Dynamic deuterium NMR in liquid crystalline solvents: Ring inversion of cyclohexane-d12

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Abstract
The application of deuterium NMR spectroscopy of molecules dissolved in liquid crystalline solvents to study intramolecular dynamic processes is demonstrated using the ring inversion kinetics of cyclohexane-d12. The spectra of C6D12 dissolved in phase V and in hexyloxyazoxybenzene were studied over the temperature range −36 to +115 °C, and were found to exhibit a very pronounced line shape variation with temperature. At low temperatures (<−10 °C) the spectrum consists of two symmetric doublets due to the axial and equatorial sites. The doublet splittings are due to the quadrupole interactions of the corresponding deuterons. Upon increasing the temperature the lines broaden until finally the spectrum transforms into a single doublet with a spacing corresponding to the mean quadrupole splitting of the axial and equatorial deuterons. In an Appendix the line shape theory for two interacting nuclei of spin I = 1 which undergo mutual exchange is presented, and this theory is used to quantitatively interpret the experimental spectra of cyclohexane-d12. The kinetic parameters derived from these results are ΔH≠ = 10.3 kcal/mole, ΔS≠ = +0.5 e.u., and 1/τ(at 300 °K) = 1.5×105 sec−1. The results are compared with corresponding data obtained in isotropic solvents.