Mechanisms of antioxidant activity: The DFT study of hydrogen abstraction from phenol and toluene by the hydroperoxyl radical

Results from theoretical study of hydrogen abstraction from phenol and toluene by the hydroperoxyl radical are reported. The free radical reactions considered have rates differing by six orders of magnitudes yet have nearly equal reaction heats. The corresponding potential energy profiles and reaction mechanisms are studied using the DFT B3LYP/6-311+G(2d,2p) method. The calculations correctly predict the large difference in activation energies, ΔEa, for the reactions considered, both in gas phase and nonpolar solvent. However, quantitatively the ΔEa is overestimated by 11-12 kJ/mol. The difference of the computed bond-dissociation energies can be compared to ΔEa. The electron structure analysis of the reaction intermediates shows that the conventional H-atom transfer is described by differing qualitative mechanisms for the considered processes. The phenol-peroxyl reaction has the features of proton-coupled electron transfer, while the toluene-peroxyl reaction is closer to neutral H-atom transfer. The found difference of the reaction mechanisms gives a new perspective for rationalization of the highly differing reactivity of phenolic antioxidants and hydrocarbons towards peroxyl radicals.