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(3 df,2p) levels was 5.0 kcal/mol. With regard to the Delta H degrees(f,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. (C) 2016 Elsevier B.V. All rights reserved.