QUANTITATIVE DESCRIPTION OF THE FACIAL DISCRIMINATION OF MOLECULES CONTAINING A PROCHIRAL GROUP BY NMR IN A CHIRAL LIQUID-CRYSTAL

The quantitative description of the facial discrimination in molecules containing a prochiral group oriented in a polypeptide liquid crystal line system is reported. From the analysis of proton, carbon-13, and d euterium NMR spectra df ethanol dissolved in an organic solution of po ly-gamma-(benzyl-L-glutamate) (PBLG), the full molecular order matrix and the principal axis system were determined. It is shown that five n onzero independent order parameters are necessary to describe correctl y the molecular ordering of ethanol in this medium, which clearly demo nstrates that in this chiral liquid crystal, the symmetry of the order ing in a prochiral molecule is broken. Due to the nonequivalence in te rms of the dipolar couplings of the two enantiotopic protons of ethano l, it is possible to determine the sign and the magnitude of geminal s calar couplings between them. This new approach to obtain the geminal (2)J(HH) constants is tested on a series of linear alcohols and compar ed to values derived from (2)J(HD) of the same molecules by the isotop ic substitution method.