The computational results for curcumin at the B3LYP/6-31G(d,p) level show that the enol form of curcumin is more stable than the diketo form because of an intramolecular hydrogen bond, which extends the conjugation effect in the enol chain, formed in the enol structure. Cis-diketone form can not be obtained, presumably due to the strong repulsion between the carbonyl dipoles aligned in parallel. According to the phenolic O—H bond dissociation en- thalpy, curcumin in its most stable form can be suggested to be a relatively good antioxidant. In order to avoid overcoming H-bond interaction and to improve the antioxidant activity of curcumin, a catechol moiety was incor- porated into curcumin for designing a novel antioxidant. It is found that the designed molecule is much more effi- cient to scavenge radical than curcumin, comparable to vitamin E. Moreover, the ionization potential of the de- signed molecule is similar to that of curcumin, indicating that the designed molecule can not display the prooxidant effect.
