## 1 Introduction  
The quercetin molecule (3, 3', 4', 5, 7--pentahydroxylarone, C15H10O7) is an important flavonoid compound, which is found in many foods and plants, in particular in honey [1], and is known to act as a natural drug molecule with a wide range of treatment properties--antioxidant, anti-toxic, etc.--and is also involved in drug delivery from the site of administration to the therapeutic target [2; 3; 4; 5; 6; 7; 8; 9]. The structure of the quercetin contains two aromatic A and B rings linked through the C ring containing oxygen and five OH hydroxyl groups attached to the 3, 3', 4', 5 and 7 positions (see Scheme 1) [10; 11; 12; 13; 14; 15]. In a previous study [16], by using the quantum-mechanical (QM) calculations at the MP2/6-311++G(2 df,pdf)/B3 LYP/6-311++G(d,p) level of QM theory together with [PERSON]'s \"Quantum Theory of Atoms in Molecules\", for the first time, all possible conformers were established, corresponding to local minima on the potential energy hypersurface of the isolated quercetin molecule. Altogether, 48 stable conformers were established, which have been divided into four different conformational subfamilies by their structural properties: subfamily I--conformers **1**-**12**; subfamily II--conformers **13**-**18**, **20**, **23**, **24**, **26**, **29**, and **30**; subfamily III--conformers **19**, **21**, **22**, **25**, **27**, **28**, and **31**-**36**; subfamily IV--conformers **37**-**48**[16]. It was shown that these 48 stable conformers (24 planar structures (C\({}_{\rm s}\) point symmetry) and 24 non-planar structures (C\({}_{\rm 1}\) point symmetry)) represent a comprehensive set of the theoretically possible structures. Conformers of quercetin are polar structures with a dipole moment, which varies within the range from 0.35 to 9.87 Debye for different conformers with different direction for each. Their relative Gibbs free energies are arranged within the range from 0.00 to 25.30 kcal-mol-1 under normal conditions in vacuum. One half of these structures (24 conformers) possesses planar structure (C\({}_{\rm s}\) point symmetry), whereas the other half (24 conformers) does not have symmetry at all (C\({}_{\rm 1}\) point symmetry) (C3C2C1'C2' = 40.9-44.3 degree; C2C3 OGH = 9.4-16.3 degree). We also defined their physico-chemical characteristics, in particular, structural, energetic, and polar, which are necessary for understanding of the biological mechanisms of action of this molecule. Intramolecular specific contacts have also been explored in detail. [PERSON]'s \"Quantum Theory of Atoms in Molecules\" analysis shows that conformers of the quercetin molecule differ from each other by the intramolecular specific contacts (two or three), stabilizing all possible conformers of the molecule--H-bonds (both classical OH... O and so-called unusual CH... O and OH... C) and attractive van der Waals contacts O... O. Energies of these cooperative intramolecular specific contacts have been estimated [16]. Also, it was theoretically modeled the conformational interconversions [17; 18; 19; 20; 21] in the 24 pairs of the conformers of the quercetin molecule through the rotation of its almost non-deformable (A+C) and B rings around the C2-C1' bond through the quasi-orthogonal transition state (TS) with low values of the imaginary frequencies (28-33/29-36 cm\({}^{-1}\)) and Gibbs free energies of activation in the range of 2.17 to 5.68/1.86 to 4.90 kcal-mol-1 in the continuum with dielectric permittivity \(\varepsilon=1/\varepsilon=4\) under normal conditions. Also, we studied the changes of the number of physico-chemical characteristics of all intramolecular specific contacts--hydrogen bonds and attractive van der Waals contacts during these conformational rearrangements. This study is a logical development of the previous investigations [16; 17; 18] and is devoted to the novel interconversions between the conformers of the quercetin molecule due to the rotations of the O3'H and O4'H hydroxyl groups around the exocyclic C-O bonds outside.