Visualization of enantiomers through deuterium NMR in cholesterics. Optimization of the chiral liquid crystal solvent

Abstract

Thermotropic cholesteric liquid crystals made of mixtures of cholesteric esters and of a Δχ <0 nematic (ZLI2806) may be used as NMR solvents to visualize enantiomers. Proton decoupled deuterium NMR spectra of a racemic mixture show that the quality of the spectra strongly depends on the temperature and on the nature and the concentration of the ester. A relationship between the spectral resolution and the presence of textural defects in the mesophase is also discussed. The best results, both for spectral resolution and enantiomeric separation, were obtained with a 45/55 by weight mixture of cholesteryl butyrate and ZLI2806 at 368 K.