Molecular and Crystalline Vibration Characteristics of Baicalin Investigated By Terahertz Spectroscopy and Density Functional Theory

Baicalin (C
₂₁
H
₁₈
O
₁₁
) is the main ingredient of the traditional Chinese herbal medicine Scutellaria Baicalensis Georgi, with proven bacteriostatic, anti-inflammatory, anti-hypertensive, and anti-allergic effects. It is widely used in a variety of popular nonprescription drugs, and the consumer safety and quality assurance of these drugs have become an increasingly pressing issue. In order to employ terahertz technology as a nondestructive inspection approach for such purposes, we have investigated by terahertz time-domain spectroscopy baicalin's terahertz vibration spectrum, obtaining unique characteristic absorption peaks at 0.787, 1.059, 1.403, 1.598, and 1.668 THz, which can be used as characteristic fingerprints for molecular detection and identification. To shed light on the origin of these spectral features, density functional theory (DFT) calculations were performed on this compound molecule, which provided clear evidence of its molecular and crystalline vibrational nature. The crystalline structure of baicalin was obtained in a single crystal culture and single-crystal X-ray diffraction experiment. The DFT calculations involving structural optimization and energy calculation based on both a single molecule and the crystalline structure produced vibrational spectral features that were consistent with the experimental results, concomitantly with complete, unambiguous assignment of all the vibrational modes. Systematic analysis suggests that the characteristic absorption peaks of baicalin are derived from both intramolecular and intermolecular interactions.