NMR

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NMR

• Nuclear Magnetic Resonance

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1H, 13C, 15N, 19F, 31P
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One Level is more populated than the
other Therefore a Resulting Net Magnetization
will be Macroscopically Observable
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CW Continous Wave
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The Exact Resonance Frequency Varies with the
Chemical and Electronical Environment of the
Nucleus This is Called the Chemical Shift d
(measured in ppm)
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The Chemical Shift is Measured Relative to a
Reference or Standard ppm parts per million
d(w-wref)/wref 106 The most common standard
for 1H and 13C NMR is
TMS Tetramethylsilan
CH3 CH3 Si - CH3 CH3
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NMR is
quantitative The intensity is
measured as the integral below the resonance
signal. The intensities are proportional to the
number of protons, that contribute to the signal.
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Each Particular Arrangement of Nuclei has a
Characteristic Coupling Pattern n
Neighbors (n1) Lines
A B
B A B
C B A
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example
CH3
CH2

• No spin-spin coupling is observed if
• protons are separated by four or more single
  bonds, i.e.,  
• H-C-C-C-H
• protons are equivalent, i.e.,within a CH3 or CH2
  group

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Pick the molecule that gives rise to the
following 1H NMR spectrum!
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Assign the Signals to the Correct Hydrogens
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(almost) pure a D Glucose
50 - 50 a/b D Glucose
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FT NMR Pulse Echo NMR
Broadband Excitation Record Transient
Response FID Free Induction Decay Extract
Frequencies by Fourier Transform to get NMR
Spectrum
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More Complex Experiments Measurement of
Relaxation Times (Lifetimes) Inversion Recovery
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13C NMR Spectroscopy Sensitivity low only 1.1
natural abundance Complicated 1H coupling
pattern, no couplings between 13Cs (due to low
abundance) S/N can be improved by 1H broadband
decoupling and FT averaging
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3JHH coupling constants contain Geometrical
Information on Torsion Angles Karplus Relation
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Two Dimensional NMR Spectroscopy A Signal as a
function of two times becomes after 2D
FT Spectrum as a function of two frequencies
Usually presented as contour plot
COSY Correlated SpectroscopY
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Cyclosporin A
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Hb
CH3g/d etc
HN
Ha
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complex spectra can be resolved using
2dimensional NMR methods