Topological Analysis of the Electron Localization Function (ELF) Applied to the Electrophilic Aromatic Substitution

The topological analysis of the electron localization function ELF provides a partition of the molecular space into basins of attractors which have a clear chemical signification. The hierarchy of these basins is given by the bifurcation of the localization domains. In the case of π-donor substituents (OH, NH 2, F, CH 3, C 6 H 5, Cl), the aromatic domain is first opened close to the substituted carbon and then in the vicinity of the meta carbon; whereas for attractor substituents (CN, CHO, NO 2, CF 3 and CCl 3 ), it is first opened in the ortho and para positions. The orienting effects of the electrophilic substitutions are correlated with these bifurcations. The experimental favored positions always correspond to the locally electronegative carbons (i.e., those which keep their shell structure at the higher ELF values). This suggests that the local Pauli repulsion plays a noticeable role in the orienting effects which are complementary to the charge transfer effect involved in standard quantum chemical pictures.