequency vibrational analysis of three free anthraquinones via terahertz spectroscopy

In this study, terahertz time-domain spectroscopy (THz-TDS) was used to obtain the terahertz absorption spectra of three free anthraquinones (Chrysophanol, Emodin, Physcion) in the frequency range of 0.2-4.3 THz. The results show that terahertz spectroscopy is an effective detecting such compounds. Meanwhile, the theoretical spectrum using density functional theory calculations agrees well with the experimental spectrum. A modal decoupling method was used to identify each low-frequency vibrational mode and determine the average contribution of different atoms and groups. Modal decoupling provides a better understanding of molecules' mixed vibrational modes and enables quantifying the atoms' vibrational contributions. Results show that the substituent group facilitates the transition between the fundamental vibrational modes; subsequently, the sub-stituent group shifts the vibrational centre of gravity of the three molecules and affects the vibrational contri-bution of hydrogen bonds. Furthermore, insignificant Emodin absorption is related to the nearly symmetrical structure formed by the substituents. The feasibility of terahertz analysis of differential molecular structures has also been confirmed.

Automated summary

In this study, terahertz time-domain spectroscopy (THz-TDS) was used to effectively detect the terahertz absorption spectra of three free anthraquinones. The theoretical spectrum agreed well with the experimental spectrum, and modal decoupling method provided a better understanding of molecules' mixed vibrational modes and enabled quantifying the atoms' contributions. Results showed that the substituent group facilitated the transition between fundamental vibrational modes and affected the vibrational contribution of hydrogen bonds. The feasibility of terahertz analysis of differential molecular structures was also confirmed.