Terahertz Spectroscopic Study And Low Frequency Vibrational Analysis Of Anthraquinones

Anthraquinone is a natural coloring agent found in Polygonaceae rhubarb and other higher plants such as Cyperaceae,and can also be synthesized artificially and is used in the synthesis of raw materials for hydrogen peroxide and as a cooking agent for paper making.Typical free anthraquinones such as Chrysophanol and Emodin have become valuable due to being rediscovered to have important medicinal studies.In this type of compounds,the similarity of both macroscopic physical and microstructural properties makes the search for differences particularly difficult.The collective vibrational rotation information of drugs and biological macromolecules as well as the vibrational frequencies of weak interactions with neighboring molecules are in the terahertz band,and terahertz spectroscopy,which has the advantages of containing molecular structure information and being less destructive to drugs,is adopted based on complementing the current detection means,which can help further understanding of complex physicochemical processes such as molecular force mechanisms and structural characterization of compounds in drug development.This paper takes free anthraquinone as the object of study,terahertz time-domain spectroscopy system as the detection means,and combines density generalization theory and modal decomposition method to carry out relevant research,the details of which are as follows:(1)The terahertz time-frequency domain spectral data of five free anthraquinones were obtained by THz-TDs,and then their absorption coefficients were extracted by optical parameter models.Among them,Chrysophanol,Emodin,Physcion and Rhein have 9characteristic absorption peaks with different characteristics,with rhubarbin peak showing the least obvious.The experimental results showed that the different strong and weak absorption regions and characteristic fingerprint peaks of the five compounds at 0.2-4.5 THz could be used as the basis for their qualitative discrimination.(2)Theoretical spectral validation of rhubarb phenols,rhodopsin,and rhodopsin methyl ether by density generalized function theory.In this paper,based on the Generalized gradient approximations,Perdew-Burke-Ernzerhof energy exchange correlation generalized function,the simulation results show a high match with the experimental results by the reasonable choice of truncation energy,K point and pseudopotential.The vibration patterns showed that:Chrysophanol was dominated by unbalanced oscillation in the range of 0.2-4.5 THz;Emodin was dominated by rotational motion or translation of the molecule as a whole in 0.2-2 THz,oscillation in 2-3 THz,and mixed vibration of methyl rotation with other atoms in 3-4.5 THz;Physcion showed higher vibrational activity of peripheral groups,such as methyl and methoxy.Meanwhile,the vibrational modes of similar peak positions among the compounds are different,which shows that the small structural differences bring about great differences in the overall vibrational modes of the molecules.(3)The vibration mode is decomposed into three intermolecular translations and librations,and intramolecular vibrations based on the modal decoupling method,with a total of seven fundamental vibration components.The results showed that the intermolecular forces decreased and the intramolecular vibrations gradually increased with the increase of frequency.Among them,the intramolecular force increment of Chrysophanol showed the smoothest performance;Emodin vibration coupling was the highest,and the individual modes showed the most libration components;the intermolecular vibrations of Physcion accounted for the most,which combined with the multiple oscillatory vibration modes throughout the observable frequency band,indicated that there was a positive correlation between the libration contribution and the vibration modes of swing.The average vibrational contributions of the atoms and groups of the three molecules in question were found to be as follows: the vibrational center of gravity of Chrysophanol shifts with respect to Emodin and Physcion with the action of substituents,where rhodopsin forms an approximately symmetrical vibrational structure,which results in a small out-of-plane distortion force affecting the increase of the overall intramolecular vibration.At the same time the action of the substituent makes the intramolecular hydrogen bonding more pronounced,which may be related to the shift of the vibrational center of gravity.The contribution of atoms forming hydrogen bonds indicates that the intramolecular hydrogen bonding forces are extremely small and more pronounced for intermolecular hydrogen bonds than for intramolecular hydrogen bonds.