Substituent effects on O--H bond dissociation enthalpies and ionization potentials of catechols: a DFT study and its implications in the rational design of phenolic antioxidants and elucidation of structure-activity relationships for flavonoid antioxidants.

Density functional theory (DFT) on B3LYP/6-31G(d,p) level was employed to investigate the substituent effects on O--H bond dissociation enthalpies (BDEs) and ionization potentials (IPs) of catechols. It was revealed that the ortho hydroxyl of catechol was effective for the reduction of the O--H BDE; however, the group had little influence on the IP. The para substituent effects upon O--H BDEs and IPs for catechols were roughly the same as those for monophenols, and this gave the catechol moiety more potential than monophenol to be used as a lead compound in rational design of phenolic antioxidants. In addition, the 1,4-pyrone effects on O--H BDEs of catecholic rings A or B of flavonoids were also investigated. Although 1,4-pyrone extended the conjugation system of flavonoids, it was not beneficial to reduce the O--H BDE as a result of its electron-withdrawing property. Thus, 1,4-pyrone was unlikely to be favorable to enhance the H-abstraction activity of flavonoids.