2 H QUOSY 2D-NMR Experiments in Weakly Aligning Systems: From the Conventional to the Ultrafast Approach

Ultrafast 2D NMR for the analysis of complex mixtures

2D NMR is extensively used in many different fields, and its potential for the study of complex biochemical or chemical mixtures has been widely demonstrated. 2D NMR gives the ability to resolve peaks that overlap in 1D spectra, while providing both structural and quantitative information. However, complex mixtures are often analysed in situations where the data acquisition time is a crucial limitation, due to an ongoing chemical reaction or a moving sample from a hyphenated technique, or to the high-throughput requirement associated with large sample collections. Among the great diversity of available fast 2D methods, ultrafast (or single-scan) 2D NMR is probably the most general and versatile approach for complex mixture analysis. Indeed, ultrafast NMR has undergone an impressive number of methodological developments that have helped turn it into an efficient analytical tool, and numerous applications to the analysis of mixtures have been reported. This review first summarizes the main concepts, features and practical limitations of ultrafast 2D NMR, as well as the methodological developments that improved its analytical potential. Then, a detailed description of the main applications of ultrafast 2D NMR to mixture analysis is given. The two major application fields of ultrafast 2D NMR are first covered, i.e., reaction/process monitoring and metabolomics. Then, the potential of ultrafast 2D NMR for the analysis of hyperpolarized mixtures is described, as well as recent developments in oriented media. This review focuses on high-resolution liquid-state 2D experiments (including benchtop NMR) that include at least one spectroscopic dimension (i.e., 2D spectroscopy and DOSY) but does not cover in depth applications without spectral resolution and/or in inhomogeneous fields.

Molecular enantiodiscrimination by NMR spectroscopy in chiral oriented systems: Concept, tools, and applications

The study of enantiodiscriminations in relation to various facets of enantiomorphism (chirality/prochirality) and/or molecular symmetry is an exciting area of modern organic chemistry and an ongoing challenge for nuclear magnetic resonance (NMR) spectroscopists who have developed many useful analytical approaches to solve stereochemical problems. Among them, the anisotropic NMR using chiral aligning solvents has provided a set of new and original tools by making accessible all intramolecular, order-dependent NMR interactions (anisotropic interactions), such as residual chemical shift anisotropy (RCSA), residual dipolar coupling (RDC), and residual quadrupolar coupling (RQC) for spin I > 1/2, while preserving high spectral resolution. The force of NMR in enantiopure, oriented solvents lies on its ability to orient differently in average on the NMR timescale enantiomers of chiral molecules and enantiotopic elements of prochiral ones, leading distinct NMR spectra or signals to be detected. In this compendium mainly written for all chemists playing with (pro)chirality, we overview various key aspects of NMR in weakly aligning chiral solvents as the lyotropic liquid crystals (LLCs), in particular those developed in France to study (pro)chiral compounds in relation with chemists needs: study of enantiopurity of mixture, stereochemistry, natural isotopic fractionation, as well as molecular conformation and configuration. Key representative examples covering the diversity of enantiomorphism concept, and the main and most recent applications illustrating the analytical potential of this NMR in polypeptide-based chiral liquid crystals (CLCs) are examined. The latest analytical strategy developed to determine in-solution conformational distribution of flexibles solutes using NMR in polypeptide-based aligned solvents is also proposed.