2. Introduction
• Nuclear Magnetic Resonance (NMR) is a
spectroscopy technique which is based on the
absorption of electron magnetic radiation in the
radio frequency region by nuclei of atoms.
• The radio frequency radiation has the frequency
range of 4-900 MHz corresponded to the
wavelength region of 75-0.5 m.
• For example we can have NMR signals for the
following atoms:-
• 1H and 13C.
3. Uses of NMR
• To record differences in the magnetic
properties of various magnetic nuclei present.
• To study different kind of environment within
the molecule.
• To study which atoms are present in
neighbouring groups.
• To deduce the position of nuclei within the
molecule.
4. Spinning Nucleus (Principle)
• Nucleus of the hydrogen atom(proton)
behaves as a tiny spinning bar magnet as it
possesses both electric charge and mechanical
spin.
• Any spinning charged body , generate a
magnetic field.
5.
6. Effect of an external magnetic field
• Proton responds to the influence of an
external magnetic field by aligning itself with
that field.
• Proton can only adopt two orientations to the
external magnetic field.
• Aligned with the field (lower energy state or
parallel).
• Opposed to the field (higher energy state or
antiparallel).
7.
8. Spin Quantum Number
• Spin Quantum no.(I) is related to the atomic
and mass number of the nucleus.
• Elements with odd mass number or odd
atomic number have nuclear spin.
Mass N0. Atomic No. Spin No.(I) Eg.
odd Odd/even ½,3/2,5/2..... 1H,11B
even even 0 12C,16O
even odd 1,2,3,... 2H,14N
9. Shielding and Deshielding
Upfield:-
The nucleus feels weaker magnetic field.
Shielding:-
A nucleus whose chemical shift has been
decreased due to the addition of electron
density , magnetic induction or other effects.
10. Deshielding
• Downfield:-
• The nucleus feels stronger magnetic feild.
• Deshielding:-
• A nucleus whose chemical shift has been
increased due to removal of electron density,
magnetic induction or other effects.
11. Chemical Shift
• Chemical shift is difference between the
absorption position of the sample proton and
absorption position of reference standard.
• Variation of position of NMR absorption due
to the electronic shielding and deshielding.
12. Measurement of chemical Shift
• For protons and 13C ,the accepted reference is
Tetramethylsilane , TMS.
• Low boiling point.
• Soluble in most organic solvents.
• Not soluble in water.
13. • The chemical shift numbers (signals) of Hs depend on
the surrounding (neighbours) atoms.
• The higher chemical shift are for most deshielded Hs
and lower chemical shift are for most shielded Hs.
• In the above spectrum which is for ethylbenzene that
Hs in the benzene ring are highly deshielded giving the
NMR signal around 7.1 ppm.
• The Hs in CH2 are close to the benzene ring appearing
at 2.6 ppm.
• The Hs in Ch3 are far away from the benzene ring ,
more shielded , giving NMR signal at 1.6 ppm.
14.
15. 1H NMR chemical shift
• Protons in different environments experience
different degrees of shielding and have
different chemical shifts.
17. electronegativity
• The chemical shift simply increases as the
electronegativity of the attached element
increases.
• The greater the electronegativity of the
substituents , the more deshielding of protons
and hence greater is the chemical shift of
those protons.
18. Interpretation of NMR spectra
• Numbers of signals- indicates how many
different kind of protons presents.
• Position of signals-indicates magnetic
environments of protons.
• Relative intensity of signals-proportional to
numbers of protons present.
• Splitting of signals- indicates the number of
nearby nuclei usually protons.
19. 1H NMR – number of signals
• The number of NMR signals equals the
number of different types of protons in a
compound.
• Protons in different environments give
different NMR signals.
• Equivalent protons give the NMR signal.
22. 1H NMR –Positions of signals
CH3-CH2-Cl
↑ ↑
H1) H(2)
The H(2) protons are deshielded because they are
closer to the electronegative cl atom, so they
absorb downfield from H(1).
BrCH2CH2F
↑ ↑
H(1) H(2)
Because F is more electronegative than Br, the H(2)
protons are more deshielded than Ha protons. And
absorb downfield.
23. • ClCH2CHCl2
• ↑ ↑
H(a) H(b)
The larger number of electronegative Cl atoms
deshields H(b) more than H(a), so it absorbs
downfield from H(a).
24. 1H NMR- Intensity of signals
• The area under an NMR signal is proportional
to the number of absorbing protons.
• An NMR spectrometer automatically
integrates the area under the peaks , and
prints out a stepped curve on the spectrum.
• Modern NMR spectrometers automatically
calculate and plot the value of each integral in
arbitrary units.
25. Example
• Multiplicity of NMR signals:-
• The NMR signals of Hs can have multiplicity
depending on numbers of neighbour Hs.
• Number of multiplicity= n+1 n is the number
of neighbour Hs.
• Multiplicity Intensity Ratio
Singlet (S) 1
Doublet(D) 1:1
Triplet(t) 1:2:1
Quartet(Q) 1:3:3:1
Quintet 1:4:6:4:1
26. Spin- Spin Coupling
• Splitting in other words.
• Protons on nearby carbons will interact and
split (division) each others resonance into
multiple peaks (multiples) n+1 rule:-
• Equivalent protons that have n equivalent
protons on the adjacent carbon will be split
into n+1 peaks.
27. Limitations of NMR
• It is limited to the measurement of nuclei
with magnetic moments.
• It may be less sensitive than other
spectroscopic and chromatograhic methods of
analyses.
• Requires computer modelling- time
consuming and expensive.
28. Applications of NMR
• Material Science:- A powerful tool in research of polymer
chemistry and physics.
• Chemical Analysis:- A matured technique for chemical
identification of chemicals whether synthetic or natural.
• Hydrogen bonding :-A unique technique for the DIRECT
detection of hydrogen bonding interactions.
• Drug screening and design :-Particularly useful for
identifying drug leads and determining the conformations
of the compounds bound to enzymes, receptors, and other
proteins.
• Protein hydration :-A power tool for the detection of
interior water and its interaction with biomacromolecules.
