Hovering above the π‐electron cloud, an anion positioned over hexafluorobenzene along the C6 axis as shown here interacts with the permanent quadrupole of the arene. Crystallographic and computational evidence demonstrate that anion–π interactions exist and are energetically favorable.

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Several π-complexes of cations and anions with aromatic rings have been optimized at the MP2/6-31++G** level of theory. Different aspects of the cation-π interaction have been compared to those of anion-π, including changes in the aromaticity of the ring upon complexation, charge-transfer effects using the Merz-Kollman and "atoms-in-molecules" (AIM) charges, and the contribution of correlation and dispersion energies by comparing the complexation energies computed at the HF, B3LYP, and MP2 levels of theory. In this paper, we study three aromatic systems that allow direct comparisons, free from other influences, of the cation-π versus anion-π interactions, which are the 1,3,5-trifluorobenzene (TFB), s-triazine (TAZ), and 2,5-dichloropyrazine (DCP). These compounds are able to π-interact favorably with either anions or cations because of their very small quadrupole moments.

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Several structures of pi complexes of isocyanuric acid and of several thio derivatives with anions have been computed by using high level ab initio calculations. The nature of the complexes has been studied by means of the method of molecular interaction potential with polarization (MIPp) and Bader's theory of atoms-in-molecules. These molecules form favorable complexes with anions and can be used as binding units for building receptors for the molecular recognition of anions. In several cases, the anion-pi interaction has been demonstrated experimentally by means of X-ray crystallography.