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lanthanide reagents for nmr spectrum
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Shift Reagents
Why are the shift reagents used in NMR spectroscopy? Presence of paramagnetic impurities in
the sample: shortens the relaxation times, which causes
line broadening () useful in integration in 13C NMR: suppresses
the NOE () causes a shift of the signals, but not equally
for all resonances useful in spectral analysis ()
Lanthanide shift reagents (LSRs) Paramagnetic ions of Ni and Co were the
first used shift reagents a big drawback: severe line broadening On 1969 Hinckley discovered that
paramagnetic lanthanide ions gave shifts without significant line broadening
Example: 90 MHz 1HNMR spectrum of 1-hexanol in the presence of chelate complex Eu(III)-tris(dipivaloylmethanate)
remarks: all the protons become
less shielded; all the CH2 groups become separated
the shifts increase with the proximity of the protons to the OH group of alcohol
Eu(III)-tris(dipivaloylmethanate)
How does this shift effect occur?
interactions between nuclear spins and the spin of the unpaired electrons of paramagnetic ions
two types of interactions: the contact interactions the pseudocontact interactions
Both types of interaction depend on: the formation of a complex between the
substrate S and the paramagnetic metal ion L In solution there exists a dynamic equilibrium
between the free components and the complex:
L + S LS Example of a complex:
RO
HEu(DPM)3
The contact term is based on the contact interaction: the spin
density of the unpaired electron is transferred to the substrate molecule
the electron spin density is not the same at all positions of the observed nuclei throughout the molecule
in saturated compounds the most affected 13C nuclei are those in - and -positions relative to the complexing center (e.g. O, N or S)
In conjugated systems more distant positions could be affected as well
The contact term is very important in 13C NMR
The pseudocontact term this type of interaction is of greater importance
in 1H NMR then the contact term the name pseudocontact is used to describe a
dipolar interaction between the magnetic dipole field of the unpaired electron and that of the observed nucleus
the interaction is transmitted through space
Geometry of the complex the shift in the resonance
frequency of the observed nuclei depends on the geometry of the complex:
DDip= K(3cos2 -1)/r3K- constant which depends on
the magnetic dipol moment of the paramagnetic metal ion
the equation is valid if the complex is symmetrical about the L-O axis
L
O CR
H
r
Conclusion about 1H shifts in the presence of lanthanide reagents
Dip= K(3cos2 -1)/r3
the shift effect decreases in inverse proportion to r3
it is independent on the observed nuclides
can be positive or negative (depending on the sign of (3cos2 -1) term)
Applications simplifying complicated spectra
separation of overlapping signals easier assignment integration of signals which are otherwise
overlapped decoupling experiments
determining accurate geometrical data for the LS complex and hence for the molecule of interest
Applications (cont.) troubles with olefinic and aromatic protons - they do not
show lanthanide-induced shifts (do not form complexes with lanthanide ions)
however, a solution is found: silver(I) ions they make complexes with pi-electrons if they are added in the form of AgFOD to the solution
containing the substrate and the LSR, shifts are observed for olefines and arenes
obviously, the silver ions are able to transmit the shift effect
O O
CF2CF2CF3(CH3)3C
1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octandione(FOD)
Chiral LSR Enantiomers are indistinguishable by NMR not possible to determine if the sample is a
pure enantiomer or a racemate by using a chiral reagent and making
diastereoisomers or diastereomeric complexes, it becomes possible
example: mixture of 1-phenylethylamine's enantiomers
LSR: the chiral complex Eu(TFC)3
Chiral solvents Interactions between solutes and solvents can
induce shifts as well our focus: interactions between chiral
compounds and chiral solvents (CSAs, chiral solvating agents)
a racemate (S(+) and S(-)) dissolved in a chiral solvent (e.g. L(-)) gives the two solvation diastereomers (S(+)L(-) and S(-)L(-))
this can lead to separate resonances in the NMR spectrum
The most common CSAs chiral
acids amines alcohols
fluorinated sulfoxides cyclic compounds
C CF3H
OH
2,2,2-trifluoro-1-phenylethanol
C CH3H
NH2
1-phenylethylamine
Mixture of achiral solvent + chiral reagent + chiral substrate
A shift effects are observed very often in such mixtures
example: 1H NMR spectrum of mixture of the racemate of 1-phenylethylamine solvent: CDCl3/DMSO (+)-2-methoxy-2-(trifluoromethyl)phenylacetic
acid
C CF3CH3O
COOH
(+)-2-methoxy-2-(trifluoromethyl)phenylacetic acid
Influencing factors The induced shifts depend on:
the chosen solvent the substrate (analytes) the complexing strength temperature concentration ratio
The effect is not observed for solvent signals reason: fast exchange of solvent molecules
between complexes with both substrate enantiomers
Shift ReagentsWhy are the shift reagents used in NMR spectroscopy?Lanthanide shift reagents (LSRs)PowerPoint PresentationHow does this shift effect occur?Both types of interaction depend on:The contact termThe pseudocontact termGeometry of the complexConclusion about 1H shifts in the presence of lanthanide reagentsApplicationsApplications (cont.)Chiral LSRLysbilde 14Chiral solventsThe most common CSAsMixture of achiral solvent + chiral reagent + chiral substrateLysbilde 18Influencing factors