Terahertz time-domain spectroscopy was used to identify and analyze the low-frequency vibrational modes of three free anthraquinones, revealing the vibrational contributions of different atoms and groups.
A recent study published in the Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal reports on the successful identification and low-frequency vibrational analysis of three free anthraquinones, namely Chrysophanol, Emodin, and Physcion, found in Rhubarb roots using terahertz time-domain spectroscopy (THz-TDS) (1).
Led by Quancheng Liu and Liping Shang from the Southwest University of Science and Technology in Mianyang, China, the research aimed to distinguish between the three structurally similar compounds, which have different medicinal effects, to prevent medicine abuse.
The team found that THz-TDS is an effective method for detecting such compounds, with the theoretical spectrum using density functional theory calculations agreeing well with the experimental spectrum. They also used a modal decoupling method to identify each low-frequency vibrational mode and determine the average contribution of different atoms and groups, which enabled a better understanding of molecules' mixed vibrational modes and quantified the atoms' vibrational contributions.
THz-TDS is a technique that utilizes terahertz radiation to study the properties of materials. It involves generating a broadband pulse of terahertz radiation and measuring the time-resolved electric field of the pulse. This allows for the determination of the material absorption and refractive properties in the terahertz frequency range.
Results showed that the substituent group facilitates the transition between the fundamental vibrational modes, shifting the vibrational center of gravity of the three molecules and affecting the vibrational contribution of hydrogen bonds. They also found that Emodin's nearly symmetrical structure formed by the substituents resulted in insignificant absorption.
Terahertz spectroscopy is a particular fingerprint method that can differentiate between similar structure compounds, such as isomers and different polar groups, making it a useful tool in identifying medicinal components in Rhubarb roots. Furthermore, understanding the motions of crystals in the terahertz region helps to understand the network structure, interactions, and functionality of crystals.
Although density functional theory calculations are considered necessary to predict these vibrational motions, understanding these 3D motions in a 2D visualization remains challenging. However, the study's use of individual mass-weighted eigenvectors for each atom provides a more detailed understanding of the resonance modes.
In conclusion, this study confirms the feasibility of terahertz analysis of differential molecular structures and highlights the potential of terahertz spectroscopy in identifying medicinal components in natural products.
(1) Hou, S.; Liu, Q.; Deng, H.; He, J.; Zhao, W.; Wu, Z.; Zhang, Q.; Shang, L. Identification and low-frequency vibrational analysis of three free anthraquinones via terahertz spectroscopy. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2023, 293, 122439. DOI:
