How to observe NMR?
Time Domain
Frequency Domain
t
FT
υ
F
Continuum of Frequency
Excite all Resonances at
the same time
Short and high power
pulse irradiation at Fo MHz
Net
Magnetization
flipped to
xy plane
Bo
1
Excited Spin
How to observe NMR?
Precessing Spin
1D NMR Experiment
Receiver at position x picked up a varying electromagnetic signal
2
How to observe NMR?
Intensity decreases with time. Why?
Free Induction Decay (FID)
FT
Time Domain
Frequency Domain
3
Modern NMR Experiments
Multi-pulses: 90 degree , 180 degree or shaped pulses
Multi-channels : 1H, 15N, 13C (one channel one freq) with gradient z
Multi-dimensional: t1, t2, t3 (one dimension one t)
4
What is Multi-pulsed NMR
Experiments?
t
Short Pulse
Quantum
States
Mz
Bo
Bulk
Magnetization
(not quantized)
Excited Spin
5
Models for the description of NMR experiments:
1. Quantum Model: Density Matrix Analysis, Product operator
formalism, very powerful but difficult to use.
2. Vector Model: Classical formalism, too simple but easy to
visualize.
Net
Magnetization
flipped to
xy plane
Bo
x
x
y
y
inverted
Mz
90-x
My
Mz
180y
Spin Gymnastics!
By varying the power, time length (angle)
or phase (direction) of the pulse
-Mz
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Spin Gymnastics
Interconversion rules between different M
directions by applying 90 degree pulses
Phase of pulse
(thumb position of your right hand)
Starting
M direction
Mz
90y
Mx
Use Right Hand Rule
Mz
90z
Mz
Mx
90x
Mx
My
90y
My
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What is Multi-pulsed NMR Experiments?
Magnetization can be transferred between nuclei through
cross-talking by:
1. bonding: J coupling
(H-H, H-C, H-N, etc…)
2. Close in space: dipolar coupling
dAB
HA
C—H
N—H
HB
Therefore information related to the other nuclei
(e.g. chemical shifts δ, bonding, J-couplings, distances dAB)
can be encoded in the NMR signals as well.
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Mulit-Dimenstional NMR?
Single pulse 1D NMR Experiment
1. Preparation: Recycle delay for relaxation
2. Applying pulse
3. Acquisition of FID
Raw NMR data FID = F(t1) n1 points
t1
2D NMR Experiment
relaxation
J or δ coding in t
Exchange info
get FID
Raw NMR data FID = F(t1, t2) (n1 x n2 points)
One dimension One t
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Mulit-Dimenstional NMR?
F(t1)
F(t1,t2)
1D
FT
FT
F(w1)
F(w1,w2)
2D
F(t1,t2,t3)
FT
F(w1,w2,w3)
3D
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N2 points
COSY: Correlation Spectroscopy
F(t1,t2)
FT(t2)
Sine wave
in t2
N1
F(t1,w2)
FT(t1)
F(w1,w2)
Ha J-coupled to Hb
δa
δb
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Homonuclear Experiment: Single channel, single resonance
2D COSY (directly J-coupled)
TOCSY or HOHAHA
(Total Correlation Spectroscopy)
(directly or indirectly J-coupled)
2D NOESY (thru space dAB)
(NOE Spectroscopy)
2D ROESY (Rotating frame NOESY)
(thru space dAB)
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Heteronuclear Experiment: Double resonance (e.g H, C)
H,C-Hetero-COSY (directly H-C J-coupled)
DEPT (Distortionless Enhancement
via Polarization Transfer)
Detecting C
Inverse experiment: magnetization is transferred back to the more
sensitive H
HSQC (Heteronuclear Single Quantum
Correlation) (directly H-C J-coupled)
HMQC (Heteronuclear Multi-Quantum
Correlation) (directly H-C J-coupled)
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Heteronuclear Experiment: Triple resonance
Triple resonance: H, C, N
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Why NMR? So informative, so many applications and so much FUN!
2nd advantage of pulsed FT NMR: allow multi-dimensional NMR
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