A NEW SEMIEMPIRICAL METHOD OF ESTIMATION OF ACTIVITY AND BOND-DISSOCIATION ENERGIES OF ANTIOXIDANTS

Experimental data on reactions of peroxyl radicals with antioxidants (phenols, aromatic amines, organic hydroxylamines and thiophenols) have been analysed on the basis of a semiempirical parabolic model of the transition state. Parameters were estimated that describe the activation energy of any reaction of a fixed class as a function of the reaction enthalpy. The equations of the parabolic model were used to calculate the bond dissociation energies of antioxidants using experimental data on rate constant ratios. The empirical parameters for reactivity of antioxidants towards peroxyl radicals were analysed and the following physical factors were found to be the most important in their influence on the activation energy of an antioxidant reaction with a peroxyl radical: enthalpy of reaction, force constants of the forming and dissociating bonds, triplet repulsion in the transition state, electron affinity of the atoms of the reaction centre, steric hindrance, pi and p-electrons near the reaction centre and the radii of the atoms forming the reaction centre.