Bond dissociation energies and enthalpies of formation of flavonoids: A G4 and M06-2X investigation

By means of G4 theory and M06-2X calculations we investigated the thermochemistry and C–OH bond ruptures of quercetin, fistein, lutelin, caffeic acid, pinocembrin and p-coumaric acid. Notwithstanding the fact that we observed a good agreement for the prediction of the weakest O–H bond, the mean absolute deviation (MAD) between the bond dissociation energies (BDE) computed at the G4 and M06-2X/6-311+G(3df,2p) levels was 5.0 kcal/mol. With regard to the ΔHof,0 computed at the M06-2X/6-311+G(3df,2p) level, the MAD was 10.0 kcal/mol. Therefore, thanks to error cancellation the error found for the O–H BDE was smaller. Intramolecular hydrogen bond is one of the key factors stabilizing the oxygen radicals formed when the CO–H bonds are broken. Finally, we propose the following enthalpies of formation at 0 K: −222.0, −174.5, −180.2, −134.4, −109.7 and −91.9 kcal/mol for quercetin, fistein, lutelin, caffeic acid, pinocembrin and p-coumaric acid, respectively.