CONFORMATIONAL-ANALYSIS OF CROWN-ETHERS .2. 15-CROWN-5

An extensive conformational search of 15-crown-5 was performed with th e MM3 force field yielding 3623 conformers ranging in steric energy fr om 21.40 to 56.24 kcal mol(-1). Free energies and conformer population are reported for the gas phase. A continuum dielectric model is used to estimate the conformer populations in a variety of solvents. The th ree conformers observed in crystal structures containing 15-crown-5 mo lecules that do not form inner sphere complexes with metal ions are pr edicted to be among the most populated conformations in condensed phas es. Comparison of the structural features of the crystallographically observed conformers with those of the energy minimized MM3 structures demonstrates that the MM3 force field well reproduces that experimenta lly observed structural features. Predicted conformer populations are consistent with C-13 NMR and vibrational spectra. (C) 1997 Elsevier Sc ience B.V.