Application of a 1H δ-resolved 2D NMR experiment to the visualization of enantiomers in chiral environment, using sample spatial encoding and selective echoes

We present the application of a 2D broadband homodecoupled proton NMR experiment to the visualization of enantiomers. In a chiral environment, the existence of diastereoisomeric intermolecular interactions can yield - generally slight - variations of proton chemical shifts from one enantiomer to another. We show that this approach, which relies on a spatial encoding of the NMR sample, is particularly well suited to the analysis of enantiomeric mixtures, since it allows, within one single 2D experiment, to detect subtle chemical shift differences between enantiomers, even in the presence of several couplings. This sequence, which uses semiselective radio-frequency (rf) pulses combined to a z-field gradient pulse, produces different selective echoes in various parts of the sample. The resulting homonuclear decoupling provides an original delta-resolved spectrum along the diagonal of the 2D map where it becomes possible to probe the chiral differentiation process through every proton site where the resulting variation in the chemical shift is detectable. We discuss the advantages and drawbacks of this approach, regarding other experiments which provide homodecoupled proton spectra. This methodology is applied to the observation of enantiomers of (1) (+/-)2-methyl-isoborneol coordinated to europium (III) tris[3-(trifluoromethyl-hydroxymethylene)-(+)camphorate] in isotropic solution, and (2) (+/-)3-butyn-2-ol dissolved in a chiral liquid-crystal solvent, in order to show the robustness of this pulse sequence for a wide range of chiral samples. Copyright (c) 2009 John Wiley & Sons, Ltd.