MOLECULAR-ORBITAL APPROACH TO ANTIOXIDANT MECHANISMS OF PHENOLS BY AN ABINITIO STUDY

An ab initio molecular orbital theory has been applied to a study of the hydrogen abstractions of phenolic antioxidants in the chain process of autoxidation. The optimum structures of 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 enthalpy (DELTA-H) and activation energy (E(a)) obtained, it was found that the rates of the reaction of peroxyl radical with phenolic antioxidant were faster than those with organic substrate in the propagation, and that the effect of the aromatic ring of the antioxidants not only stabilized a product state but also decreased an energy level in the transition state. The para-substituent effect that an electron-releasing substituent increased the antioxidant activity, whereas an electron-withdrawing one decreased it, was recognized. The relationship between DELTA-H and E(a) values followed the Evans-Polanyi rule. The transition states in the hydrogen abstractions with lower E(a) values prefer reactant-like structures, whereas those with higher E(a) values prefer product-like ones.