Conformational properties of monosubstituted cyclohexanes in their thiourea inclusion compounds and in solution: variable-temperature one-dimensional and two-dimensional carbon-13 NMR investigations

Abstract

Temperature-dependent changes in high-resolution solid state C-13 NMR spectra of thiourea inclusion compounds containing monosubstituted cyclohexane guest moleculeS (C6H11X: X = CH3, NH2, OH, Cl, Br, I) have demonstrated that a ''chair-chair'' ring inversion process occurs for these guest molecules inside the thiourea tunnel, and the energy barriers for this process have been determined. The relative populations of the axial and equatorial conformers of the guest molecules have been determined directly from the NMR spectra recorded (at low temperature) under slow exchange conditions. On this basis, the guest molecules studied can be divided into two classes: those with X = Cl, Br, and I have a predominance of the axial conformer (relative populations of equatorial conformer almost-equal-to 0.05-0.15), whereas those with X = CH3, NH2, and OH have a predominance of the equatorial conformer (relative populations of equatorial conformer almost-equal-to 0.82-0.97). In addition, conformational properties of these monosubstituted cyclohexanes in solution have been reinvestigated via variable-temperature C-13 NMR measurements. It is shown, from the reported results for methylcyclohexane, that two-dimensional exchange spectroscopy can successfully identify minor conformers with relative populations as low as 0.01 in solution and 0.03 in the solid state.