It is well known that the selectivity of a receptor for an anion depends on the compatibility of the cavity size of the receptor and the size of the anion. In this work the macrobicyclic [H6L(X)]5+ (X−=F, Cl, Br); are studied theoretically and compared with [H6L′(F)]5+ having a smaller cavity size. It was shown that the ideal match between the sizes of the protonated azacryptand and the fluoride ion exists in the [H6L′(F)]5+ complex but the [H6L]6+ is a better receptor than [H6L′]6+ in solution. Thus the results clearly indicate that in some special cases a better receptor is not one whose cavity size has better compatibility with the anion size.
