Solid state NMR studies and density functional theory (DFT) calculations of conformers of quercetin

This work presents the first comparative analysis of two crystallographic modifications of quercetin (3,3', 4', 5,7-pentahydroxyflavone). The existence of dihydrate and unhydrated forms of quercetin in the solid state is confirmed by several experimental techniques e.g. X-ray diffraction of powders, DSC, TGA, and NMR. Our studies allow an understanding of the complexity of quercetin samples obtained from different sources. A PASS-2D experiment is employed to establish principal values of C-13 chemical shift tensors for both modifications. Solid state NMR spectroscopy and DFT GIAO calculations provide unique information about NMR shielding and electron density distribution for different conformers. It has been concluded that changes of conformation and hydrogen bonding pattern have great influence on bond order parameters of quercetin. Theoretical calculations and experimental data do not exclude the existence of the syn conformer of quercetin, which so far was not considered in the condensed phase.