Nuclear magnetic resonance (NMR) spectroscopy can detect certain atomic nuclei that have spin, including the carbon-13 isotope. While carbon-12 does not produce a signal in NMR due to having no spin, carbon-13 accounts for about 1.1% of naturally occurring carbon and can be detected. Carbon-13 has a very weak signal that is difficult to detect due to its low natural abundance and sensitivity being 1/5700 of hydrogen-1. However, the development of Fourier-transform NMR and signal averaging techniques have allowed the detection and analysis of carbon-13 NMR spectra.

2. Presented By:
Arif Iqbal Shahid
Roll No.:
108

3. Nuclear Magnetic Resonance
concern the magnetic properties of
certain atomic nuclei. It concern
the atoms having Spin Quantum
Number ( I ).
1

4. Some atoms have no spin. Atoms
having even atomic number and mass
number (12C, 16O).
Some other nuclei have spin. Atoms
having either odd atomic number or
mass number or both (1H,13C,19F).
2

5. 12C have no Magnetic Spin
and produces no NMR signal.
13C have Magnetic Spin I= ½
13C accounts for only 1.1% of
naturally occurring carbon
3

6. The low probability of adjacent 13C
atoms leads to no detectable carbon-carbon
splitting
No coupling between 13C and C.
 13C have weak signal
13C have strong coupling with H
 13C have weak coupling with C
4

7. Two techniques have been developed
to detect the 13C isotope in an organic
sample by NMR:
Signal averaging (increases instrument
sensitivity)
Fourier-transform NMR (increases
instrument speed)
5

8. increases instrument sensitivity. Any
individual 13C NMR spectrum is
extremely “noisy”, but when hundreds
of individual runs are added together
by computer and then averaged, a
greatly improved spectrum results.
6

9. 7

10. increases instrument speed.
In the FT-NMR technique, all
signals are recorded simultaneously.
All 1H and 13C in the sample
resonate at once, and the complex
composite signal is manipulated
using so-called Fourier transforms
before it can be displayed.
8

11. 9

12. The sensitivity of 13C is only
1/5700 of 1H;
This sensitivity problem is
overcome with Fourier
Transform (FT)
NMR instrumentation
10

13. It is unlikely that a 13C would be adjacent
to another 13C, so splitting by carbon is
negligible.
13C will magnetically couple with
attached protons and adjacent protons.
These complex splitting patterns are
difficult to interpret.
11

14. The CMR Spectra is
recorded in decoupling
condition.
The nuclei are decoupled
by double irradiation
12

15. Two Types of decoupling:
Homonuclear decoupling:
Decoupling of
same nuclei (C and C) or (H and H).
Heteronuclear decoupling:
Decoupling of
different nuclei C and H.
Broad Band Decoupling
Off resonance decoupling
13

16. Broad Band
Decoupling:  A sample is irradiated with two different radio frequencies.
 One to excite all 13C nuclei.
 A second broad spectrum of frequencies to cause all
hydrogen in the molecule to undergo rapid transitions
between their nuclear spin states.
 On the time scale of a 13C-NMR spectrum, each hydrogen is
in an average or effectively constant nuclear spin state, with
the result that 1H-13C spin-spin interactions are not
observed; they are decoupled.
 Thus, each different kind of carbon gives a single, unsplit
peak.
14

17. The Coupling can be achieved by off setting
the high power proton decoupler by about
1000-2000 Hz up field or about 2000-3000
Hz downfield from the frequency of TMS
without using the noise generator.
Groups signals
－CH3 quartets
－CH2－ triplets
Doublets
C H
A quarternary Carbon Singlet
15

18. Proton off-resonance decoupling
Broad band decoupling
16

19. The number of different signals
indicates the number of different kinds
of carbon.
The location (chemical shift)
indicates the type of functional group.
The peak area indicates the numbers
of carbons (if integrated).
17

20. 18

21. C-13 NMR has d 0 to 220 ppm (1HNMR
d 0 to 12 ppm)
CMR spectra is recorded under proton
decoupling condition
In the CMR multiplets, no need to
determine area ratios.
19

22. Hybridization of 13C atom:
–sp3 C signal is in the range 0-90 
–sp2 C signal is in the range 110-220

–C=O signal is at the low-field end, in
the range 160-220 
20

23. Characteristic Carbon NMR Chemical Shifts (ppm)
(CH3)4Si = TMS = 0.00 ppm (singlet) CDCl3 (solvent) = 77.0 ppm (triplet)
RCH3 0 – 40 RCH2Cl 35 – 80 benzene ring 110 – 160
RCH2R 15 – 55 R3COH 40 – 80 C=O ester 160 – 180
R3CH 20 – 60 R3COR 40 - 80 C=O amide 165 – 180
RCH2I 0 – 40 RCCR 65 – 85 C=O carboxylic acid 175 – 185
RCH2Br 25 - 65 R2C=CR2 100 - 150 C=O aldehyde, ketone 180 – 210
21

24. 22

25. Electro negativity of nearby atoms:
C bonded to O, N,
or halogen absorb downfield because O, N,
or halogen pull electrons away from
nearby 13C atoms, decreasing their
electron density and “deshielding” them.
23

26. 24

27. Example
25

28. Example
CH
H
H
CCH2CH2CH2CH3
H H
200 180 160 140 120 100 80 60 40 20 H
CH O
CH
C
O
C
CH2 CH2
CH3 CH2
26

29. Thank You