AMMONIUM CATION COMPLEXATION BY 18-CROWN-6 UNDER REALISTIC CONDITIONS- SIMULATIONS USING PROPER POTENTIALS

Macrocyclic (crown) polyethers exhibit a high degree of specificity fo r 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 o f 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 perturbati on theory in conjunction with NPT Monte Carlo simulations at 25-degree s-C and atmospheric pressure. The interaction between ammonium and 18- crown-6 is characterized by an intermolecular potential derived from e lectronic density functional theory calculations within the Kohn-Sham, local density approximation formalism. We discuss our results in term s of the potential of mean force between the crown and substrate as a function of center-of-mass separation R(c), equilibrium binding consta nts which are compared to experiment, conformational changes of the cr own upon complexation, and, lastly, solvent structure around the compl ex.