Molecular mechanics calculations for ethers. The conformations of some crown ethers and the structure of the complex of 18-crown-6 with benzylammonium thiocyanate

Abstract

The White–Bovill alkane–alkene force field (WBFF) has been slightly modified and extended for the calculation of geometrical and thermodynamic properties of saturated compounds containing the isolated ether or the 1,4-dioxa grouping. The new force field reproduces the heats of formation of 14 ethers with a mean deviation of 0.86 kcal mol^–1 between observed and calculated enthalpies. Bond lengths, bond angles, and torsion angles are satisfactorily reproduced with mean deviations of 0.010 Å, 1.0°, and 4.3° respectively over a set of 103 observations. The force field has been used to investigate the detailed relationship between the strain energies and the corresponding equilibrium geometries of various conformations of 9-crown-3, 12-crown-4, and 18-crown-6. The determination of the structure of the complex of 18-crown-6 with benzylammonium thiocyanate, by X-ray crystallography, is described and molecular mechanics calculations are used to comment upon the conformations adopted by 18-crown-6 in a range of complexes.