2D-NMR Strategy Dedicated to the Analysis of Weakly Ordered, Fully Deuterated Enantiomers in Chiral Liquid Crystals
Résumé
Methods for the assignment of the quadrupolar doublets in the deuterium NMR spectra of weakly ordered, perdeuterated or partially deuterated enantiomers dissolved in chiral liquid crystals are described which use robust 2D correlation NMR experiments. To overcome a lack of resolution in deuterium tilted Q-COSY 2D spectra in such materials, we propose and explore a correlation 2D sequence which is based on deuterium−carbon 2D correlation spectroscopy. The technique results in a 13C−2H contour plot and allows the full resonance assignment of overcrowded deuterium 1D spectra using carbon−deuterium correlations. The 2H autocorrelation and 13C−2H correlation experiments are applied in the case of a racemic mixture of 2-ethylhexanoic acid-d15 dissolved in a polypeptidic chiral oriented solvent. The performance and the limits of both techniques are presented and discussed. For the last step of the assignment procedure, we propose a simple method for obtaining two coherent sets of quadrupolar splittings, one for each enantiomer.