Factors Influencing the Antioxidant Activities of Phenols by an Ab Initio Study

Abstract

Abstract An ab initio molecular orbital theory has been applied to the elucidation of the hydrogen abstraction mechanism of phenolic antioxidants in the chain process of autoxidations. The optimum structures of o-, m-, and p-substituted phenols, of peroxides, and of those compounds in the transition states were obtained with a Hartree-Fock/STO-3G basis set. From the values of the enthalpy (ΔH), activation (Ea), and OH bond dissociation (D(O–H)) energies obtained, it was found that these three parameters indicate a good relationship with each other; particularly, the relation between the ΔH and Ea values follows the Evans-Polanyi rule. The electron-releasing substituents in the o- and p-positions in phenols decrease the activation parameters for hydrogen abstraction, while those in the m-position increase. The electron densities on the ipso-carbon, oxygen, and hydrogen of the OH substituent and their bond populations are obtained. Variations of the electron densities from the reactants to the transition states lead to a clarification of the reaction mechanism as an antioxidant. Namely, the gain or loss of electrons in the reaction states may be correlated to the experimental data, 13C chemical shifts of the ipso-carbon of the OH substituent, and the values of the induction period as an antioxidant activity.