Conformational analysis of crown ether analogs in solution:: cis-cyclohexano-8-crown-3 as studied via low-temperature 13C and 1H NMR spectroscopy and molecular mechanics calculations

Two conformations of the cis-cyclohexano-8-crown-3 molecule were detected at 185 K. From relative C-13 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 C-13 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 greater than or equal to 1% were identified. Copyright (C) 1999 John Wiley & Sons, Ltd.