Study of N-H, O-H, and S-H bond dissociation enthalpies and ionization potentials of substituted anilines, phenols, and thiophenols

In this article, the study of 44 compounds representing various p-and m-substituted anilines, phenols and thiophenols is presented. These serve as model structures of amine, phenol and thiophenol types of antioxidants. Molecules and their radical structures were studied using DFT and semi-empirical PM3 and AM1 quantum chemical methods in order to calculate the N-H, O-H and S-H bond dissociation enthalpies (BDEs) and ionization potentials (IPs) which belong to the most important characteristics of antioxidants. Selected compounds BDEs were obtained using ab initio MP2 method, too. Calculated BDEs were compared with available experimental values to ascertain the suitability of used approaches for BDEs and IPs calculation. Applied methods reproduce experimental BDEs of anilines and phenols satisfactorily. Semi-empirical methods do not provide correct BDE values for thiophenols. However, they reliably reproduce the substituent effect on BDE. MP2 calculations overestimate individual BDEs, however for anilines and thiophenols they provide the range of substituent induced BDE changes in the best agreement with experiments. Comparison with experimental data shows that AM1 and PM3 methods slightly overestimate and DFT significantly underestimates the IP values. HF method provides the most reliable IPs. The results indicate that dependences of N-H, O-H, and S-H BDEs and IPs of the studied compounds on Hammett constants of the substituents are linear.