Ion vs. ion pair receptor: NMR and DFT study of the interaction of Thallium and Cesium ions and ion pairs with meso-octamethylcalix[4]pyrrole

► Thallium is strongly bound by calixpyrrole as a bare cation. ► The bond of cesium as a bare cation is weak. ► In cooperation with bromide ion, cesium is strongly bound as an ion pair.