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,pi-donor substitue nts (OH, NH2, F, CH3, C6H5, Cl), the aromatic domain is first opened close to the substituted carbon and then in the vicinity of the meta carbon; wher eas for attractor substituents (CN, CHO, NO2, CF3 and CCl3), it is first op ened in the ortho and para positions. The orienting effects of the electrop hilic substitutions an 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). T his suggests that the local Pauli repulsion plays a noticeable role in the orienting effects which are complementary to the charge transfer effect inv olved in standard quantum chemical pictures.