Conformational analysis of crown ether analogs in solution:cis-cyclohexano-8-crown-3 as studied via low-temperature13C and1H NMR spectroscopy and molecular mechanics calculations

Abstract

Two conformations of the cis-cyclohexano-8-crown-3 molecule were detected at 185 K. From relative 13C NMR peak area measurements, the conformation with the O— CH2— O unit equatorial was found to be favoured by 4.4 ± 0.4 kJ mol−1. In the spectrum of the minor conformer at low temperature, a 9.6 ppm 13C chemical shift difference is present between the two methine carbons. Transannular 1,4 H–H interactions are deemed to be partly responsible for this effect. Merck Molecular Force Field (MMFF) calculations indicate a preference of ca 1.8 kJ mol−1 for the most populated conformation. A boat–chair conformation is present in the 1,3,6-trioxocane ring of the major conformer whereas a twist boat–chair is preferred in the minor form. Two other conformations with populations of ≥ 1% were identified. Copyright © 1999 John Wiley & Sons, Ltd.