Density functional theory (DFT) using PBE, PBE1PBE, B3P86, B97-1, and BHandHLYP functionals as well as MP2 calculations employing the TZVP basis set were used to study the formation of four host–guest complexes with formula ½H3L X2þ (X ¼ F, Cl, Br, and I). The result of calculations on the structural parameters of ½H3L Cl2þ shows that the calculated data with BHandHLYP functional have the best agreement with the experimental data and all ¯ve DFT methods give the better result than MP2. The largest amounts of interaction energies between the halide anions and protonated macrocycle were also calculated by PBE functional. The results show the following trend for the formation of complexes in the gas phase: ½H3L F2þ > ½H3L Cl2þ > ½H3L Br2þ > ½H3L I2þ. Formation of the complexes in solution also shows the above trend where the ½H3L Br2þ and ½H3L I2þ complexes have positive G0 aq values and probably cannot be formed.
