A series of meso-tetraaryl calix[4]pyrrole receptors have been used as a model system to quantify chloride- interactions in solution (see picture; green balls are chloride ions). The free energy values are generally indicative of a repulsive interaction; their magnitude depends on the substituent on the aromatic ring.
![Quantitative evaluation of anion-pi interactions in solution](https://www.iciq.org/wp-content/uploads/2014/03/Quantitative-evaluation_eng_noticies_articles_2008_44.gif)