On the directionality of anion–π interactions

Abstract

The directionality of two important noncovalent interactions involving aromatic rings (namely anion–π and cation–π) is investigated. It has been recently published that the anion–π interactions observed in X-ray structures where the anion is located exactly over the center of the ring are scarce compared to cation–π 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–π complexes of chloride with either hexafluorobenzene or trifluoro-s-triazine. We have compared the results with the directionality of the cation–π interaction in the sodium–benzene complex. The results are useful to explain the experimental differences between both ion–π interactions. We have also computed the van der Waals radii of several halide anions and we have compared them to the neutral halogen atoms.