Monohalocyclohexanes in liquid crystalline solution: the molecular behaviour of chloro- and iodo-cyclohexane

Dynamic ²H NMR spectroscopy is used to study selectively and perdeuteriated samples of chloro- and iodo-cyclohexane dissolved in a liquid crystalline solution. The equatorial and axial conformers are found to exist in a dynamic equilibrium with relative amounts of about p_e = 0.7 and p_a = 0.3, respectively, as taken from the ²H NMR lineshapes and 2D exchange spectra. The quantitative analysis of variable temperature ²H NMR spectra provides the kinetic parameters of the underlying ring inversion process. The activation enthalpies of this internal process are given by ΔH = 48.1 ± 1.0 and 40.6 ± 2.3 kJ mol^-1 for chloro- and iodocyclohexane, respectively. These values resemble those reported from earlier studies of isotropic solutions. The molecular order parameters, as well as the orientation of the main ordering axis, for the axial and equatorial conformers are determined as functions of temperature. These molecular quantities exhibit a strong dependence on the actual conformational state and size of the substituent, while polar effects of the substituents seem to play no important role.