NMR Study of Rotational Barriers and Conformational Preferences. I. Cyclohexyl Fluoride

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Abstract
The equatorial:axial ratio and rate of conformational isomerization of cyclohexyl fluoride has been measured by observation of the 19F resonance as a function of temperature (—87.6° to +29.5°) for a 25 vol.% solution in CCl3F. Rate information is readily obtained over a wide temperature range, since the equatorial and axial fluorines differ in chemical shift by 20.5 ppm; the equatorial and axial α protons differ in chemical shift by only 0.46 ppm. Interpretation of the spectra in terms of isomerization rates is complicated by the following: (a) the equatorial and axial conformers are present in unequal proportions; (b) their ratio changes with temperature; and (c) 19F–H coupling produces fine structure and/or peak broadening. The general expression for spectral line shape resulting from exchange between two unequal, chemically shifted spin populations is plotted (using the IBM 7090 computer), chemical shift and conformer ratio data being obtained from the low temperature 19F spectra. The observed spectra fit the calculated spectra very closely, and can be used with their aid to obtain the rates for axial‐to‐equatorial and equatorial‐to‐axial isomerization. Excess line broadening resulting from 19F–1H coupling was eliminated by double resonance. Modifications to the rate expressions arising from the differing entropies and enthalpies of the axial and equatorial conformers are presented. Theoretical expectations for the activation entropies and enthalpies are discussed and compared to observed values.