Theoretical investigations on the enhancing effect of the cation-π interaction on the halogen bond in the M∙∙∙HCCX∙∙∙NH₃ (M = Li⁺, Na⁺, Cu⁺, Ag⁺, Au⁺; X = Cl, Br) complexes.

The M∙∙∙HCCX∙∙∙NH₃ (M = Li⁺, Na⁺, Cu⁺, Ag⁺, Au⁺; X = Cl, Br) complexes were designed to study the influence of cation-π interaction on the X∙∙∙N halogen bonds under M05-2X/aug-cc-pVDZ(PP) level. In comparison with the HCCX∙∙∙NH₃ complexes, the bond distances of the halogen bonds have decreased, and the interaction energies become more negative. The results show that the X∙∙∙N halogen bonds have been strengthened by the cation-π interactions. For different cations, the enhancing effect is more intensive in the order of Au⁺ > Cu⁺ > Ag⁺ > Li⁺ > Na⁺, which indicates that transition metal cations can enhance the halogen bond in a stronger manner. Molecular electrostatic potential and second-order perturbation stabilization energy were calculated to deepen the discussion. In addition, atoms in molecules analysis was performed and the electron density shift was studied.