Ring inversion of fluorocyclohexane in its solid thiourea inclusion compound

R K Harris, A Nordon, K D M Harris

Research output: Contribution to journalArticle

10 Citations (Scopus)

Abstract

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. 

Original languageEnglish
Pages (from-to)15-24
Number of pages10
JournalMagnetic Resonance in Chemistry
Volume37
Issue number1
DOIs
Publication statusPublished - Jan 1999

Keywords

  • NMR
  • solid-state NMR
  • magic-angle spinning
  • fluorine-19
  • carbon-13
  • fluorocyclohexane
  • inclusion compound
  • ring inversion
  • chemical exchange
  • monosubstituted cyclohexanes
  • conformational properties
  • temperature-gradients

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