SPIN SPIN coupling, decoupling and shift reagents

1. Dept. of Pharmaceutics 1
Seminar on…..
Spin-Spin Coupling
De-Coupling &
Shift Reagent
Presented by:-
NIVEDITHA G
1st sem Mpharm

2. Spin Spin Coupling
The interaction between the spins of the
neighbouring nuclei in a molecule may cause
the splitting of lines in the NMR spectrum.
This phenomenon of signal splitting is called
SPIN-SPIN COUPLING or SPLITTING
This splitting is caused by the influence of
the spin states of neighbouring protons
transmitted through chemical bonds.
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4. The number of lines (multiplicity) observed in
the NMR signal for a group of proton is not
related to number of proton in that group ; the
multiplicity of line related to the number of
proton in neighbouring group.
A simple rule (n+1) is used to find the
multiplicity.
n= no. proton in neighbouring group.
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5. SPECTRUM OF ETHYL IODIDE (CH3CH2I)--
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6. Methylene protons are split into a quartet (four peaks) &
The methyl group is split into triplet (three peaks).
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7. Cause of Splitting--
 Splitting of signal can be determined by the environment of
the absorbing proton with respect to neighbouring proton.
 Because of different spin states of neighbouring protons ,
the actual magnetic field experienced by given proton may
be modified in number of way.
 Absorbing proton may experience each of the modified field
& it’s absorption signal may be split up into different peaks.
 This is the spin-spin coupling of absorbing and neighbouring
proton which is responsible for splitting of peak.
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Fig:-Splitting by one adjacent proton.

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Fig:-Splitting by two adjacent proton.

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12. Spin-Spin De-Coupling
The process of removing the spin spin splitting
between spins of a proton is called decoupling.
In this technique, radio frequency source is needed
to irradicate the proton at the necessary frequency
& recording of spectrum is done in the same way,
it is called “double resonance” or “double
irradiation”
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13. Since multiplet collapses to a singlet in the
process.It is also called spin decoupling.
 Irradiate
CH3 - CH2 – CH2 OH  CH3 – CH2 – CH2OH
Triplet sextet triplet triplet quartet
 Irradiate
CH3 – CH2 - CH2OH  CH3 – CH2 – CH2OH
Singlet Singlet
 Irradiate
CH3 - CH2 - CH2OH  CH3 – CH2 – CH2OH
Triplet Triplet
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14. 1. Field sweep method:
 In this method both the applied frequency (Vl)
&the irradiation frequency (V2) are Kept
constant the magnetic field is varied until a chara
cteristic change occurs in the lines of particular-
multiplies.
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METHODS OF DECOUPLING

15. 2. Frequency sweep method:
 In this method the applied magnetic field and the
irradiation frequency (VI) is kept constant and the
applied frequency is varied the irradiation
frequency (V2) be changed after each scan,
permitting an extensive investigation of spin spin
coupling.
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16. Types of decoupling
Off resonance de- coupling.
Proton decoupling or noise decoupling.
Inter nuclear double resonance decoupling
Coherent and broadband decoupling.
Spin tickling
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17. Off resonance de- coupling
Off resonance decoupling can be define as partial
decoupling of one type of nuclei while observing the
spectrum of another type of nuclei by applying
radiofrequency field not exactly equal to the larmour
frequency of that nucleus i.e. the sample is irradiated at a
frequency close to but not coinciding with the resonance
frequency of protons as a result the multiplets become
narrowed not removed all together as in fully decoupled
spectra.
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18. Irradiate the sample at two frequencies of proton
resonance.
The first frequency to affect the proton magnetic resonance
i.e. 80MHz.
The second frequency to resonance all proton at the same
time E.g. 80MHz.
Decoupling is carried out by Appling the decoupling signal
which has all the H+ frequencies (i.e. around 80-100Hz)
and their fore a form of radio frequency noise thus the
obtained spectrum is proton decoupled or noise decoupled.
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Proton decoupling or noise decoupling

19. Shift Reagent
Shift reagents, provides a useful technique for
spreading out PMR absorption patterns which
normally overlap, without increasing the strength
of the applied magnetic field.
The PMR spectrum of n-hexanol is reproduced in
figure, which is normal record.
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20. When the same spectrum is recorded after the
addition of a soluble europium (111) complex, Eu
(DMP)3i.e, the shift-reagent the spectrum is
spread out over a wide range of frequencies so that
it is new simplified almost to first order.
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22. REFEREENCES
Y.R. Sharma. Elementary Organic
Spectroscopy. 4th Ed. 2008;201-32.
G.R. Chatwal. Instrumental methods of
chemical analysis.5th Ed. 2008;2.198-2.202.
Internet sources.
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THANKS