13C CP MAS NMR and GIAO-CHF/DFT calculations of flavonoids: Morin, kaempferol, tricin, genistein, formononetin and 3,7-dihydroxyflavone

13C CP MAS NMR spectra of the flavonoids: morin, kaempferol, 3,7-dihydroxyflavone, tricin and isoflavones: genistein and formononetin were recorded to characterize solid-state conformations. Intramolecular hydrogen bonds forming five-, six- and seven-membered rings are present in the two morin molecules in the crystals – their 13C resonances have been assigned with the aid of the calculated shielding constants. Linear relationships between the calculated shielding constants σDFT (ppm) and chemical shifts (δCPMAS, ppm) were obtained for all studied compounds. Higher correlation coefficients suggest that the conformation with “clockwise” orientation of both OH groups is more probable in the solid 3,7-dihydroxyflavone, whereas in the solid formononetin the OH and OCH3 substituents are directed “anticlockwise”. The barrier to the rotation of phenyl ring B decreases in the order: morin (2′-OH, 3-OH) > kaempferol (3-OH) > tricin.