This document provides an overview of carbon-13 (13C) nuclear magnetic resonance (NMR) spectroscopy. It discusses the characteristics of 13C, including its low natural abundance and magnetic moment. It also describes the difficulties in 13C NMR spectroscopy related to sensitivity. The document outlines the features of 13C NMR spectra, such as chemical shift range and lack of 13C-13C coupling. Additionally, it explains proton-coupled 13C NMR spectroscopy and techniques to simplify complex spectra, such as decoupling, higher magnetic fields, and chemical shift referencing.

1. Seminar on c-13 nmr Spectroscopy
Presented by ,
NIVEDITHA G
1st M.Pharm
Dept. Of Pharmaceutics
NARGUND COLLEGE OF PHARMACY

2. INTRODUCTION
 Carbon-13 (13C) is a natural and stable
isotope of carbon.
 12C is the most abundant isotope of carbon
and is inactive since it has nuclear spin of
Zero.
 13C however has odd mass and is active with
nuclear spin ,with I=½
 In general any nucleus with a nuclear spin
½ will give rise to NMR signal provided the
appropriate magnetic field and radio
frequency radiations are applied.
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3. DIFFICULTIES ENCOUNTERED IN 13C NMR
1.Natural Abundance:-
 The natural abundance of 13C is only 1.1%
than that of 12C.
 As a result magnetic resonance of 13C is
much weaker i.e sensitivity is only 1.6% than
that of 1H nuclei which is not adequate for its
study.
2.Magnetic moment and Gyro magnetic ratio:-
 The magnetic moment of 13C is much weaker
due to low natural abundance of 13C nucleus.
 The Gyro magnetic ratio of 13C is 1.4043 as
compared to 5.5854 of a proton 1H .
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4. CHARACTERISTIC FEATURES OF 13C NMR
 The Chemical shift of CMR is wider (d is 0-
240 relative to TMS) in comparison to PMR (d
is 0-14 relative to TMS) .
 13C -13C coupling is negligible because of low
natural abundance of 13C in the compound.
The proton- coupled spectra, the signal for
each carbon of magnetically equivalent is
split by the proton directly to that carbon
and the n+1 rule is followed.
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5. 13C- 13C COUPLING
 The concentration of carbon-13 in a
compound is very small.
 Therefore chances of having more than one
carbon-13 nucleus in the same molecule are
small.
 Hence homonuclear coupling is rarely
observed.
 Therefore 13C- 13C coupling is ignored.
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6. PROTON COUPLED 13C SPECTRA
(13C-1H COUPLING)
Protons attached to carbon-13 atoms will
couple carbon nucleus to split the resonance
peaks observed for these carbon atoms
Fig shows how attached hydrogen atoms will
split a carbon-13 nucleus.
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7. SPIN-SPIN COUPLING OR SPLITTING
 The magnetic interaction that occurs between 2
or more proton, most often through the
intervening electrons there by result in splitting
of spectral lines is called spin coupling.
 It is related to number of possible spin
orientation of neighboring protons.
 The spacing of adjacent peaks in the multiplets
is a direct measure of spin coupling of protons
and is known as spin coupling constant,
designated as ‘J’.
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8.  If the NMR spectrum is very complex and
difficult to interpret by spin coupling the
simplification may be achieved by two ways:-
Use of higher magnetic field with high
homogeneity
Spin-spin decoupling
1.The chemical shifts are field dependent,
where as coupling constants are not
independent of field, therefore by use of
higher magnetic field we may simplify the
spectra.
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9. SPIN-SPIN DECOUPLING
 It is a technique for irradiating a nucleus with a
strong radiofrequency signal at its resonance
frequency while scanning other nuclei to detect
which ones are affected by decoupling from
irradiated nucleus.
 The process of removing the spin-spin splitting
between spins of a proton is called decoupling.
 Decoupling of a particular nucleus means
eliminating magnetic effect of that nucleus from
reminder of the molecule.
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10. DECOUPLING
 The decoupling simplifies 13C spectra by
removing the multiplet.
Principle:-
In addition to basic NMR spectrometer it
requires tunable radio frequency source to
irradiate a particular nucleus at a desired
frequency, while scanning nucleus.
It is also known as double resonance as
sample is irradiated by two frequencies:-
 Observation frequency
 Decoupling frequency
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11. TYPES OF DECOUPLING
 Homonuclear decoupling
 Heteronuclear decoupling
Homonuclear decoupling :- It is a selective
decoupling where the type of nucleus being
decoupled is same as that being observed.
Ex:- Decoupling one proton while observing other
proton.1H[1H]
Heteronuclear decoupling:-It is occurs when
decoupled nuclei are different from the observed
one.
