1H NMR analyses of enantiomeric mixtures using chiral liquid crystals. (January 2018) - Farjon, Jonathan
Abstract: Nuclear Magnetic Resonance in chiral liquid crystalline media is a powerful tool for deciphering mixtures of enantiomers. 1 H NMR appears to be the most sensitive technique for analyzing enantiomers but remains very challenging for the analysis of chiral solutes interacting with weakly orienting media. Indeed, probing 1 H networks even for small sized molecules is difficult because in oriented media, partially averaged anisotropic interactions such as residual dipolar couplings contribute to broaden NMR signals and lead to crowded spectra. For this reason, using 1 H NMR for analyzing enantiomeric mixtures has long required a complex and tedious analytical process to extract a qualitative and quantitative information about each enantiomer. Several methods have been developed in the last decade to overcome this difficulty. SElective ReFocussing (SERF) based techniques allowed the possibility to get access to high resolution spectra and measure accurately every homonuclear 1 H total couplings for each enantiomer. Among them, the Gradient encoded SERF (G-SERF) pulse sequence is an advanced tool for a rapid analysis of whole networks of residual dipolar couplings. Moreover, other multi-dimensional experiments have been developed to acquire homonuclear correlation spectra in which complex proton lineshapes of fully coupled systems are simplified. Finally, heteronuclear correlation methods that eventually combine the signal dispersion of another heteronucleus with the homonuclear decoupling of the proton dimension, were successfully implemented to separate lines from 1 H sites in each enantiomer. Graphical abstract: Image 1 Highlights: NMR has become a tool of choice for analyzing enantiomers in chiral liquid crystals. The resolution of 1H spectra was not suitable for studying such complex samples. Pure shift and T-edited methods have been developed to increase spectral resolution. These methods were implemented in homo- and hetero-nuclear correlation spectra. They brought significant improvements to the analysis of enantiomeric mixtures.