On the directionality of anion-pi interactions

The directionality of two important noncovalent interactions involving aromatic rings (namely anion-pi and cation-pi) is investigated. It has been recently published that the anion-p interactions observed in X-ray structures where the anion is located exactly over the center of the ring are scarce compared to cation-pi interactions. To explain this behavior, we have analyzed how the interaction energy (RI-MP2/aug-cc-pVDZ level of theory) is affected by moving the anion from the center of the ring to several directions in anion-p complexes of chloride with either hexafluorobenzene or trifluoro-s-triazine. We have compared the results with the directionality of the cation-pi interaction in the sodium-benzene complex. The results are useful to explain the experimental differences between both ion-pi interactions. We have also computed the van der Waals radii of several halide anions and we have compared them to the neutral halogen atoms.