Substituted cyclohexanes in liquid crystalline solution: the molecular behaviour of bromocyclohexane

Selectively and perdeuteriated samples of bromocyclohexane, dissolved in a liquid crystalline solution, are studied by dynamic 2H NMR spectroscopy. The analysis of variable temperature 2H NMR spectra between 238 and 333 K, as well as 2D exchange NMR experiments provide the assignment of the various NMR lines in terms of the equatorial and axial bromocyclohexane conformers that exist in a dynamic equilibrium in relative amounts of about pe=0.7 and pa= 0.3, respectively. By taking into account the molecular geometries of the two conformers (as obtained from molecular mechanics calculations) it is possible to derive the molecular order parameters of both species as a function of temperature. The observed temperature dependent lineshape changes can be attributed to the ring inversion process of the solutes, i.e. a mutual exchange of both conformers. The lineshape analysis yields an activation enthalpy for this internal process of DeltaH=44.5 +/- 4.5 kJ mol-1, in close agreement with earlier findings for isotropic solution.