This document provides an overview of C-13 NMR spectroscopy. It discusses the background of C-13 NMR, how it compares to H-1 NMR, chemical shifts, coupling and decoupling techniques, the NOE effect, Fourier transform methods, and examples of restricted rotation. The key points covered are: - C-13 NMR provides information about chemically nonequivalent nuclei and their chemical environments. It differs from H-1 NMR in abundance, chemical shift range, and coupling. - Tetramethylsilane (TMS) is used as the reference standard for both H-1 and C-13 NMR. - Coupling between nuclei allows their environments to be determined, while decoupling provides separate spectra for each

1. • Introduction
• Background to C-13 NMR
• Comparison C 13 & H NMR
• Chemical shift
• Coupling & Decoupling of C 13 NMR
• NOE effect
• FT technique
• Restricted rotation
• Reference
16/27/2013

2. Nuclear magnetic resonance spectroscopy, most commonly known as NMR
spectroscopy, is a research technique that exploits the magnetic properties of certain
atomic nuclei to determine physical and chemical properties of atoms or the
molecules in which they are contained. It relies on the phenomenon of nuclear
magnetic resonance and can provide detailed information about the structure and
chemical environment of molecules
300 mhz 900 mhz
2

3. C-12 C-13
Spin number = zero
Even mass and even
Charge
NMR inactive
Spin number = ½
Odd mass & odd charge
NMR active
Nature abundance of C-13-
-is very low
3

4. Both are given information about the number of chemically
nonequivalent nuclei.
Both are given information about the environment of nuclei .
H - 1 C - 13
Abundance 99 % 1 . 1 %
Chemical shift 0 – 15 ppm 0 – 220 ppm
reference TMS TMS
Coupling Yes No
4

5. Can use TMS (tetramethylsilane) for organic solvents; functions as both H-1 and C13
reference.
Pri. Alkyl 0 – 40 ppm
Sec. alkyl 10 – 50 ppm
Ter. alkyl 15 – 50 ppm
Alkyl halide 10 – 65 ppm
Alcohol & ether 50 – 90 ppm
Alkenes aromatics 100 – 170 ppm
Amides 150 – 180 ppm
Carboxylic acid or Aldehydes &ketones 160 – 185 or 183 – 215 ppm
5

6. 6

7. 7

8. 8

9. 9(Free induction decay)

10. 10
• In this method the hydrogen nuclei are “saturated”, a
situation where there are as many downward as there are
upward transitions .
• During the time the carbon-13 spectrum is being determined,
the hydrogen nuclei cycle rapidly between their two spin
states (+1/2 and -1/2) and the carbon nuclei see an average
coupling (i.e., zero) to the hydrogens.
• The hydrogens are said to be decoupled from the carbon-13
nuclei.

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{ Y = gama }

12. Where is the magnetogyric ratio of the nucleus being
irradiated , and is the nucleus being observed
NOE is the enhancement of the signal and it must be added
to the original signal strength :
Total predicted intensity ( max ) = 1+ NOEmax
y
y
irr
obs
When a specific nucleus is magnetically excited and its
neighbor is at equilibrium, relaxation occurs between the
two nuclei. Because of this relaxations, the intensity of the
nucleaus that was at equilibrium changes. For a carbon and
its neighboring proton it is possible to observe an almost
four-fold increase in signal stranght resulting from
decoupling.
12

13. At room temperature a near sample of di methyle formamid
show two peaks because the rate of rotation around the
Hindered partial double bond is slow . At 123 degree C , the
Rate of exchange of the two groups is rapid enough so
that the two peaks merge .
CH
CH
3
3
N C
O
H
…….
_
(b)
(a)
+
O
_
(b)
D M F ( dimethyl formamide )
3
3
13

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You don't have to know much about NMR to realize that the
presence of three signals of equal intensity suggests that the three
methyl groups in N,N-dimethylacetamide are different.
If the formula in the inset represented the true structure of N,N-
dimethylacetamide, you should expect free rotation around the
sigma bond between the nitrogen atom and the carbonyl carbon.
This rotation would make the two methyl groups attached to the
nitrogen atom identical.
In that case, the NMR spectrum of N,N-dimethylacetamide should
contain two signals, one for the methyl group attached to the
carbonyl carbon, and a second for the two methyl groups attached
to the nitrogen. The intensity of the second signal should be twice
that of the first. Clearly that is not the case. Something must be
restricting the rotation around the C-N bond.
Restricted Rotation in DMA

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In our discussion of resonance theory, we learned that one of
the structural features required for resonance involved a non-
bonded pair of electrons adjacent to a pi bond. This situation
occurs in amides, where the lone pair of electrons on the
nitrogen atom interacts with the pi bond of the carbonyl group
Structure C represents the resonance hybrid that is formed by mixing structures A and B.
The partial double bond in the hybrid structure C implies a barrier to rotation around the
bond between the nitrogen atom and the carbonyl carbon atom. If that barrier is great
enough, the two methyl groups attached to the nitrogen atom would be Non equavilent;
one of them would always be adjacent to the methyl group attached to the carbonyl
carbon, while the other would be next to the carbonyl oxygen atom. Then the NMR
spectrum of N,N-dimethylacetamide should contain three signals, one for the methyl
group attached to the carbonyl carbon, and one for each methyl group attached to the
nitrogen. The intensities of the three signals should be the same

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17. F T instrument :-
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• Spectroscopy by Pavia ,Lampman ,Vyvyan
• Spectroscopy by Kemp
NMR spectroscopy and polymer microstructure Alan Tonelli
Nmr Spectroscopy of Polymers Roger N. Ibbet
Proton and Carbon NMR Spectra of Polymers, Volume. 2,.
Quang-Tho. Pham, Roger Petiaud, and Hugues Waton, Wiley,
Proton and Carbon NMR Spectra of Polymers, Vol.1, Heyden,
Philadelphia 1981. Silverstein rm, Bassler gc,
Carbon-13 NMR Spectroscopy of Biological Systems Nicolau
Beckmann
...
.
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