Ex:- Decoupling of 1H while observing 13C. 13C[1H]
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12. Heteronuclear decoupling is of different types:-
 Proton decoupling or Noise decoupling
 Broad band decoupling
 Off resonance decoupling.
Proton decoupling or Noise decoupling:-
The coupling from 13C-1H makes the CMR spectrum very
complex.The coupling constant for 13C-1H is very large,
hence there is an overlap of multiplets.
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13. Principle
The spectra is recorded by irradiating the
sample at two frequencies:-
 First radiofrequency is used to affect
carbon magnetic resonance while
simultaneous exposure.
 Second signal causes all the protons in
resonance at the same time.
 As a result there is no coupling and each
proton appears as a single unsplit peak at he
position corresponding to its chemical shifts.
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14. SPECTRA IS OBTAINED AS FOLLOWS
Irradiate the sample at two frequencies of proton
resonance.
First to affect proton magnetic resonance i.e 80
MHz
Second to resonance all proton at the same time.
13C-1H coupling effect in spectrum can be
eliminated by decoupling the hydrogen nuclei .
Decoupling is carried out by applying the
decoupling signal .
This has all the H+ frequencies and is therefore a
form of radiofrequency noise.
Thus the obtained spectrum is proton or noise
decoupled.
Ex:- Proton decoupled spectra of Diethyl phthalate
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16. BROAD BAND DECOUPLING
 In this, all the protons resonance can be reduced
to get sharp CMR peaks each directly reflecting a
13C chemical shift.
The NMR spectrum of nucleus “A” is split by nucleus
“B” because A can see B in different magnetic
orientation .
The splitting will be maintained only if the life time
of B in any spin states is sufficient long for coupling
to be measured.
 If the double irradiation is sufficiently intense, the
transition will be rapid and lifetime of specific spin
states is shortened.
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17.  13C NMR spectra can be simplified by
simultaneous decoupling of all the protons of
carbon, therefore it is called as Broad band
decoupling.
 This technique simplifies the spectrum and
increases the peak height of spectra.
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18. OFF RESONANCE DECOUPLING
 The technique that is used to obtain typical proton
decoupled spectra that all peak becomes singlets.
 It is achieved by off setting the high power proton
decoupler by:-
 About 1000-2000Hz upfield or 2000-3000Hz
downfield from the frequency of TMS without using
noise generator.
 In this, sample is irradiated at a frequency close to
resonance frequency of protons.
 Consequently, the multiplets become narrow and not
removed altogether as in fully decouple spectra.
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19.  In this methyl carbon atoms appear as
quartets, methylene as triplets, methines as
doublets and quarternary as singlet.
 In conclusion, off resonance decoupling
simplifies the spectrum by permitting some
splitting information.
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20. Ex:- Off resonance CMR spectra of 1,2,2-
trichloropropane.
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21. RELAXATION PHENOMENON
 The transitions occurring from one spin state
to another is directly proportional to state
from which transition takes place.
 However while lower energy state is more
populated than upper energy state, upward
transition predominates slightly.
 Hence absorption of energy from radiation
beam takes place.
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22. The excited nucleus can undergo energy loss
by transition from high energy state to low
energy state is called Relaxation.
 The energy difference E can be reemitted by
nucleus as radio frequency radiation.
 There are 2 types of relaxation process:-
1. Spin-Lattice relaxation
2. Spin-Spin relaxation
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23. CHEMICAL SHIFTS IN 13C NMR
Chemical shifts of 13C nuclei are expressed in
units of parts per million(ppm).
The range of 13C chemical shifts is much greater
200ppm compared to 20ppm range of proton
chemical shifts.
Carbons of the methyl groups of TMS are used for
reference.
Because of wide spread of signals it is unlikely
that two 13C nuclei will have identical chemical
shifts.
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24. PROPERTIES
 13C spectra has long relaxation times means
no integrations.
 Normal 13C spectra are broadband, proton
decoupled, so the peaks show as single lines.
 Number of peaks indicates the number of
types of carbon.
 Decoupling simplifies the spectrum and
increases peak height.
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25. The chemical shift correlation chart is divided
into 4 sections:-
 Saturated carbon atoms with hydrogen appears
at highest field(upfield) nearest to TMS (8-
60ppm) .
 Second chart demonstrates the effect of
electronegative atoms (40-80ppm).
 Third includes alkenes and aromatics ring
carbon atoms (100-175ppm).
 Fourth contains carbonyl carbons appearing at
lowest field (downfield) values (155-220ppm).
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26.  Elementary Organic Spectroscopy by
Y.R.Sharma.
 Internet source
 Instrumental methods of chemical analysis by Dr
B.K.Sharma.
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