Chemical shift and application of NMR

1. NUCLEAR MAGNETIC RESONANCE [NMR]
CHEMICAL SHIFTS,FACTORS AFFECTING
CHEMICAL SHIFTS AND APPLICATIONS OF
NMR
PRESENTED BY :-
NIVEDITHA G
Ist sem M PHARM
DEPT. OF PHARMACUETICS
NARGUND COLLEGE OF PHARMACY
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2. WHAT IS CHEMICAL SHIFT
The Shift in the Protons of NMR Absorptions which
Arises due to Shielding or Deshielding
of the protons by the electrons is called
Chemical Shift
The position of the Signals in the Spectrum helps
us to know the nature of proton viz, Aromatic,
Aliphatic, adjacent to some Electron attracting
or Electron releasing groups , each of the types
of protons will have different environment
and thus they at different applied field strength
2

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It was first observed in Ethyl Alcohol by Packard
In 1951 , It was found that the Precessional
Frequency of all the protons in the same
external field is not the same and depends on
Number of factors.
Ethyl Alcohol shows 3 signals corresponding
To 6 protons
It was concluded that 3 different signals were
Due to 3 different chemical environments
In ethanol viz. CH3CH2 and OH

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When a molecule is placed electrons are in a
magnetic field , its electron are caused to
circulate and thus ; they produce secondary
Magnetic fields ( Induced magnetic field )
There are two types of protons:-
1. Shielded Protons :-
Those protons at which the induced
magnetic field Opposes the applied magnetic
fields and thus the proton felt less magnetic field.

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2. Deshielded Protons :-
Those protons at which the induced magnetic
field Reinforces the applied magnetic field and
thus proton felt higher magnetic field.

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Shielding shifts the Absorption of Upfeild
Deshielding shifts the absorption Downfeild
To get an effective field strength necessary
For absorption

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MEASUREMENT OF CHEMICAL
SHIFT
For Measuring chemical Shift, of various protons
In a molecule, The signal for TMS (Trimethyl
silicate ) is taken as reference
CH3
|
CH3-SI-CH3
|
CH3

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The difference in absorption position of the proton
With respect to TMS Signal is Called Chemical
Shift (δ) Delta
It is not measured in Gauss, measured in equivalent
Frequency units which is then divided by
Frequency of the spectrometer used this gives value
of (δ)
WHY TMS IS USED AND MOST CONVINENT
1. It is miscible with almost all organic compounds
2. Highly Volatile And readily removed via system
3. It will not take part in Intermolecular Association
with sample.

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FACTORS AFFECTING CHEMICAL
SHIFT
Following the factors which influence the
Chemical shift
1. Inductive Effect
2. Vander Wall’s De Shielding
3. Anisotropic effect or Space effect
4. Hydrogen Bonding
4. It is chemically inert, magnetically isotopic,
volatile, and soluble in organic solvents
5. All of its hydrogen atoms remain in identical
Environment, they are more strongly shielded
Than the protons in any pure organic compound

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1) Inductive Effect:- A proton is said to be
deshielded if it is attached with an
electronegative atom or group.
Greater the electronegativity of the atom, greater is
the deshielding caused to the proton
Consider the following compounds
i)CH3 CH2 F ii) CH3-CH2-CL
b a b a
Two signals are expected for each of the two
Compounds, deshielding for protons ‘a’ in
Compound (i)

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Is more than that for similar protons in
compound
 As the distance from electronegative atom
Increases, the deshielding effect due to it
Diminishes.
Proton ‘b’ are comparatively less deshielded
And hence will resonate at the comparatively
Lower than value of δ

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2) Van Der Wall’s Deshielding :-
In overcrowded molecules, it is possible that
some proton may be occupying sterically
hindered position.
Clearly, electron cloud of a bulky group
(hindering group) will tend to repel the electron
cloud surrounding the proton. This such a proton
will be deshielded and will resonate at slightly
higher value of δ than expected in the absence of
this effect
Ex:- morphine, alkaloids, steroids etc.

