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DOUBLE RESONANCE IN NMR
PRESENTED BY GUIDED BY
PRIYANKA GOSWAMI Dr. RAJA CHAKROBARTY
M.PHARM ASSOCIATE PROFESSOR
DEPARTMENT OF PHARMACY DEPARTMENT OF PHARMACY
ASSAM DOWN TOWN UNIVERSITY ASSAM DOWN TOWN UNIVERSITY
• Decoupling is a special technique used in NMR spectroscopy to avoid the splitting of the signals
by eliminating partially or fully the observed coupling.
• Which involve the irradiation of a proton with sufficiently intense radiofrequency energy, so that
it prevents the coupling with the neighbouring proton and gives spectral line as a singlet.
• Decoupling can help to determine structures of chemical compounds.
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
• Decoupling causes the multiplet to collapse to a doublet or singlet and give spectra which are
easy to interrupt
• If we examine the proton NMR spectrum of pure ethanol , it will look like the 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 turn affect
the NMR spectrum
• The base react with the alcohol, forming the alkoxide molecule, if we add a bit of water in which
the H is replace by deuterium so that we can labile the water it will react with alkoxide 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.
• When the ethanol molecule and the base/acid exchange the hydrogen atom, there will be an equal
probability of the new hydrogen having either a +1/2 or -1/2 spin, as this exchange takes place
the average spin of hydrogen will be zero.
• This means that since the spin of hydroxide hydrogen is zero, it will not undergo spin-spin
interaction with the adjacent methylene hydrogen. in such case, the methylene hydrogen is said to
be spin decoupled.
DECOUPLING BY IRRADIATION
• Spin decoupling is useful phenomenon. It can be used by an NMR spectrum to enhance spectral
signals to get a clear distinction between certain signals
• The range for coupling constant for cis 6-12 that for trans molecule is 12-18 Hz. 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 involve 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, thereby removing coupling between methyl hydrogen
and H1 and H2.