Ring inversion of fluorocyclohexane in its solid thiourea inclusion compound

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 F-19 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 C-13-{H-1,F-19} spectra were studied. It is concluded that the average activation parameters (from the F-19 work) for the forward and backward ring inversion processes are Delta H double dagger=39.4+/-2.6 kJ mol(-1) and Delta S double dagger = - 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 F-19 measurements over the use of C-13 spectra is emphasized.