Shift reagents in NMR spectroscopy

Metal enolates found varied application in chemical analysis. An outstanding group are certain lanthanide enolates used as shift reagents in NMR spectroscopy. The analytical methods discussed in Section IV are based on formation of a metal enolate for separation, detection, identification and determination of metal ions or the use of a metal enolate as ancillary reagent to improve analytical quality. Of special relevance in analytical chemistry are the metal /3-diketonates, M(dik) , derivatived from deprotonated /3-diketones (dikH),... 

USE The salts in cathode ray tube coatings for color television receivers. Europium has a very high cross-section for the capture or thermal neutrons which is of value in the construction of electric atomic power stations. Organic derivs of europium are used as shift reagents in NMR spectroscopy C. C. Hinckley, J. Am. Chem. Soc. 91, 5160... 

While the coordination chemistry of lanthanide ions in water is extensive, it is of course not limited to this solvent. In the 1970s, considerable interest developed in the synthesis of lanthanide ion complexes soluble in apolar solvents for their use as lanthanide shift reagents in NMR spectroscopy. Typically, such complexes were neutral and based on chelating 1,3-diketonate ligands, one of their attractive features being that not only did they cause normally overlapping resonances to be spread out and resolved but that they could readily be prepared from optically active ligands and thus used to... 

Reuben J 1973 Paramagnetic lanthanide shift reagents in NMR spectroscopy principles, methodology and applications. Prog Nuc Magn Res Spec 9 1-70... 

Shift-reagents in NMR spectroscopy. EUL3 (L = chelate) Porphyrins [125-128,129] [130]... 

Due to the paramagnetism of most of the REM ions they are used as shift reagents in NMR spectroscopy. The J-diketonates and other chelate complexes of Eu(III) are exploited most frequently [125-127]. These compounds are rather soluble in most of organic solvents and the shift of NMR signals promoted by them is not accompanied by their broadness. At the same time it should be taken into account that in some cases the europium complexes cause the intramolecular rearrangement of the studied compound [128]. 

Sherry AD and Geraldes CFGC (1989) Shift reagents in NMR spectroscopy in lanthanide probes. In-. Biinzli J-CG and Chopin GR (eds) Life, Chemical and Earth Sciences, Theory and Practice, Amsterdam Elsevier. 

Von Ammon R and Eisher RD (1972) Shift reagents in NMR spectroscopy. Angewandte Chemie, International Edition in English 11 675-692. 

NMR polarimetry Technique using chiral shift reagents in NMR spectroscopy for the determination of enantiomeric purity of natural semiochemicals. 

Flockhart, B.D., Lanthanide Shift Reagents in NMR Spectroscopy, Crit. Rev. Anal. Chem., 1976, p. 69. Atta-ur-Rahman, NMR Basic Principles, Springer, New York, 1986. 

J. Reuben, in Progress in Nuclear Magnetic Resonance Speetroscopy , Vol. 9, Pt. 1 Lanthanide Shift Reagents m NMR Spectroscopy Principles, Methodology and Applications , Pergamon Press, Oxford, 1973. 

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. 

Table T. Lanthanide Shift Reagents (LSRs) used in NMR Spectroscopy...

Stable metal complexes can be favorably formed when a bidentate metal-binding site is available, such as a- and -diketone moieties which are the tautomeric forms of a- and /3-ketoenols. Some /S-diketonate complexes of paramagnetic lanthanides such as Pr(III), Eu(III) and Yb(III) have been extensively utilized as paramagnetic shift reagents for structural assignment of molecules with complicated stereochemistry prior to 2D techniques in NMR spectroscopy. Their syntheses and application are discussed in separate chapters in this volume. The examples below provide some dynamic and structural basis for better understanding of metal enolates in biomolecules and biochemical processes. 

Another possibility of producing explicable signal displacements is the addition of complexing reagents. Bose et al. (41) reported that titanium tetrachloride in deuteriochloroform can be used as a shift reagent in H and l3C NMR spectroscopy, and applied this method to coumarin and some angular furanocoumarins (42). 

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

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