Nuclear Magnetic Resonance (NMR) spectroscopy involves absorbing radio frequency radiation by atomic nuclei in a magnetic field. NMR can be used to study the magnetic properties and local chemical environments of different nuclei, deduce molecular structure, and identify atoms in neighboring groups. The number and positions of NMR signals provide information about the number of different proton types in a molecule and their magnetic shielding. Signal intensities correlate with proton numbers, and splitting patterns indicate neighboring protons. NMR has applications in materials science, chemical analysis, studying hydrogen bonding and drug design.

1. Nuclear Magnetic Resonance
-Roll no:-3

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.

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).

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.

15. 1H NMR chemical shift
• Protons in different environments experience
different degrees of shielding and have
different chemical shifts.

16. ethylbenzene

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.

20. • CH3-O-CH3
• ↑ ↑
• H(a) H(a)
• All equivalent Hs 1 NMR signals.
• CH3-CH2-Cl
• ↑ ↑
• H(a) H(b)
• 2 types of Hs 2 NMR signals.

21. • CH3-O-CH2-CH3
• ↑ ↑ ↑
• H(a) H(b) H(c)
• 3 types of Hs and 3 NMR signals

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.

29. THANK YOU