MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 37, 15È24 (1999) Ring inversion of ýuorocyclohexane in its solid thiourea inclusion compound¤ Robin K. Harris,1 * Alison Nordon1 and Kenneth D. M. Harris2 1 Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK 2 School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 22 May 1998 ; revised 27 July 1998 ; accepted 28 July 1998 ABSTRACT : The rate of ring inversion of Ñuorocyclohexane in the solid inclusion compound that it forms with thiourea was studied using a variety of NMR techniques. Special use was made of 19F NMR spectra acquired with high-power proton decoupling. Bandshape analysis was used in the range 237È300 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 spinÈlattice relaxation in the rotating frame showed a minimum at 300 K, yielding an additional value for the ring inversion rate. Variable-temperature triple- channel 13CÈM1H, 19FN spectra were studied. It is concluded that the average activation parameters (from the 19F work) for the forward and backward ring inversion processes are *Ht \ 39.4 ^ 2.6 kJ mol~1 and *St \[ 12 ^ 11 J mol~1 K~1. The near equality of the axial and equatorial populations makes accurate determi- nation 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 Ñuorocyclohexane in other media. The superiority of the 19F measurements over the use of 13C spectra is empha- sized. 1999 John Wiley & Sons, Ltd. ( KEYWORDS : NMR ; solid-state NMR ; magic-angle spinning ; Ñuorine-19 ; carbon-13 ; Ñuorocyclohexane ; inclusion compound ; ring inversion INTRODUCTION The conformation and ring inversion of cyclohexane derivatives have been the subject of many studies. NMR methods have proved to be especially productive because, whereas at room temperature and above the observed solution-state spectra are generally averages over the ring inversion process, at accessible low tem- peratures the ring inversion rates are comparable to rel- evant chemical shift separations. This allows NMR bandshapes to be used to determine the activation ener- gies and entropies of ring inversion. Moreover, it is gen- erally feasible to decrease sample temperatures sufficiently to obtain superimposed spectra of the separate axial and equatorial conformations of mono- substituted cyclohexanes, thus giving information on conformational preferences. It is of considerable interest to examine how such preferences and inversion rates are a†ected by the local chemical environment (e.g. by solvent or by phase) and in this regard we are particu- larly interested in the e†ect of constraining cyclohexane derivatives within solid host structures.1,2 Fluorocyclohexane is particularly suitable for NMR studies since, under conditions of proton decoupling, * Correspondence to : R. K. Harris, Department of Chemistry, Uni- versity of Durham, South Road, Durham DH1 3LE, UK E-mail : r.k.harris=durham.ac.uk ¤ Dedicated to Professor John D. Roberts on the occasion of his 80th birthday. Contract/grant sponsor : EPSRC ; Contract/grant number : L02906. the axial and equatorial conformations should each give rise to a single line in the 19F spectrum at low tem- peratures, i.e. when ring inversion is in the slow- exchange regime. In fact, ring inversion in Ñuorocyclohexane has been studied by 19F NMR in the gas phase3 and in solution,4 by 1H NMR in solution4,5, by 13C NMR in solution6,7 and by 13C NMR as a guest in its solid thiourea inclusion compound.2 For organic solids containing both Ñuorine and hydrogen, 19F NMR has been inhibited until recently by the sup- posed difficulty of high-power proton (dipolar) decoup- ling (given that 19F and 1H resonance frequencies only di†er by ca. 6%). However, commercial equipment is now available which allows such experiments to be undertaken efficiently. As part of a series of investiga- tions using such techniques,8h14 we have examined 19F NMR spectra of a diverse range of compounds in the solid state. A 19F NMR study of ring inversion in the ÑuorocyclohexaneÈthiourea inclusion compound is of particular interest since data are available for the inver- sion of Ñuorocyclohexane2 and also chloro-, bromo- and iodo-cyclohexanes1 in their solid thiourea inclusion compounds from 13C NMR spectra. In the case of the ÑuorocyclohexaneÈthiourea inclusion compound, the 13C NMR work resulted2 in a surprisingly high value for the entropy of activation. Therefore, in the study described in the present paper, in addition to taking advantage of the intrinsic sensitivity of 19F NMR with variable-temperature bandshape-Ðtting experiments, we have extended the temperature range of the investiga- tion of the ring inversion process by using selective ( 1999 John Wiley & Sons, Ltd. CCC 0749-1581/99/010015È10 $17.50