1H, 13C MAS NMR and DFT GIAO study of quercetin and its complex with Al(III) in solid state

Quercetin (Q) as a pentahydroxy flavonoid, has three possible chelating sites competing in complexation processes. (1)H and (13)C MAS NMR spectra were recorded for solid quercetin and its Al(III) complex (AlQ). (1)H MAS spectrum of quercetin shows a broad resonance at ca. 12 ppm that confirms the existence of intramolecular C5-OH … O=C4 hydrogen bond. Such a signal is absent in the spectrum of AlQ, which is in accordance with other spectroscopic data and the suggested model for the solid-state structure of the complex. DFT GIAO calculations were used to verify the experimental (13)C CPMAS NMR data and to suggest the best model structure for the complex AlQ. The calculated shielding constants for different conformers of isolated quercetin molecules, quercetin trimer as taken from the X-ray data, and different model structures for possible Al(III) complexes were compared with the (13)C CPMAS NMR experimental values. The results demonstrate the importance of intermolecular interactions when dealing with structures in solid state and the successful application of the combined DFT GIAO and (13)C CPMAS NMR approach. All data confirm that the chelating site of Q in the solid complex AlQ involves the deprotonated C5-OH and the C4=O groups at ring C, in contrast to the available studies performed in solution.