Ab initio HF and density-functional theory studies on the structure and vibrational frequency of huperzine A

• Published in: Journal of molecular structure. Theochem Vol. 488; no. 1; pp. 21 - 28
• Main Authors: Zhu, Wei-Liang, Jiang, Hua-Liang, Gu, Jian-De, Chen, Jian-Zhong, Shen, Jing-Kang, Chen, Kai-Xian, Ji, Ru-Yun, Cao, Yang
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.1999
• Subjects: Ab initio HF; DFT-B3LYP; Huperzine A; Normal Mode Analysis; Vibrational Spectrum; Normal Mode Analysis; DFT-B3LYP; Ab initio HF; Huperzine A; Vibrational Spectrum
• ISSN: 0166-1280; 1872-7999
• DOI: 10.1016/S0166-1280(98)00605-8

Huperzine A, an alkaloid used as acetylcholinesterase inhibitor isolated from traditional Chinese herb, was studied using semiempirical method AM1, ab initio Hartree–Fock (HF), and density-functional theory (DFT) B3LYP method at different basis set levels. The calculated results showed that the three-ring structure of HupA is rigid and the pyridone ring is planar. However, the hydrogen atom positions of its amide and amino groups will shift when the molecular environment changes, especially for the amino group. HF/4-31G calculated results revealed that the amino group can rotate at room temperature. The investigation also indicated that the B3LYP/6-31G* method is better than AM1 and HF/6-31G* for studying infra-red (IR) spectrum of HupA and its analogues. The predicted vibrational bands at B3LYP/6-31G* level are in good agreement with the observed spectrum except the vibrational modes which relate to the atoms of amide and amino groups. The reason for the differences of structure and vibrational bands is probably that these groups can form intermolecular hydrogen bonds in the crystal structure, which will affect the force properties and the vibrational frequency.