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

✍ Scribed by G. W. Buchanan; A. B. Driega; R. C. Laister; K. Bourque

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 100 KB
• Volume: 37
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199906)37:6<401::aid-mrc469>3.0.co;2-z

✦ Synopsis

Two conformations of the cis-cyclohexano-8-crown-3 molecule were detected at 185 K. From relative 13 C NMR peak area measurements, the conformation with the O-CH 2 -O unit equatorial was found to be favoured by 4.4 š 0.4 kJ mol 1 . In the spectrum of the minor conformer at low temperature, a 9.6 ppm 13 C 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 ½1% were identified.