09/09/2014
SPIN COUPLING IN NMR
SPIN DECOUPLING
IN NMR
In order to prevent coupling between
nuclei one needs to shortens the life
time τ1. This can be achieved by
simultaneously irradiating the nucleus
to be decoupled by a second
frequency ν2. Irradiation will saturate
the particular nucleus.
In order for spin coupling to occur the life time τ1
must satisfy the following condition:
τ1 > 1/J (s)
For a typical proton coupling of 10 Hz, this means
that the life time τ1 must be greater than 0.1 sec. In 1H and 13C NMR spectroscopy this condition is
usually satisfied and the couplings can therefore be
observed. Spin decoupling
in 1H NMR
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09/09/2014
SPIN DECOUPLING IN 2-CHLOROPYRIDINE
BROAD BAND DECOUPLING IN 13C
NMR In BROAD BAND DECOUPLING an rf
generator provides a band of frequencies
extending over the entire 1H-NMR spectrum.
When JC-H couplings are eliminated, however,
it becomes very difficult to assign 13C peaks
with proper Carbon atoms in the molecule. COUPLING CONSTANTS
JHH (Hz)
JCH(Hz)
JCC(Hz)
1J
276
125-250
30-80
2J
0-30
-10-+20
<20
3J
0-18
1-10
0-5
BROAD BAND
DECOUPLING
Broad band decoupling increases signal intensity
and simplifies the spectrum.
However, it becomes much
difficult to identify the specific carbon atoms and there may be some
distortions in signal
intensities.
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09/09/2014
Decoupled
13C-NMR
SPIN DECOUPLING
IN 13C-NMR
spectrum of diethylphthalate
Completely coupled
Off resonance
H3 C-CH=CH-COOH
4 3
2
1
C-H coupling
OFF-RESONANCE DECOUPLING
Completely decoupled
Yields simplified spectrum yet retains residual 13C-H
coupling.
Achieved by offsetting the central frequency of BB
proton decoupler by about 1000-2000 Hz upfield
or about 2000-3000 Hz downfield from the proton
frequency of TMS.
This results in residual coupling of protons directly
bonded to the 13C atoms while the long range coupling
is lost.
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09/09/2014
13C
13C
SPECTRUM NMR EXAMPLE
13C-NMR
EXAMPLE
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09/09/2014
EXAMPLE COMPOUND
CHarom, JHP = 0
OH, JHP = 0
NMR Active nuclei:
CH3, JHP=0.6 Hz
CH-P, JHP = 28.7Hz
CH2, JHP=8.0 Hz
1H, I = 1/2
13C, I = 1/2
31P, I = 1/2
17O, I = 5/2
CH2
31P
Spectrum
JPH = 8.0 Hz
JPH Decoupled spectrum
CH
CH2
JHH = 7.1 Hz
JPH Undecoupled spectrum
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09/09/2014
JPH
1 ppm = 600 Hz
-OCH2
CH2CH3
1 ppm = 400 Hz
1 ppm = 200 Hz
Presaturation using
digital filter
INTEGRATION OF COMPOUND 1
H2O signal presaturated
D2O/H2O
D2O
CDCl3
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09/09/2014
DETECTION OF SIGNAL
T1 TIMES FOR COMPOUND 1
13C
NMR OF COMPOUND 1
1H DECOUPLED)
13C
CHEMICAL SHIFTS AND JP-C COUPLING
CONSTANTS
Chem Shift (ppm)
147.1 d 125.5 s 110.9 s 106.5 d 65.2 d 16.7 d JCP (Hz)
2.3
0 0 201.3
7.2
5.5
Assignment
Carom.O
Carom. Cl
Carom. H
CH-P
OCH2
CH3
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09/09/2014
13C
OF COMPOUND 11
13C
SIGNALS OF Carom.Cl
GATED DECOUPLING
2J
Doublet of doublet
1H
3J
CCH
= 5.4 Hz
CCCH = 7.9 Hz
BB DECOUPLED
Decoupled spectrum
13C
SIGNAL OF METHYL GROUP 31P
NMR OF COMPOUND 1
BB Decoupled
(only 3JPOCC present
Selective decoupling
of OCH2 protons
Undecoupled
1J
= 127.7 Hz (q)
POCC = 5.5 Hz (d)
2J
CCH = 2.7 Hz (t)
3J
CH
2J
HCP
3J
HCOP
= 28.7 Hz (d)
= 8.0 Hz (t)
Proton undecoupled
Proton decoupled
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