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.

The three-electron bond in radical anions of the H n XYH m - type, with X, Y = Cl, S, P, Si, F, O, N, C and n, m = 0−2, has been investigated from the topological analysis of the electron localization function (ELF) at the BH&HLYP level. It is shown that the topological modifications arising within the bonding region upon vertical electron attachment are of three different types, according to the vertical electron affinity (vEA) of the neutral compound:  for vEA smaller than −16 kcal mol-1 the bonding population remains unchanged, as in H4P2, for negative vEA greater than −16 kcal mol-1 the bonding population decreases, as in H2S2, and for positive vEA the bonding population disappears, as in Cl2. However, after relaxation of the geometry, the formation of the three-electron bond is accompanied in all cases by the disappearance of the X−Y bonding basin (which is the signature of the covalent bond in the neutral parent molecule). From a quantitative point of view, the topological approach also allows us to characterize the transfer of charge and spin densities that arises upon these processes toward the lone pairs basins of the X and Y atoms. Finally, to quantify the electron fluctuation between the two moieties, an index of delocalization has been defined from the analysis of the variance of the lone pairs population. This index increases approximately linearly with the dissociation energy D e of the radical anions, provided that they are separated into a group of weakly bonded ones (D e < 18 kcal mol-1) and a group of strongly 3e-bonded ones (D e > 18 kcal mol-1).