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 [H3L⋯X]2+ (X = F-, Cl-, Br-, and I-). The result of calculations on the structural parameters of [H3L⋯Cl]2+ shows that the calculated data with BHandHLYP functional have the best agreement with the experimental data and all five 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: [H3L⋯F]2+ > [H3L⋯Cl]2+ > [H3L⋯Br]2+ > [H3L⋯I]2+. Formation of the complexes in solution also shows the above trend where the [H3L⋯Br]2+ and [H3L⋯I]2+ complexes have positive values and probably cannot be formed.
