In this work the selectivity of hexaprotonated aliphatic azacryptand 1,4,8,11,14,18,23,27-octaazabicyclo[9.9.9]nonacosane (H62326+) for halide anions in both the gas phase and solution are studied theoretically. The calculated interaction and formation energies, in agreement with the experimental observations, show the following trend for the formation of complexes in the gas phase: [F⊂H6232]5+ > [Cl⊂H6232]5+ > [Br⊂H6232]5+ > [I⊂H6232]5+. The theoretical studies on the formation of these complexes in solution also show the above trend. There is a good correlation between the experimental and calculated stability constants for studied complexes. In addition, the origin of the increased selectivity of H62326+ for chloride and its decreased selectivity for fluoride, in comparison with a similar but smaller protonated azacryptand, H62226+, is reported. Thus the results show that the selectivity is a balance of numerous aspects affecting the binding and the influence of such factors as steric, electronic or solvent effects cannot be underestimated.
