IR spectrum and normal mode analysis of the anti-Alzheimer's disease natural product Huperzine A: A quantum chemistry density-functional theory (DFT)investigation

Quantum chemistry density-functional theory (DFT)B3LYP method with 6-31G* b asis set has been empolyed to study the electronic structure and IR spectru m of Huperzine A. The calculation result showed that the characteristic of the predicted IR bands was in general consistent with the experimental spec trum. 45 vibration modes were assigned clearly from the total of 102 vibrat ion bands. The strongest IR-intensive band corresponds to the stretching vi bration of the C=O bond of the pyridone ring, and the highest frequency ban d belongs to the pyridone N-H stretch. The investigation showed that the ob vious differences between the calculated bands and the experimental spectru m existed at the bands involving the hydrogen atoms of amino and pyridone a mide groups, which could form intermolecular hydrogen bond with other Huper zine A in the crystal structure. The hydrogen bonds can not only affect the orientation of these hydrogen atoms, but also can affect the force propert y of the chemical bond, which can change the vibrational frequencies.