Spectroscopy enantiomer discrimination

Stretched gelatin gels as chiral alignment media for the discrimination of enantiomers by NM R spectroscopy. Angew. Chem. Int. Ed. 2005, 44, 3145-3147. 

NMR spectroscopy cannot normally be used directly for discriminating enantiomers in solution. The NMR signals for most enantiomers are isochronic under achiral conditions. However, NMR techniques can be used for the determination of enantiomer compositions when diastereomeric interactions are introduced to the system. 

The isolated enantiomers S (M ) and R (Mr) of a chiral molecule M exhibit the same spectral features since their physical properties are identical. However, their aggregation with a chiral chromophore of defined configuration (Cr/s) leads to the formation of two diastereomeric complexes with different spectral properties, i.e., and [C /yM ]. The lcR2PI spectroscopy is able to discriminate between Mj and by measuring the spectral shift of the diastereomeric [C /yM ] and [Cj5/5-Mj ] complexes with respect to that of the bare chromophore Cr/s- It is convenient to define the diastereomeric clusters as homochiral when the chromophore and the solvent have the same configuration, and heterochiral in the opposite case. 

In this section an overview of the numerous methods and principles for the discrimination of enantiomers is given. First, the interaction principles of the polymer-based methods adapted from chromatographic procedures are illustrated. The discrimination of enantiomers was achieved some decades ago by using different types of stationary materials, like cyclodextrins or polymer-bonded amide selectors. These stationary-phase materials have successfully been appointed for label-free optical sensing methods like surface plasmon resonance (SPR) or reflectometric interference spectroscopy (RIfS). Furthermore, various successful applications to optical spectroscopy of the well-established method of fluorescence measurements for the discrimination of enantiomers are described. 

Hargcr ct a), reported the discrimination by NMR spectroscopy of both enantiomers of alkylphcnylphospinic amides The observed spectra can be rationalized in terms of dimerization through hydrogen bonds [Fig. 4a (top)]. In this system there was no marked preference for homochiral or heterochiral dimerization, and the partnerexchanging was fast on the NMR time scale. In an cnantiomcrically unbalanced sample, the major enantio-... 

At low temperature, the H NMR spectra for [Ln(DTPA)], where Ln = Pr, Eu and Yb, display 18 signals, which coalesced to 9 signals upon the increase of temperature. The exchange was studied by EXSY spectroscopy [176,177], Ln binding of the three nitrogens of the diethylenetriamine backbone results in chirality of the central nitrogen, the two enantiomers cannot be discriminated... 

Analysis of natural abundance deuterium distribution in organic molecules, an important step in the study of kinetic isotope effects associated with enzyme-catalyzed reactions, by the use of chiral anisotropic media has been explored. An aspect of this analysis is the discrimination of the enantiotopic deuterons in prochiral molecules and the quantification of isotopic fractionation on methylene prostereogenic sites. Towards this an approach has been presented which is based on the use of natural abundance 2-dimensional NMR experiments on solutes oriented by chiral liquid crystalline solvents and the separation of the deuterium signals based on the quadrupolar interaction. The case of 1,1 -bis(phenylthio)hexane derived by cleavage from methyl linoleate of safflower has been used to illustrate the method with (D/H)pro-R and (D/H)pro-S measured at the same methylene position of a fatty acid chain. Enantiomers of water soluble materials can be observed using deuterium NMR spectroscopy in the lyotropic mesophase formed by glucopon/hexanol/buffered water. ... 

Modem 2D techniques are applied, for example, in NMR on chiral solutes in chiral liquid crystalline solvents. NMR spectroscopy on such chiral host/chiral guest systems can be used for the spectral discrimination of enantiomeric solutes because the enantiomers show differential orientational ordering in the chiral environment. Based on this differential ordering effect (DOE), enantiomeric purity or excess can be quantitatively measured [92]. The DOE may also help to understand the intermolecular interactions involved in the chiral recogni-... 

One of the most common methods employed for analysis of chiral compounds is NMR spectroscopy [82, 83]. Enantiomers cannot be discriminated in an achiral medium because the resonances of enantiotopic nuclei are isochronous. However, diastereoisomers may be distinguished as the nuclei resonances are anisochronous. In NMR, nuclei can be classified as isochronous or anisochronous. Where diastereotopic protons show the same chemical shift, they are said to be equivalent or isochronous, and where they have different chemical shifts, the protons are described as anisochronous [84]. As long as there is a large enough... 

Succinoglycan, a sinorhizobial exopolysaccharide, is composed of an octasaccharide subunit. Succinoglycan monomers were isolated and used as chiral shift reagents with NMR spectroscopy for chiral discrimination of catechin. NMR signal splittings were observed in the interactions of saccharides with the enantiomers of catechin. Both chiral separation and discrimination of catechin depend on the presence of succinate substituents of the linear monomeric octasaccharide in capillary electrophoresis and NMR spectroscopy, suggesting that succinylation of sinorhizobial octasaccharide is decisive for the effective chiral separation and discrimination of catechin. 

Many crown ethers or aza crown ethers, the latter of which have one or more nitrogen atoms in the cavity, have been evaluated for their ability to cause enantiomeric discrimination in NMR spectroscopy. In most cases, only a few substrates have been examined. Also, most of these crown ethers involve multistep syntheses and are not commercially available. The one exception is (18-crown-6)-2,3,11,12-tetracarboxylic acid 72, (Figure 50.21) both enantiomers of which are commercially available. Comparative studies usually show that 72 is more effective as a chiral NMR solvating agent than other crown ethers. ... 

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