nucler magnetic resonance

1. BY
K.SUMA LATHA

2. TYPES OF DOUBLE RESONANCE
 A. Simple decoupling
 B. Differene decoupling

3. A.SIMPLE DECOUPLING
 Spin decoupling is a remarkably powerful tool for simplifying a
spetrum and for determining the relative positions of protons in a
molecule.
 This technique involves irradiation of a proton or a nucleus with an
intense radiofrequency signal at its resonance frequency to eliminate
completely the absorbed coupling to the neighbouring protons.
 A signal for a particular proton or a set of equivalent protons is split
up into a multiplet under the influence of the neighbouring protons
under different environments.

4. • Consider the compound
• In which and are in different environment. Clearly, two doublets
corresponding to each proton should be observed at different field strengths
in its NMR spectrum.
• If we irradiate Ha with appropriate frequency energy so that the rate of its
transitions between the two energy state (spin state ) become much larger,
then the life time of this nucleus in any spin state will be too short to resolve
coupling with Hb .
• In such a case, will appear as a singlet (not doublet). Similarly ,if we
irradiate with strong radiofrequency energy, then due to its rapid
transitions between the two spin states, will have one time average view
of Hb.

5.  Thus Hb will come to resonance only once and Ha
will also appear as a singlet. However, the time (dt)
required to resolve the two lines of a doublet is
related to the separation between the lines that is
coupling constant J.
 Hence, the formation of doublet in the above
example is possible if each spin state of Ha has a life
time greater than dt.
 With double irradiation, said life time beome still
less. So due to the increase in the rate of transition,
coupling is not possible and thus singlets result.

7. SPIN DECOUPLING:
 In this tehnique, we make simultaneous use of two two
radiofrequency sources .
 In addition to normal NMR instrument, a second turnable
radiofrequency sources is needed to irradiate Ha at the
necessary frequency and the recording of the spectrum is
done in the same way.
 Since the multiplet collapses to a singlet in the process is
called Spin coupling.

8. HETERONUCLEAR DECOUPLING:
 When the experiment involves nuclei of different elements
it is known as heteronuclear decoupling .
 There is one limitation in homonuclear decoupling that is the two signals
must be well separated, or the irradiating frequency V2 will disturb the
signal V1.
 The technique works for all kinds of multiplets, so that a double doublet
will collapse to a doublet when the sample is irradiated the resonance
frequency of one of the protons to which it is coupled.
 In a complex molecule, it is possible to irradiate successively at the
frequency of each of the signals in the spectrum, to plot the spectrum in
each case and to look at all the spectra to find which signals lose coupling
in each experiment.

9.  In this way the full coupling relationships can be
collected about the connections between methyl,
methylene and methine groups with in a
molecule.Consider NMR SPECTRUM OF
FURFURAL(furan 2-aldehyde) which is quite complex.

10.  This results in the collapse of each of the four line
multiplet (for Hb and Hc) into the doublet

11.  . The value of Jbc is very small as compared to Jac and
Jab in the 12 line spectrum.

12. LIMITATIONS:
 Due to greater strength of irradiaion used for
decoupling.
 Strong decoupling tehnique is not suitable when the
hemial shift between the two protons is small.So spin
tickling technique is applied.

13. (B)DIFFERENCE DECOUPLING:
 In an FT istrument, it is possible to use decoupling to
removal a buried signal. Shows a narrow part of the
methylene region of the spectrum of the following
steroid(A).

14.  The signal is in fact a composite of the multiplets from
your protons, one of them H7,overlapping
inextricably. When the signal of H6, which is further
downfield than the ones is irradiate,the signal from H7
 loses one of its coupling and the signal changes in
(b).

16.  Since the spectrum is in the comuter in digital form, it is
possible to subtract the original spectrum(c) from the
decopled spectrum(b). The result is plottedin (a).

17.  The other three protons were not coupled to H6alfa
and were unaffected by the decoupling.
 The signal left from H7alfa has both the coupled and
decoupled signal in it.
 The original coupled signal is down and the partly
decoupled signal is up.
 Evidently H7alfa is coupled equally to each of the
protons H6beta and H7beta with a coupling constant
of 13 Hz, leading to a quartet and to H6alfa with a
coupling constant of 4.3 Hz,doubling that quartet.

18. REFFERENCE:
 H.kaur