Quantitative Description of the Facial Discrimination of Molecules Containing a Prochiral Group by NMR in a Chiral Liquid Crystal

Abstract

The quantitative description of the facial discrimination in molecules containing a prochiral group oriented in a polypeptide liquid crystalline system is reported. From the analysis of proton, carbon-13, and deuterium NMR spectra of ethanol dissolved in an organic solution of poly-γ-(benzyl-l-glutamate) (PBLG), the full molecular order matrix and the principal axis system were determined. It is shown that five nonzero independent order parameters are necessary to describe correctly the molecular ordering of ethanol in this medium, which clearly demonstrates that in this chiral liquid crystal, the symmetry of the ordering in a prochiral molecule is broken. Due to the nonequivalence in terms of the dipolar couplings of the two enantiotopic protons of ethanol, it is possible to determine the sign and the magnitude of geminal scalar couplings between them. This new approach to obtain the geminal 2JHH constants is tested on a series of linear alcohols and compared to values derived from 2JHD of the same molecules by the isotopic substitution method.