Anisotropic 1D/2D NMR in Molecular Analysis
Résumé
Contrarily to isotropic solutions, the molecular tumbling regime no longer exists in oriented solvents/media made of lyotropic liquid crystals (LLCs) or compressed/stretched gels. Under this condition, any analyte dissolved in these systems (chiral or not) adopt an orientational order with respect to the magnetic field of a NMR spectrometer. This leads to very informative intramolecular anisotropic NMR interactions such as the residual dipolar coupling (RDC), the chemical shift (RCSA) and finally the residual quadrupolar coupling (RQC) for spin I > ½, that can be succefully exploited for many applications useful to chemists. In this Chapter 12, the various key theoretical and practical/technical aspects are presented as well as the spectral specificities of 1D/2D multinuclear NMR in weakly aligned chiral solvents (both LLCs and gels) using a large collection of chemically different organic analytes. Some additional concepts related to stereochemistry analysis (as chirality, prochirality or enantiotopicity) and their spectral consequences in anisotropic NMR are also examined. For illustrating the outstanding analytical potential of the anisotropic NMR spectroscopy, the most significant and contributive applications, such as the determination of enantiopurity of chiral mixtures, the study of the site-specific natural isotopic fractionation, the analysis of prochiral compounds or molecular dynamic processes. A special attention will be paid also to the recent and creative approaches developed for the structure and configuration elucidation based on RDC, RCSA and RQC obervables or their combination. Key or pioneering examples will be examined.
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