Carbon 13

Presented By:
Arif Iqbal Shahid
Roll No.:
108

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

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

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

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

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

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

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

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

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

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

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

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

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

Proton off-resonance decoupling
Broad band decoupling
16

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

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

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

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

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

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

Carbon 13

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Carbon 13