The LIS reagents work only on molecules with Lewis basic sites. Lanthanide complexes can be 7, 8 or even 9 coordinate. This means that and substrate to reagent complexes can be formed, and indications of such complexes have been found, for example in nonlinear plots of chemical shift vs mole fraction of shift reagent. The high coordination numbers of the lanthanide complexes also allow chelation to occur when a substrate has more than one suitably situated basic site, as in the case of ortho-dimethoxybenzene below. Shift reagents are not used just to simplify spectra. They are especially valuable for distinguishing geometric isomers, such as cis-trans isomers of double bonds.
They have sometimes been used for conformational analysis, but this use is constrained by the likelihood that complexation to the lanthanide could cause changes in the conformation of the molecule.
14: Nuclear Magnetic Resonance
However, these conclusions are strictly valid only for the europium complexes, not for the free compounds. At normal temperatures the complexation and decomplexation rates of typical lanthanide shift reagent-substrate complexes are fast on the NMR time scale, so that averaged signals are obtained. Thus addition of 0.
Chiral Shift Reagents. One of the most useful applications of lanthanide shift reagents is in the determination of optical purity by the use of chiral ligands on the lanthanide. Some of the more effective reagents developed are Eu facam 3 tris 3-trifluoroacetyl-d-camphorato europium III and Eu hfbc 3 tris 3-heptafluorobutyryl-d-camphorato europium III H. Goering J.
Often sufficient separation between the R and S enantiomers can be obtained so that the enantiomeric purity can be determined directly by NMR integration. This means that 13 C NMR spectra change much less dramatically than do proton spectra, since the chemical shift range is much larger.
Spectral simplification is typically not very dramatic, but shift reagents have been useful for assignment of carbon NMR signals in complex natural products e. Wenkert HCA , 58 , Using Lanthanide Shift Reagents. Here are some considerations in the practical use of LIS reagents. Remember that CDCl 3 contains a few percent of ethanol-d 6 as stabilizer; this must be removed by filtration through a short plug of alumina or silica gel. If the material has been adapted instead of reproduced from the original RSC publication "Reproduced from" can be substituted with "Adapted from".
In all cases the Ref.
NMR Spin Coupling
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Chapter 14. Nuclear Magnetic Resonance
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