Interconversion kinetics and ordering of 1,4- and 1,1-dimethylcyclohexane in liquid-crystalline solutions by 1D and 2D deuterium NMR

Abstract

Deuterium NMR spectra of perdeuteriated 1,4-dimethylcyclohexane-d ₁₆ and 1,1-dimethylcyclohexane-d ₁₆ dissolved in the nematic solvent ZLI 2452 are reported for the temperature range −40 to +80°C. Between −30 and +60°C the spectra exhibit characteristic exchange broadening and coalescence due to the ring inversion process. In the extreme slow exchange regime, peak assignment and determination of relative signs of the deuterium quadrupole interactions were made using 2D exchange spectroscopy and structural parameters derived from molecular mechanics calculations. In the intermediate temperature range the lineshapes were interpreted quantitatively in terms of the ring interconversion kinetics yielding the kinetic equations, k = 1.38 × 10¹³ exp (−45.2/RT)s⁻¹ for 1,4-dimethylcyclohexane, and k = 4.05 × 10¹³ exp (−49.0/RT)s⁻¹ for 1,1-dimethylcyclohexane, where R is in kJ mol⁻¹. The complete ordering matrix of both compounds was determined over the whole temperature range of the measurements.