A theoretical study on the importance of steric effects, electronic properties, interaction and solvation energies in the ‘host–guest’ chemistry of protonated azacryptands and halide anions

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.