Ammonium cation complexation by 18-crown-6 under realistic conditions: Simulations using proper potentials

Macrocyclic (crown) polyethers exhibit a high degree of specificity for cation (e.g. metal, ammonium) binding. The strength and specificity of this binding depend not only upon the intrinsic interaction between the crown and its substrate but also upon the moderating influences of the solvent. In this study, we investigate the complexation between an ammonium cation and 18-crown-6 in polar and apolar solvents (water and carbon tetrachloride, respectively). We use statistical perturbation theory in conjunction with NPT Monte Carlo simulations at 25°C and atmospheric pressure. The interaction between ammonium and 18-crown-6 is characterized by an intermolecular potential derived from electronic density functional theory calculations within the Kohn-Sham, local density approximation formalism. We discuss our results in terms of the potential of mean force between the crown and substrate as a function of center-of-mass separation R_c, equilibrium binding constants which are compared to experiment, conformational changes of the crown upon complexation, and, lastly, solvent structure around the complex.