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3) Space Effect/ Anisotropic Effect:
Circulation of – π electrons about nuclei generates
induced field.
Induced Field - OPPOSE (shielding) or REINFORCE
(Deshielding) the applied field at the proton
Depending on location of proton in SPACE.
π-electrons have pronounced effect on chemical shift
of protons

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Alkenes :
An alkenes group is so oriented that the
plane of the double bond is at 90º (right angle) to
the direction of the applied field.
The induced circulation of π electrons
generates a secondary magnetic field, which is
diamagnetic around the carbon atoms, but
paramagnetic in the region of the alkenes
protons.
Thus the protons will feel greater field
strength and hence resonance occurs at lower
applied field.

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Alkynes :
In alkynes, electronic circulation around
triple bond takes place in such a way that the
protons experience diamagnetic shielding effect.
When the axis of the alkynes group lies
parallel to the direction of the applied field, the Π
electrons are induced to circulate around the axis
in such a way that the induced magnetic field
opposes the applied field.
Thus the protons feel smaller field strength
(shielding) and hence resonance occur at higher
applied field.( low δ value)

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Aromatic compounds:
In case of benzene, loops of π electrons are
delocalized cylindrically over the aromatic ring.
These loops of electrons are induced to
circulate in the presence of the applied field
producing ring current.
The induced current is diamagnetic
(opposing the applied field) in the centre of the
ring and is paramagnetic outside the ring.
Thus the aromatic protons (around the
periphery of the ring) experience a magnetic field
greater than the applied field.

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4) Hydrogen Bonding
The hydrogen atom exhibiting the property of
hydrogen bonding absorbs at a lower field
compared to other
The proton if attached to highly Electronegative
atom they have smaller electron density around it
If less shielded the field by proton is more and
resonance occurs at downfield
Example : In case of OH group attached to ethanol

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HO-CH2-CH3
wo
low
field
high
field

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APPLICATIONS OF NUCLEAR
MAGNETIC RESONANCE
 DETECTION OF HYDROGEN BONDINGS
Intermolecular hydrogen bondings shifts
The absoprtion for a concern proton downfield
The extent of hydrogen bonding shifts the
Varies with the some factors
Intramolecular bondings also shifts the
Absorption downfield

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 DETECTION OF THE AROMATICITY
Protons attached Benzoyl, polynucelar and
Heterocyclic compounds, whose π electrons will
Follow Huckel’s rule [4 n + 2 ] rule , where
N = 1,2,3 ( whole number )are extremely
deshielded due to circulating the current ring of π
electrons
As the result , the signal for the aromatic proton
appear at low field than the observed compared
even for benzene, from this the aromatic
character of the compound under the
investigation can be predicted

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 DISTINCTION OF CIS – TRANS ISOMERS AND
CON FORMERS
The Cis and Trans isomers of a compound can be
Easily distinguished as the concenned protons
Have the different values of the chemical
Shifts as coupling constants
Ha
/ /
C = C C = C
/ /
Ha Hb Hb
CIS TRANS

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 DETECTION OF ELECTRONEGATIVE ATOM
OR GROUP
It is known that presence of electronegative group
Or atom in the neighborhood of proton
Cause deshielding and signal is shifted downfield
Grater the electro negativity of adjacent atom,
Smaller the value of absorption of concerned
Proton

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 DETERMINATION OF SOME DOUBLE BOND
CHARACTER DUE TO SOME RESONANCE
In some compounds molecules acquire a little
Double bond character due to resonance
Due to this, different signals are expected
These signals are due to the restricted rotation
of the formed double bond, which changes
The geometry of molecule
Ex:- N,N Dimethyl formamide

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 IN MEDICAL DIAGNOSTICS
One of the most important applications of
NMR in medical sciences is that of Magnetic
Resonance Imagining ( MRI), in which data
from pulsed radiofrequency excitation of solid
Or semisolid objects are subjected to Fourier
Transmission and converted to 3D of the
interior of the objects
It has found an easy way to detect cancer using
This technique is based upon the iron oxide
Nanoparticles which show upon the MRI Scan
Because iron has magnetic properties

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REFERENCE :-
1. Instrumental methods of chemical analysis
B.K Sharma
2. Elementary organic spectroscopy Y R Sharma

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Thank You