Composite ab initio method (ONIOM-G3B3) and density functional theory (B3LYP) were evaluated against 179 experimental C—H, O—H and N—H bond dissociation enthalpies of diverse organic molecules to verify their performance. It was found that the ONIOM-G3B3 predictions were in good agreement with the experimental values for these three types of bond dissociation enthalpies, and the accuracy was about 5.9 kJ/mol. Finally, armed with this powerful tool two interesting classes of natural antioxidants including vitamin E and catechins in tea were studied. Discussions were made about the antioxidant activity, radical scavenge mechanism and structure-activity relationships.

Key words: ONIOM-G3B3, density functional theory, bond dissociation enthalpy, vitamin E, catechin