A new era of the prototropic tautomerism of the quercetin molecule: A QM/QTAIM computational advances.

In this study for the first time we have revealed and investigated in details 123 different prototropic tautomers of the most stable conformer of the quercetin molecule using quantum-mechanical calculations at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of QM theory. We have found that in the most energetically favorable prototropic tautomer mobile hydrogen atoms are localized at the О3, О3', О4', О5, and О7 exocyclic oxygen atoms. Molecular tautomers are in the range of the Gibbs free energies from 0.0 to 69.8 kcal·mol-1, while zwitterionic ones - from 30.1 до 172.8 kcal·mol-1 at normal conditions. It was also reliably established that the weakest point causing the decyclization of the molecule is its C ring - this reaction is launched by the transition of the proton from the C8H group to the endocyclic O1 oxygen atom. All prototropic tautomers, except two cases, are joined by the intramolecular cooperative specific interactions (from 1 to 5) - H-bonds and attractive van der Waals contacts, which have been revealed and characterized by QTAIM analysis. Communicated by Ramaswamy H. Sarma.