How amino and nitro substituents direct electrophilic aromatic substitution in benzene: an explanation with Kohn–Sham molecular orbital theory and Voronoi deformation density analysis

Abstract

The substituent effect of the amino and nitro groups on the electronic system of benzene has been investigated quantum chemically using quantitative Kohn–Sham molecular orbital theory and a corresponding energy decomposition analysis (EDA). The directionality of electrophilic substitution in aniline can accurately be explained with the amount of contribution of the 2p_z orbitals on the unsubstituted carbon atoms to the highest occupied π orbital. For nitrobenzene, the molecular π orbitals cannot explain the regioselectivity of electrophilic substitution as there are two almost degenerate π orbitals with nearly the same 2p_z contributions on the unsubstituted carbon atoms. The Voronoi deformation density analysis has been applied to aniline and nitrobenzene to obtain an insight into the charge rearrangements due to the substituent. This analysis method identified the orbitals involved in the C–N bond formation of the π system as the cause for the π charge accumulation at the ortho and para positions in the case of the NH₂ group and the largest charge depletion at these same positions for the NO₂ substituent. Furthermore, we showed that it is the repulsive interaction between the π_HOMO of the phenyl radical and the π_HOMO of the NH₂ radical that is responsible for pushing up the π_HOMO of aniline and therefore activating this π orbital of the phenyl ring towards electrophilic substitution.