Chemical shift and application of NMR.ppt

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
IstsemM PHARM
DEPT. OF PHARMACUETICS
NARGUND COLLEGE OF PHARMACY
1

2. WHATIS 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 3different signals were
Due to 3different 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 )
Therearetwotypesofprotons:-
1.ShieldedProtons:-
Those protons at which the induced
magnetic fieldOpposes the applied magnetic
fields and thus the proton felt less magnetic field.

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2.DeshieldedProtons:-
Thoseprotonsatwhichtheinducedmagnetic
fieldReinforcestheappliedmagneticfieldand
thusprotonfelthighermagneticfield.

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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 ISUSEDANDMOST 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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FACTORSAFFECTINGCHEMICAL
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)VanDerWall’sDeshielding:-
In overcrowded molecules, it is possible that
some proton may be occupying sterically
hindered position.
Clearly,electroncloudofabulkygroup
(hinderinggroup)willtendtorepeltheelectron
cloudsurroundingtheproton.Thissuchaproton
willbedeshieldedandwillresonateatslightly
highervalueofδthanexpectedintheabsenceof
thiseffect
Ex:-morphine,alkaloids,steroidsetc.

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3) Space Effect/ Anisotropic Effect:
Circulation of –πelectrons 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:
Analkenesgroupissoorientedthatthe
planeofthedoublebondisat90º(rightangle)to
thedirectionoftheappliedfield.
Theinducedcirculationofπelectrons
generatesasecondarymagneticfield,whichis
diamagneticaroundthecarbonatoms,but
paramagneticintheregionofthealkenes
protons.
Thustheprotonswillfeelgreaterfield
strengthandhenceresonanceoccursatlower
appliedfield.

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Alkynes:
Inalkynes,electroniccirculationaround
triplebondtakesplaceinsuchawaythatthe
protonsexperiencediamagneticshieldingeffect.
Whentheaxisofthealkynesgrouplies
paralleltothedirectionoftheappliedfield,theΠ
electronsareinducedtocirculatearoundtheaxis
insuchawaythattheinducedmagneticfield
opposestheappliedfield.
Thustheprotonsfeelsmallerfieldstrength
(shielding)andhenceresonanceoccurathigher
appliedfield.(lowδvalue)

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Aromatic compounds:
Incaseofbenzene,loopsofπelectronsare
delocalizedcylindricallyoverthearomaticring.
Theseloopsofelectronsareinducedto
circulateinthepresenceoftheappliedfield
producingringcurrent.
Theinducedcurrentisdiamagnetic
(opposingtheappliedfield)inthecentreofthe
ringandisparamagneticoutsidethering.
Thusthearomaticprotons(aroundthe
peripheryofthering)experienceamagneticfield
greaterthantheappliedfield.

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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-CH
2
-CH
3
w
o
low
field
high
field

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APPLICATIONS OFNUCLEAR
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
Intramolecularbondings also shifts the
Absorption downfield

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DETECTION OF THE AROMATICITY
Protons attached Benzoyl, polynucelar and
Heterocyclic compounds, whose πelectrons will
Follow Huckel’s rule [4n + 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