A study of the energetics of antioxidant action of p-phenylenediamines

A study of seven N,N'-substituted p-phenylenediamine antioxidants is presented. Antioxidants and their radical structures were studied using the PM3 quantum-chemical method in order to calculate the enthalpies of formation of the molecules and of the radicals formed by hydrogen atom abstraction from three possible reaction centres: two amine nitrogen atoms and the aliphatic carbon atom adjacent to the nitrogen atom. The results obtained are in agreement with experimental study of antioxidants effectiveness in polyisoprene rubber and indicate that the effectiveness depends not only on the reaction enthalpies of hydrogen atom abstraction from the reaction centres, but also on the partial charges of the corresponding atoms. (c) 2005 Elsevier Ltd. All rights reserved.