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Nmr Spectroscopy Nmr Spectroscopy
Chemistry 330
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ObjectivesObjectives
Nmr basics chemical shifts relaxation times
2-Dimensional Nmr experiments COSY NOESY
What kind of information do we obtain?
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Nmr BasicsNmr Basics
The signal in a nmr spectrum arises from transitions between nuclear spin states. 1H, 13C, 31P all have a nuclear spin quantum
number, I = 1/2. The total number of spin states
2I + 1 = 2
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Energy Levels in the Spin 1/2 Energy Levels in the Spin 1/2 SystemSystem
-1/2
+1/2
oJ
oJ
B
BhU
2
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The Nmr SignalThe Nmr Signal
(Hz)
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The Magnetisation VectorThe Magnetisation Vector
We can represent the slight excess of spins in the state by use of the nuclear magnetization vector, Mo
x
y
z
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Shielding Shielding
The presence of the bonding electrons about the nucleus gives rise to ‘electron shields’ around the nucleus Shielding constant
)1(BB oeff
)1(2
oH B
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The Nmr Spectrum with The Nmr Spectrum with ShieldingShielding
(Hz)
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The Chemical ShiftThe Chemical Shift
Resonance frequencies are field dependent
Define a field-independent parameter - the chemical shift ()
o - magnet strength in MHz
)1(2
oH B
6
o
10 x )ppm(
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The Chemical Shift SpectrumThe Chemical Shift Spectrum
(ppm)
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J-Coupling (spin-spin splitting)J-Coupling (spin-spin splitting)
What happens when we have non-equivalent protons on adjacent C atoms?
H Ha b
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The Coupling Constant The Coupling Constant
(ppm)
Jab (Hz)
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The COSY ExperimentThe COSY Experiment
COrrelation SpectroscopY What happens when we project the
following spin system in two-dimensions?
Jab JbcHa Hb Hc
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The COSY SpectrumThe COSY Spectrum
(
ppm
)
(ppm)
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The NOESY ExperimentThe NOESY Experiment
In the 2-D NOESY (nuclear Overhauser enhancement) experiment, we look for ‘through space’ dipolar couplings.
Ha
Hb
Hc
Jab > 0
Jac = 0
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The NOESY SpectrumThe NOESY Spectrum
(
ppm
)
(ppm)
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The Origin of Spin-Lattice The Origin of Spin-Lattice RelaxationRelaxation
By using a selective pulse (a 180 pulse), we can invert the populations of the nuclear spin states
y
x
z
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Spin-Lattice RelaxationSpin-Lattice Relaxation
The spin system attempts to re-establish the equilibrium magnetisation vector. We observe the magnitude of the magnetisation vector as a function of time after the inversion pulse is applied.
(180 - n - 90 - acquire)n
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1
y
x
z
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2
y
x
z
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3
y
x
z
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4
y
x
z
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5
y
x
z
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6
y
x
z
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7
y
x
z
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8
y
x
z
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Magnetisation Intensity vs. Magnetisation Intensity vs. TimeTime
-60
-40
-20
0
20
40
60
0 10 20
( arb. units)
M
(arb
. un
its)
M
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The Spin Lattice Relaxation The Spin Lattice Relaxation TimeTime
The time constant governing the decay process is the spin-lattice relaxation time, T1
M = limiting value of magnetisation intensity
M = magnetisation intensity at t =
1T
M2lnMMln
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Nuclear Overhauser Enhancement Nuclear Overhauser Enhancement (NOESY) Experiments(NOESY) Experiments
2-D Nmr NOESY experiments have been used extensively in the determination of the conformations of nucleic acids, proteins, and membranes.
The presence of the cross peak in the 2-D NOESY spectrum indicates the presence of intermolecular or intramolecular dipolar interactions (representing a spatial proximity of < 0.50 nm).
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NOESY Spectrum for NOESY Spectrum for SDS/CSDS/C44OHOH
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NOESY Spectrum for SDS/BzNOESY Spectrum for SDS/Bz
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NOESY Spectrum for NOESY Spectrum for DTAB/BzDTAB/Bz