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 σ_DFT (ppm) and chemical shifts (δ_CPMAS, 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.