Baicalin extracted from scutellaria by ethanol-water system was purified to 95.2%. Optimum resolution data of X-ray powder diffraction for the polycrystal powder were collected from an X'Pert Pro MRD diffractometer with scanning conditions of 0.0167° and 7.62 ms per step. On basis of its molecular structure in low energy conformation by density functional theory, the crystal structure was determined by a direct-space approach. The results show that the crystal structure of baicalin belongs to monoclinic, P21 space group with unit cell parameters: a＝13.02 Å, b＝17.22 Å, c＝4.49 Å, α＝γ＝90°, β＝95.40°, Z＝2, V＝1016.1 Å3. Natural bond orbital (NBO) theory analysis of the structure indicates that the heterocyclic ring in the aglycone could be an aromatic ring. The IR result shows that conjugative effect is the reason for the red shift of stretching vibration frequency for carbonyl, meanwhile hydrogen bonds lead to the red shift of stretching vibration frequency for hydroxy groups. The crystal growth characteristic of baicalin was also forecasted by the help of H-bond analysis.

Key words: baicalin, structure, X-ray powder diffraction, density functional theory, IR spectrum