A theoretical study on the selectivity of hexaprotonated form of preorganized azacyclophane C3-trenphen (H6 L 6+) for halide anions in the gas phase and solution is reported. The calculated formation and interaction energies show the following trend: [F⊂H6 L]5+ > [Cl⊂H6 L]5+ > [Br⊂H6 L]5+ > [I⊂H6 L]5+. Our calculations in both the gas phase and solution, in agreement with the experimental observations, show that the H6 L 6+ host has the largest selectivity for fluoride anion. Indeed, the stronger host–guest interaction in [F⊂H6 L]5+ complex effectively compensates the unfavourable strain energy of the host and desolvation energy of fluoride anion. Furthermore, herein, the solvent effect does not change the observed trend for gas-phase formation energies. In continuation, in order to find the origin of longer X−···aromatic ring (X−···cntr) distance than X−···bridgehead nitrogen (X−···Napical) in the present complexes, the X−···C6H6 and X−···NH3 systems were simulated, respectively. The calculated interaction energies for two latter systems in different X−···C6H6 and X−···NH3 distances clearly showed that the repulsion force between X− and nitrogen atom is larger than that between X− and aromatic ring. Thus, only when there are the strong N–H···X− interactions in top of the cage, the Napical···X− distance is considerably smaller than cntr···X− one.
