Double resonance Mpharm SEm I MPAT

1. Nuclear Magnetic Double
Resonance (Decoupling)
Presented by : Rushikesh Tidake
M.Pharm (1st Year )
Pharmacology
Rajarshi Shahu College of
Pharmacy
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2. 2
SPIN DECOUPLING
Decoupling is a special technique used in NMR
spectroscopy to avoid the splitting of the signals by
eliminating partially of fully the observed coupling.
Which involves the irradiation of a proton with
sufficiently intense radio frequency energy ,so that it
prevents the coupling with the neighbouring proton and
gives spectral line as a singlet.
Decoupling can help to determine the structure of the
chemical compounds.

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EFFECT OF DECOUPLING IN NMR SPECTRA
Decoupling is used to simplify a complex NMR
spectrum.
It is possible to irradiate each coupled protons in
the molecule to produce the spectrum with less
complexity.
Decoupling causes multiplet to collapse to a
doublet or singlet and give spectra which are easy to
interrupt.

4. 4
If we examine the proton NMR spectrum of pure ethanol,
it will look like NMR spectrum drawn above , however
most ethanol is found mixed in either acid or base,this
acid will readily react with alcohol , there by changing
the concentration of ethanol in the sample.This in term
affect the NMR spectrum.

5. 5
The base react with the alcohol,forming the aloxide
molecule , if we add a bit of water in which the hydrogen
will replace by deuterium so that we can labile the water
it will react with aloxide to form the final product ,
placing it into the proton NMR spectrum , we see that all
the 1H spectral signals disappear because it is exchanged
for deuterium.

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•When the ethanol molecule and the base or acid
exchange the hydrogen atom, there will be an equal
probability of new hydrogen having either a +1/2 or –1/2
spin,as this exchange takes palce the average spin of
hydrogen will be zero.

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•This means that since the spin of hydroxide hydrogen
is zero, it will not undergo spin-spin interaction with
adajacent methylene hydrogen in such case,the methyl
hydrogen is said to be spin decoupled.

8. 8
Decoupling by Irradiation
•Spin decoupling is useful phenomenon. It can
be used by NMR spectrum to enhance spectral
signals to get a clear distinction between
certain signals,

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•The range for coupling constant for cis molecule is 6-
12 Hz that for trans molecule is 12-18Hz. However the
given molecule contain a methyl group where hydrogen
atom undergo coupling with h1 and h2 atom. This in
turn leads to complex signal that involves much spectral
signal.
•What we can do simply the signal is irradiate the
methyl hydrogen with right frequency. This will cause
the methyl hydrogen to flip and change spins. The rapid
interconversion of spin up to spin down will lead to an
average spin of zero,therby removing coupling between
methyl hydrogen and H1 and H2.

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Thank you