A molecular mechanics study of 18-crown-6 and its complexes with neutral organic molecules

Abstract

The flexibility of 18-crown-6 has been studied by the molecular mechanics method (MM2). Calculations on 190 different potentially favourable (‘ideal’) conformations reveal that the potential energy surface of the polyether has many minima with only small energy differences. The order of the conformational energies depends mainly on the contribution of the electrostatic energy. If no electrostatic interactions are present, the most favourable conformations are the ones occuring in complexes of 18-crown-6, as observed by X-ray crystallography. However, for larger contributions of the electrostatic energy the conformation adopted by uncomplexed 18-crown-6 in the crystalline state becomes the dominating conformation. Also results are presented for calculations on six experimentally observed conformations of 18-crown-6. It is shown that the calculated geometries correspond closely with the experimental ones, notwithstanding the omission of the guest molecules in the calculations, indicating that the minima in the potential energy surface of the polyether have steep slopes. Finally, results are given for calculations on the hydrogen-bonded complexes of 18-crown-6 with urea and formamide, both using the MM2 and the MM2HB force field. The latter is a modified version of MM2, incorporating an empirical N–H ⋯ O hydrogen-bond potential. MM2HB proves to be far superior over MM2 for calculations on this type of complexes, as well as for the conformation adopted by the macrocycles as regarding the hydrogen-bond geometries.