Reagent chemical shift

The chemist can use these solvent-induced chemical shift changes to clarify complex spectra that feature overlapping multiplets. Often, by adding just a small amount (5-20%) of a benzene-cJg or pyridine-ds to the CDCI3 solution of an unknown, a dramatic effect on the appearance of the spec-tram can often be observed. The chemical shifts of peaks in the proton spectrum can be shifted by as much as 1 ppm, with the result that overlapping multiplets may be separated from one another sufficiently to allow them to be analyzed. The use of this benzene trick is an easy way to simplify a crowded spectrum. 

the low-field (60- or 90-MHz) spectrum of an organic compound, or a portion of it, is almost undecipherable because the chemical shifts of several groups of protons are all very similar. In such a case, aU of the proton resonances occur in the same area of the spectrum, and often peaks overlap so extensively that individual peaks and splittings cannot be extracted. One of the ways in which such a situation can be simplified is by the use of a spectrometer that operates at a frequency higher. Although coupling constants do not depend on the operation frequency or the field strength of the NMR spectrometer, chemical shifts in Hertz are dependent on these parameters (as Section 3.17 discussed). This circumstance can often be used to simplify an otherwise-undecipherable spectrum. 

Researchers have known for some time that interactions between molecules and solvents, such as those due to hydrogen bonding, can canse large changes in the resonance positions of certain types of protons (e.g., hydroxyl and amino). They have also known that changing from the usual NMR solvents such as CDCI3 to solvents such as benzene, which impose local anisotropic effects 

352 Nuclear Magnetic Resonance Spectroscopy Part Four 

Of the lanthanides, europium is probably the most conunonly used metal for shift reagents. Two of its widely used complexes are trM-(dipivalomethanato) europium and trw-(6,6,7,7,8,8,8-heptafluoro-2,2-dimelhyl-3,5-octanedionato) europium, fiequently abbreviated Eu(dpm 3 and Eu(fod)3, respectively. 

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These lanthanide complexes produce spectral simplifications in the NMR spectrum of any compound with a relatively basic pair of electrons (an unshared pair) which can coordinate with Eu. Typically, aldehydes, ketones, alcohols, thiols, ethers, and amines all interact  

The amount of shift a given group of protons experiences depends on (1) the distance separating the metal (Eu ) and that group of protons and (2) the concentration of the shift reagent in the solution. Because of the latter dependence, it is necessary to include the number of mole equivalents of shift reagent used or its molar concentration when reporting a lanthanide-shifted spectrum. 

Besli et alP describe a problem involving the use of CSR to determine meso and racemic forms of diastereoisomers in which the stereogenic centers of the molecules are separated by achiral spacers. In these cases, the NMR signals of both meso and racemic forms of diastereoisomers may exhibit doubling on addition of the CSR. Unequivocal assignments can be made only by characterizing the effects for separate meso and racemic forms. 

Smith et have prepared 11 chiral calix[4]arenes, calix[4]resorcarenes, and anionic cyclodextrin derivatives and investigated the properties of their lanthanide (Yb, Dy +) complexes as chiral lanthanide shift reagents (LSR). Baldovini et alP report an application of a camphor-derived chiral complex, Yb(hfc)3 (hfc = tris[3-(heptafluoropropylhydroxymethylene)-(- -)-camphorate]) to differentiate the C NMR spectra of enantiomers of bornyl acetate. 

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Chemicals from brine Chemical shift reagents Chemical shifts Chemicals in war Chemical treatments... 

Chiral chemical shift reagents for NMR analysis are also useful, and so are optical methods. 

Achiral lanthanide shifting reagents may be used to enhance the anisochrony of diastereomeric mixtures to facilitate their quantitative analysis. Chiral lanthanide shift reagents are much more commonly used to quantitatively analyze enantiomer compositions. Sometimes it may be necessary to chemically convert the enantiomer mixtures to their derivatives in order to get reasonable peak separation with chiral chemical shift reagents. 

Scheme 1-3. Determining enantiomer composition with chiral chemical shift reagent 18.

The synthesis of lanthanide chemical shift reagents has been the objective of many groups owing to their effect on NMR spectra simplification. A drawback of the commonly used reagents is their sensitivity to water or acids. Tris(tetraphenylimido diphosphinatojpraseodymium [Pr(tpip)3] has been developed as a CSR for the analysis of carboxylic acids.17 Furthermore, it has been found that dinuclear dicarboxylate complexes can be obtained through reactions with ammonium or potassium salts of carboxylic acids, and these compounds can be used to determine the enantiomer composition of carboxylic acids.18... 

Candida cyclindracea lipase circular dichroism capillary electrophoresis Cahn-Ingold-Prelog 1,5-cyclooctadiene cyclopentadienyl group m - ch I o r o pc r be n z o i c acid circularly polarized light camphorsulfonic acid chemical shift reagent... 

The same principle is involved in the use of chiral lanthanide chemical shift reagents for the determination of enantiotopic purity [44]. 

It is convenient to divide the subject into four sections. The first two, on chemical shift reagents and on relaxation studies, deal with techniques of line assignments and other facets of steroid behavior. The third, on substituent effects, lays the background for predicting steroid 13C chemical shifts and interactions of substituents with the steroid framework. In the fourth section the use of 13C NMR to solve problems in steroid stereochemistry is discussed. All chemical shift data are reported on the delta scale. 

As a matter of historical introduction it is noted that the first application (4) of a paramagnetic lanthanide chemical shift reagent involved the simplification of the proton NMR spectrum of cholesterol by means of Eu(dpm)3-2py. The applications and development of these valuable reagents are the subject of a book (5) and a recent review. (6)... 

The effectiveness of a chemical shift reagent is influenced by the steric environment of the coordination site. It has been reported (18) that the 3-hydroxyl group in methyl 3,12-dihydroxycholanate [1] complexes... 

It is probably stretching the point to consider the proton as a chemical shift reagent. However, it is convenient here to mention that the treatment of androst-4-ene-3,17-dione with 15 m H2S04 also leads to a marked deshielding of both the carbonyl and -carbon atoms with a very small effect on the cr-carbon. (21) The 17-keto group also is protonated. 

Not all NMR-active nuclei may therefore be suitable for the study of phase separation in phospholipids. Limited information can be gained on phospholipid phase transformation from 1H- and UC-NMR because of problems in resolution. Only some signals, for example the choline methyl groups of phospholipids in the outer and inner leaflets of unilamellar bilayers, can be identified by 1H- and 13C-NMR when chemical shift reagents are used. However, 13C-NMR can be applied to the study of phospholipid phase transition when the lipid is specifically enriched at the sn-2, carbonyl position [91]. 

OTHER EXAMPLES OF REVERSIBLE COMPLEXATION CHEMICAL SHIFT REAGENTS... 

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