Homolytic N–H bond cleavage in anilines: Energetics and substituent effect

In this paper, the study of aniline and 42 para- and meta-substituted anilines is presented. Substituted anilines represent model compounds of primary antioxidants from the secondary aromatic amines group. They are also widely used in the various organic syntheses. Anilines and their radicals were studied using DFT/B3LYP, DFT/PBE0 and semi-empirical SCC-DFTB methods in order to assess the substituent effect on the N–H bond dissociation enthalpies (BDEs). Calculated BDEs for para-substituted anilines are in good agreement with available experimental results. Unfortunately, published data for meta-substituted anilines are still scarce. The two DFT methods describe the effect of substituents on BDEs satisfactorily, though ΔBDEs may be overestimated in the case of electron-donating substituents. In the case of para-substituents, dependence of ΔBDE values on Hammett constants is satisfactorily linear. For substituents in meta position, the linearity is worse. SCC-DFTB may provide less reliable substituent effect description. We found very good linearity of ΔBDE vs C–N bond length or its shortening after hydrogen atom abstraction for both, para- and meta-substituted anilines. For N–H BDE, these geometry parameters represent suitable substituent effect descriptors.