Ring inversion of fluorocyclohexane in its solid thiourea inclusion compound

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The rate of ring inversion of fluorocyclohexane in the solid inclusion compound that it forms with thiourea was studied using a variety of NMR techniques. Special use was made of 19 F NMR spectra acquired with high-power proton decoupling. Bandshape analysis was used in the range 237–300 K. The occurrence of ring inversion at lower temperatures was established by the two-dimensional EXSY method and the rates thereof were determined by selective polarization inversion. Measurements of spin–lattice relaxation in the rotating frame showed a minimum at 300 K, yielding an additional value for the ring inversion rate. Variable-temperature triple-channel 13 C–{1H, 19F} spectra were studied. It is concluded that the average activation parameters (from the 19F work) for the forward and backward ring inversion processes are ΔH‡ =39.4±2.6 kJ mol−1 and Δ S‡=−12±11 J mol −1 K−1. The near equality of the axial and equatorial populations makes accurate determination of separate barriers for the equatorial→axial and axial→equatorial processes problematic. The values are discussed in relation to those for other cyclohexane derivatives in their thiourea inclusion compounds and for fluorocyclohexane in other media. The superiority of the 19F measurements over the use of 13C spectra is emphasized. © 1999 John Wiley & Sons, Ltd.