Experimental Quantification of Anion−π Interactions in Solution Using Neutral Host–Guest Model Systems

Chemical intuition suggests that anions and π-aromatic systems would repel each other. Typically, we think of cations as being attracted to electron-rich π-systems of aromatic rings, and the cation-π interaction, a well-established noncovalent interaction, plays an important role in nature. Therefore the anion-π interaction can be considered the opposite of the cation-π interaction. Computational studies of simple models of anion-π interactions have provided estimates of the factors that govern the binding geometry and the binding energy, leading to a general consensus about the nature of these interactions. In order to attract an anion, the charge distribution of the aromatic system has to be reversed, usually through the decoration of the aromatic systems with strongly electron-withdrawing groups. Researchers have little doubt about the existence of attractive anion-π interactions in the gas phase and in the solid state. The bonding energies assigned to anion-π interactions from quantum chemical calculations and gas phase experiments are significant and compare well with the values obtained for cation-π interactions. In solution, however, there are few examples of attractive anion-π interactions.