Vibrational spectra of quercetin and their interpretation with quantum-mechanical density-functional method

Experimental vibrational (Raman and IR-absorption) spectra are obtained for microcrystalline powder of quercetin in the spectral range of 400-1800 cm-1 at the room temperature. Optimized geometries of two stable isomers of quercetin molecule are calculated with a density-functional method at the level CAM B3LYP/6-311++G(d,p). The isomers have an almost planar frame structure and differ by mirror orientations of one of the rings with respect to the other rings. Vibrational spectra of the isomers are calculated in harmonic approximation at the same level of theory. The scaling factors determined experimentally for each of the two isomers have been used when comparing the calculated and experimental data. The vibrational spectra are interpreted in the whole frequency range under test. Good correlation of the experimental and calculated vibrational spectra is obtained.