Are anion/pi interactions actually a case of simple charge-dipole interactions?

Substituent effects in Cl(-)...C(6)H(6-n)X(n) complexes, models for anion/pi interactions, have been examined using density functional theory and robust ab initio methods paired with large basis sets. Predicted interaction energies for 83 model Cl(-)...C(6)H(6-n)X(n) complexes span almost 40 kcal mol(-1) and show an excellent correlation (r = 0.99) with computed electrostatic potentials. In contrast to prevailing models of anion/pi interactions, which rely on substituent-induced changes in the aryl pi-system, it is shown that substituent effects in these systems are due mostly to direct interactions between the anion and the substituents. Specifically, interaction energies for Cl(-)...C(6)H(6-n)X(n) complexes are recovered using a model system in which the substituents are isolated from the aromatic ring and pi-resonance effects are impossible. Additionally, accurate potential energy curves for Cl(-) interacting with prototypical anion-binding arenes can be qualitatively reproduced by adding a classical charge-dipole interaction to the Cl(-)...C(6)H(6) interaction potential. In substituted benzenes, binding of anions arises primarily from interactions of the anion with the local dipoles induced by the substituents, not changes in the interaction with the aromatic ring itself. When designing anion-binding motifs, phenyl rings should be viewed as a scaffold upon which appropriate substituents can be placed, because there are no attractive interactions between anions and the aryl pi-system of substituted benzenes.