The applicability of AM1 and PM3 semi-empirical methods for the study of N-H bond dissociation enthalpies and ionisation potentials of amine type antioxidants

A study of 40 para-substituted anilines is presented. These serve as model structures of amine type antioxidants. Molecules and their radical structures were studied using the AM1 and PM3 quantum chemical methods in order to calculate the N-H bond dissociation enthalpies (BDEs) and ionisation potentials (IPs) which are among the most important characteristics of antioxidants. Calculated BDEs were compared with available experimental values and the results of DFT calculations to ascertain the suitability of AM1 and PM3 methods for amine BDEs calculation. The results show that both methods reproduce experimental BDEs and DFT data satisfactorily. Comparison with experimental data shows that AM1 and PM3 methods overestimate the IP values. The results also indicate that dependences of N-H bond BDEs and IPs on Hammett constants of the substituents are linear.