Conformational equilibrium, IR and Raman vibrational spectra of the quercetin molecule in different solvents: A comprehensive quantum-chemical investigation

Abstract This study represents comprehensive investigation by the quantum-chemical calculations at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of theory of the conformational equilibrium, IR and Raman vibrational spectra of all 48 conformers of the quercetin molecule in the isolated state (e=1.0) and various solvents, such as: continuum with e=4.0, ethanol (e=24.3), methanol (e=32.7), DMSO (e=47.2) and water (e=80.4). It has been established their characteristic features, which are closely related to the microstructural mechanisms of the biological action of the quercetin molecule. It was shown that, independently from the value of the dielectric constant e of the solvent, conformational equilibrium of the quercetin molecule is determined by the co-existence of planar structures of conformers 1–8, which population consist 99 % among all 48 conformers. In the IR and Raman vibrational spectra of the conformers of the isolated quercetin molecule it was defined vibrational markers, which enable to determine their belonging to one of the four conformational subfamilies and to precisely identify each of them. It was discussed characteristics of the transformations of the vibrational spectra at the transition of the quercetin molecule into the polar solvents. It was established that altogether quercetin molecule possesses 90 vibrational modes in its vibrational spectra, including low-frequency vibrations (⁓ 4 cm−1) or so-called soft modes. It was established that different solvents lead to the increasing of the intensities of the IR and Raman vibrational spectra. It was solved the task of the detection of the belonging of the conformers of the quercetin molecules to the conformational subfamilies and precise identification each of them by the IR and Raman vibrational portraits. It was analyzed in detail the possibility of the experimental fixation of the conformational equilibrium of the quercetin molecule both in the isolated state and in the polar solutions. Possible applications of the obtained results have been also considered.