NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY
Radio waves do the trick.
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Nuclei can be thought of as tiny magnets.
Bo
No external magnetic field
Spin alignment random
With external magnetic field
Spins aligned
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When a nucleus occupying the α spin state is subjected to radio waves, an absorption can
take place.
energy
β spin state
DE
magnetic field
is applied
energy
α spin state
DE
DE
Magnetic field strength
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1H
NMR SPECTRUM
An NMR spectrum is a plot of resonance frequency vs. the intensity of rf absorption by the
sample.
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NUMBER OF SIGNALS
homotopic
enantiotopic
diastereotopic
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Exercise 1:
Identify the number of signals expected in the 1H NMR spectrum of the following
compounds.
1 signal
4 signals
4 signals
1 signal
5 signals
2 signals
2 signals
3 signals
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Exercise 2:
Determine whether the two protons shown in red are homotopic, enantiotopic, or
diastereotopic.
diastereotopic
diastereotopic
homotopic
enantiotopic
homotopic
enantiotopic
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POSITION OF SIGNALS
The x-axis is a frequency scale but is normalized to be independent of the field strength.
spectrometer operating at 300 MHz
spectrometer operating at 60 MHz
2181 Hz larger than that of TMS
436 Hz larger than that of TMS
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deshielded
shielded
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The different degree of shielding experienced by the protons is due to electron density.
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The Shoolery’s additivity rules:
0.6 + 0.3 + (20% of 2.5) = 1.4
0.6 + 2.5 + 0.3 = 3.4
0.6 + 1.5 + 0.3 = 3.4
exp = 1.488 ppm
exp = 3.505 ppm
exp = 3.505 ppm
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0.6 + 2.5 + (20% of 0.3) = 3.16
exp = 3.242 ppm
0.6 + 2.5 + 0.3 + 0.3 + (20% of 0.0, 2.5, 0.3) = 4.26
0.6 + 0.3 + (20% of 0.0, 2.5, 0.3) = 1.46
0.6 + 2.5 + 0.3 + (20% of 0.3, 0.3, 1.6) = 3.84
0.6 + 0.3 + (20% of 0.3, 0.3, 1.6) = 1.34
exp = 3.683 ppm
exp = 1.423 ppm
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INTENSITY OF SIGNALS
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SPIN-SPIN SPLITTING (COUPLING)
If n is the number of neighboring protons, then the multiplicity will be n + 1.
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What causes splitting?
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When signal splitting occurs, the distance between the individual peaks of a signal is called
the coupling constant, or J value.
It is measured in hertz and is independent of field strength.
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