Ring inversion of fluorocyclohexane in its solid thiourea inclusion compound

The rate of ring inversion of fluorocyclohexane in the solid inclusion comp ound that it forms with thiourea was studied using a variety of NMR techniq ues. Special use was made of F-19 NMR spectra acquired with high-power prot on decoupling. Bandshape analysis was used in the range 237-300 K. The occu rrence of ring inversion at lower temperatures was established by the two-d imensional EXSY method and the rates thereof were determined by selective p olarization inversion. Measurements of spin-lattice relaxation in the rotat ing frame showed a minimum at 300 K, yielding an additional value for the r ing inversion rate. Variable-temperature triple-channel C-13-{H-1,F-19} spe ctra were studied. It is concluded that the average activation parameters ( from the F-19 work) for the forward and backward ring inversion processes a re 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 po pulations makes accurate determination of separate barriers for the equator ial --> axial and axial --> equatorial processes problematic. The values ar e 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 empha sized. (C) 1999 John Wiley & Sons, Ltd.