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NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY
Radio waves do the trick.
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Nuclei can be thought of as tiny magnets.
No external magnetic fieldSpin alignment random
With external magnetic fieldSpins aligned
Bo
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When a nucleus occupying the α spin state is subjected to radio waves, an absorption can take place.
ener
gy DE
α spin state
β spin state
magnetic fieldis applied
ener
gy
Magnetic field strength
DEDE
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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
diastereotopic
enantiotopic
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Exercise 1:Identify the number of signals expected in the 1H NMR spectrum of the following compounds.
1 signal
1 signal
2 signals
2 signals
4 signals
4 signals
5 signals
3 signals
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Exercise 2:Determine whether the two protons shown in red are homotopic, enantiotopic, or diastereotopic.
homotopic
enantiotopic
diastereotopicdiastereotopic
enantiotopic
homotopic
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The x-axis is a frequency scale but is normalized to be independent of the field strength.
POSITION OF SIGNALS
spectrometer operating at 300 MHz
2181 Hz larger than that of TMS
spectrometer operating at 60 MHz
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 exp = 1.488 ppm
0.6 + 1.5 + 0.3 = 3.4 exp = 3.505 ppm
C Cl
H
H
C
H
H
H
0.6 + 2.5 + 0.3 = 3.4 exp = 3.505 ppm
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0.6 + 2.5 + (20% of 0.3) = 3.16 exp = 3.242 ppm
H
C HOC
H
H
H H
0.6 + 2.5 + 0.3 + (20% of 0.3, 0.3, 1.6) = 3.84 exp = 3.683 ppm
0.6 + 0.3 + (20% of 0.3, 0.3, 1.6) = 1.34 exp = 1.423 ppm
C
C H 3
C H 3
C Cl
O
C
H
Cl
H 0.6 + 0.3 + (20% of 0.0, 2.5, 0.3) = 1.46
0.6 + 2.5 + 0.3 + 0.3 + (20% of 0.0, 2.5, 0.3) = 4.26
C
C H 3
Cl
HC
C H 3
H
Cl
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INTENSITY OF SIGNALS
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If n is the number of neighboring protons, then the multiplicity will be n + 1.
SPIN-SPIN SPLITTING (COUPLING)
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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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