Deuterium NMR of Liquid-crystalline Samples at Natural Abundance
Résumé
Deuterium nuclei are naturally present at the level of ca 0.0155% on average in all (bio)organic compounds. Both recent instrumental (magnetic fields, cryogenic probes, and hardware) and methodological NMR developments (2H-dedicated pulse sequences) have significantly boosted the sensitivity and the analytical potentialities of natural abundance deuterium (NAD) NMR, and have opened up a large range of applications, in particular when chiral, weakly aligning media are used as NMR solvents. Interestingly, among all quadrupolar nuclei, the deuteron (second isotope of hydrogen, monovalent atom, I = 1) plays a distinctive role because it possesses an unusually small nuclear electric quadrupole moment that can be efficiently exploited. Because of its intrinsic NMR properties and its low natural abundance, deuterium can be used as a remarkable tracer for analytical purposes. In this article, we present the fundamentals of NAD NMR spectroscopy in achiral and chiral liquid-crystalline environments and its numerous contributions to molecular analysis. The most significant applications of NAD NMR using liquid crystals with low and high degrees of ordering are reviewed. Special emphasis is placed on NAD NMR in polypeptide chiral mesophases that provide powerful alternative tools for enantiomeric analysis, stereochemical investigations, and determination of site-specific natural isotope fractionation.