Conformational analysis of crown ether analogs in solution: Cis-cyclohexyl-10-crown-3 as studied via low-temperature 1H and 13C NMR spectroscopy

Two conformations of cis-cyclohexyl-10-crown-3 were detected in solution below 250 K using 1H NMR at 400 MHz and 13C NMR at 100 MHz. Chemical shift assignments were facilitated by spectra of two dideuterio derivatives. From relative peak area measurements, the conformation with the O - CH2 - CH2 - CH2 - O unit equatorial was found to be favored by 2.8 ± 0.3 kJ mol-1 at 210 K. Results of a molecular mechanics calculation were compared with the experimental findings. A remarkably large 13C chemical shift difference between conformers (ca. 10 ppm) was found for one of the methylene carbons bearing oxygen. Stereochemical factors contributing to the observed 13C shielding differences between conformers are discussed