In this article, theoretical studies on the selective complexation of the halide ions (F-, Cl- and Br-) and ion pairs (Na+F-, Na+Cl- and Na+Br-) with the cyclic nano-hexapeptide (CP) composed of L-proline have been performed in the gas phase. To calculate the dispersion interaction energies of the CP and ions, DFT-D3 calculations at the M05-2X-D3/6-31G(d) level were employed. Based on the results, Fand Na+F- make the most stable complex with the CP. The dispersion interactions between the ions and the CP are small while the electrostatic interactions are a driving force for the complex formation. Finally, natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses indicate that F- makes the most stable complex with CP due to more charge transfer and stronger bond formation in comparison to other ions.

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