The document discusses the importance of nuclear magnetic resonance (NMR) spectroscopy and the use of shift reagents to resolve signal overlap in complex organic compounds. It details the history, mechanisms, advantages, and disadvantages of lanthanide shift reagents, which enhance spectral resolution by causing chemical shifts through interactions with specific nuclei. Ultimately, these reagents simplify the interpretation of NMR spectra, allowing for more accurate structural analysis of organic and organometallic compounds.

1. CONTENTS
• Introduction
• Why need of NMR shift reagents
• Resolving problem of signal overlap
• History of lanthanide shift reagents
• Mechanism of inducement of chemical
shift
• Interaction of chemical shift reagents
Advantages
• Disadvantages
• Conclusion
• References

2. INTRODUCTION
• Nuclear magnetic resonance spectroscopy (NMR) is
the most powerful tool for organic and
organometallic compound determination.
• Structures can be determined using this technique.
• In general NMR gives information about the number
of magnetically distinct atoms of the specific nuclei
under study, as well as information regarding the
nature of the immediate environment surrounding
each nuclei.
• Because hydrogen and carbon are the major
components of organic and organometallic
compounds, proton (1H) NMR and carbon-13 (13C)
NMR are the most useful nuclei to observe.

3. • Not all the protons experience resonance at the
same frequency in a 1H NMR, so it is possible to
differentiate between them.
• This diversity is due to the existence of a
different electronic environment around
chemically different nuclei.
• Under an external magnetic field (B0), the
electrons in the valence shell are affected; they
start to circulate generating a magnetic field,
which is apposite to the applied magnetic field.
This effect is called diamagnetic shielding or
diamagnetic anisotropy

4. Why need of nmr shift reagents
• The greater the electron density around one
specific nucleus, the greater will be the induced
field that opposes the applied field, and this will
result in a different resonance frequency.
• The identification of protons is simple, however,
the NMR technique has a relatively low sensitivity
of proton chemical shifts to changes in the
chemical and stereochemical environment; as a
consequence the resonance of chemically
similar proton overlap .

5. Methods for resolving problem of signal
overlap
• use of higher frequency spectrometers
• use of shift reagents - aromatic
solvents
• lanthanide complexes

6. History of lanthanide shift reagents
• The first significant induced chemical shift using
paramagnetic ions was reported in 1969 by Conrad
Hinckley , where he used bispyridine adduct of
tris(2,2,6,6-tetramethylhepta-3,5-
dionato)europium(III) (Eu(tmhd)3), better known as
Eu(dpm)3, where dpm is dipivaloyl- methanato.
• Hinckley used Eu(tmhd)3 on the 1H NMR spectrum of
cholesterol from 347 – 2 Hz.
• Hinckley ‘s this new chemical method to improve the
resolution of the NMR spectrum was the stepping-
stone for the work of Jeremy Sanders and Dudley
Williams .

7. Mechanism of inducement of
chemical shift
• Lanthanide atoms are Lewis acids, and because of that,
they have the ability to cause chemical shift by the
interaction with the basic sites in the molecules.
• Lanthanide metals are especially effective over other
metals because there is a significant delocalization of
the unpaired f electrons onto the substrate as a
consequence of unpaired electrons in the f shell of the
lanthanide.
• The lanthanide metal in the complexes interacts with the
relatively basic lone pair of electrons of aldehydes,
alcohols, ketones, amines and other functional groups
within the molecule that have a relative basic lone pair of
electrons, resulting in a NMR spectral simplification.

8. There are two possible mechanisms by
which a shift can occur
SHIFT BY CONTACT
Transfer of electron spin
density via covalent bond
formation from the
lanthanide metal ion to
the associated nuclei.
SHIFT BY
PSUEDOCONTACT
Magnetic effects of the
unpaired electron
magnetic moment causes
the pseudocontact shift.

9. • Lanthanide complexes give shifts primarily by the
pseudocontact mechanism.
• Principal factor that influence the shift of a
specific NMR peak is the distance between the
metal ion and the proton; the shorter the
distance, the greater the shift obtained
• Direction of the shift depends on the lanthanide
complex used. The complexes that produce a shift to a
lower field (downfield) are the ones containing erbium,
europium, thulium and ytterbium, while complexes
with cerium, neodymium, holmium, praseodymium,
samarium and terbium, shift resonances to higher field.

10. Interaction of chemical shift reagents::
Lanthanide complexes interact with a relatively basic pair
of electrons ( an unshared pair ) which can coordinate
with Eu+3. Typically, aldehydes, ketones, alcohols, thiols,
ethers and amines all interact.

11. Example-
The spectra of 1- hexanol:
In the absence of shift
reagent, the spectrum
shown Only the triplet
of the terminal methyl
group the triplet of the
methylene group next
to the hydroxyl are
resolved in the spectrum.
The protons
(aside from O-H) are
found together in a broad,
unresolved group. With the
shift reagent added each
of the methylene groups
is clearly separated and
is resolved into proper
multiplet structure.

13. Advantages of using chemical shift reagents:
•Gives spectra which are much easier to
interpret.
•No chemical manipulation of the sample is
required with the use of shift reagents.
•More easily obtained
•Identifying enantiomeric mixtures in solution

14. disadvantage:
• Shift reagents cause a small
amount of line broadening
At high shift reagent concentrations this
problem becomes serious, but at most useful
concentrations the amount of broadening is
tolerable .

15. Conclusion
Thus the chemical shift reagents and solvent
induced shifts have their application in resolving
the NMR spectra of complex structures by
inducing shifts with respect to reference
compound. Thus useful in interpretation of
structures of complex organic compounds.

16. REFERENCES
• Donald L.pavia, Gary M.Lampman, Georges S.Kriz , Introduction to
spectroscopy , 3 rd edition, printed in the United States Of America,
P.322-323, 109-110.
• William Kemp, organic spectroscopy , 3 rd edition, printed by
replica press ltd., India P.102-106, 131-133, 169-170.
• C. C. Hinckley, J. Am. Chem. Soc., 1969, 91, 5160.
• file:///F:/NMR%20reagents/introduction.xhtml
•Edwin Becker, High Resolution NMR 3rd Edition