Nature of the interactions of 18-crown-6 with ammonium cations: A computational study

The complexation of alkylammonium cations with macrocyclic polyethers may find important applications in the separation of biologically important protonated amines. In this paper, we present first-principles calculations that describe the nature of the complexation for the simplest possible ammonium cation-macrocyclic polyether system; viz., ammonium ion complexation with 18-crown-6. Our quantum mechanical computations are based on Kohn-Sham density functional theory. Many degrees of freedom have been examined in order to generate a detailed potential energy hypersurface that can be used for future molecular simulations. We find the gas-phase enthalpy of complexation at 670 K to be -67.98 kcal/mol. In the most energetically stable conformation, the ammonium ion sits in the nesting orientation with the nitrogen atom located at a distance of 0.415 Å above the mean plane of the crown ether. The ammonium ion is found to have considerable translational and rotational mobility within the crown ether but is very constrained with regard to any tilting about the axis perpendicular to the crown mean plane. The results of our Kohn-Sham calculations are compared with experiment.