7- Double Resonance
1. Types of double resonance experiments
2. 1H-{1H} Homonuclear Decoupling
3. 13C-{1H} Heteronuclear Decoupling
Introduction
• The basic NMR experiment consists of placing the sample in a
static magnetic field, Bo, then subjecting it to a second RF pulse
of field, B1, at a position 1 in order to excite a particular set of
nuclei of a given magnetogyric ratio, .
• We can increase the number of elements in this experiment by
imposing a second RF field, with strength B2 at a position of 2
which perturbs the spin population and yields information
regarding spin interactions. B2 can be homonuclear (irradiation
of a nucleus of the same ), or heteronuclear (different ). This
comprises a very important group of experiments called double
resonance.
07- Double Resonance (Dayrit)
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Introduction
Parameters of a double resonance experiment:
1. homonuclear or heteronuclear;
2. position of 2 irradiation field;
3. strength of B2 irradiation field; and
4. manner of application of B2 (i.e., continuous or modulated).
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1. Types of double resonance experiments
Depending on how these parameters are selected, one can observe
any or a combination of the following phenomena:
1. weak B2: selective population transfer (SPT) or selective
population inversion (SPI): These are used to determine energy
level arrangements and are important building blocks for more
complex pulse sequences.
2. strong B2: decoupling (homonuclear or heteronuclear): This
enables spectral simplification and identification of coupling
partners. Broadband 1H decoupling is standard in 13C NMR
spectroscopy.
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Types of double resonance experiments
3. solvent suppression (1H): Used especially in aqueous solution.
4. nuclear Overhauser effect, nOe (1H): Very important for the
determination of spatial relationships.
5. saturation transfer (1H): Technique for studying chemical
exchange (migration of protons in solution).
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Thetypeof doubleresonanceexperiment depends onthestrengthof the
irradiatingfrequency, B2. Irradiatednucleus is1H. (from: Shaw, Fourier-Transform
NMRSpectroscopy)
Strengthof B2*(Hz)
(1H/2) B2Jor
(1H/2) B21/T2
(1H/2) B2J
(1H/2) B2n.J
Effects observed
Type
additional splitting
spintickling
selectiveperturbationof spin
coupling
selectivepopulation
transfer or inversion
(SPTor SPI)
completeremoval of spin
coupling
spindecoupling
changeinrelativeintensity
generalizednuclear
proportional todistancefrom Overhauser effect
irradiatednucleus
(nOe)
*(1H) =26.8x107T-1s-1; B2isinunits of T.
[(1H/2) B2]2 1/(T1.T2)
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ield(T
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its
07- Double Resonance (Dayrit)
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2. 1H-{1H} Homonuclear Decoupling
• Spin-spin coupling is a localized interaction due to the
proximity of nuclei possessing spin quantum number I >0.
• For a coupled two spin system, AX, four spin energy
levels are generated, AX, AX, AX, and AX, where
the first spin refers to the orientation of the A nucleus, and
the second spin refers to the X nucleus.
AX
AX
AX

AX
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Spin population
In a coupled spin system, the spin population will be distributed
according to the Boltzmann distribution. In the figure below, the
populations are approx: AX >  ~  > . (The bars
represent the spin populations.) The magnitude of the energy
differences depends on the strength of the coupling, and
whether the system is homonuclear or heteronuclear.
4

X2
A2
2
3

X1
1
1,2

A1
3,4
E
X
A
E

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Decoupling
Decoupling involves irradiation at the center of a multiplet with
enough power to cover the J coupling width.
Here, the transitions A1 and A2 are being irradiated. This
removes the signal due to A and equalizes the populations 1=3
and 2=4, making X a singlet.

decouple
4
X2
A2
2
3

X1
1

1,2
3,4
1,3
E
X
A1
A
E

decouple
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Theory of Homonuclear Decoupling, 1H-{1H}
• In the decoupling experiment, one shortens the lifetime of spinspin contact, , (i.e., there is an increase in the rate of spin
flipping) by the simultaneous imposition of a second RF field at
2 with sufficient power, B2, to excite the nJ multiplets of one
nucleus. This has the effect of removing this particular spin-spin
interaction from its coupling partner.
• For example, to completely decouple a 1H-1H AX doublet with J
= 5 Hz, one needs a B2 field strength of:
(/2)B2  n . J
(26.8 x 107 T-1s-1/ 2) B2  2 x 5 Hz
B2  2.34 x 10-7 T (or 2.34 x 10-3 G)
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Spin-spin decoupling for 1st order, weak coupling
AX spin system
J
AX

X
irr

J
AX X

A

X

A
AMX spin system
J
AX
J
AM
AM J
J
MX J
AX
J
J
MX AM
J
AM
irr

X

X

M

A

X
07- Double Resonance (Dayrit)

M

A
12
Pulse sequence for homonuclear (1H-1H) decouping and
selective heteronuclear (1H-13C) decoupling.
1)
H
o
m
o
n
u
c
l
e
a
rd
e
c
o
u
p
l
i
n
g
(
h
o
m
Homonuclear
decoupling,
H {1H}
13
1H}
S
e
l
e
c
t
i
v
e
h
e
t
e
r
o
n
u
c
l
e
a
rd
e
c
o
u
p
l
i
n
s
e
l
)
Heteronuclear
decoupling
Cg
{(
07- Double Resonance (Dayrit)
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3. 13C-{1H} Heteronuclear Decoupling
Jacobsen, 2007
• In a standard 13Cspectrum, we want to
decouple all of the protons
simultaneously using least energy.
• We utilize a “broadband” decoupling
sequence that covers the entire range of
1H chemical shifts (typically from -5 to
15 ppm, a width of 8000 Hz on a 400
MHz instrument).
• Current methods employ a composite
pulse sequence which is a series of
several pulses designed to give an overall
rotation that is not dependent on the
resonance offset of the 1H signals.
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Jacobsen, 2007
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Broadband 13C{1H} decoupled
No decoupling
Selective decoupling
Jacobsen, 2007
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