Features of NMR Spectrum: SHAPE of signals
Multiplicity (#peaks to which each signal is split)
depends on the number of H’s at adjacent
carbon(s)
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Features of NMR Spectrum: SHAPE of signals
Multiplicity (#peaks to which each signal is split) depends on the
number of H’s at adjacent carbon(s)
Multiplicity = (n+ 1)
n= #neighboring protons ≤ 3 bonds away
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Origin of Spin Coupling:
Nearby nuclei effect the magnetic field E
B0
Hb
Magnetic field of Hb
subtracts from the applied
field; Hb signal appears at
a higher applied field
Hb
Magnetic field of Hb adds
to the applied field; Ha
signal appears at a lower
applied field
Ha
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When a neighbor adds to H0 the E is larger, and so is the chemical shift
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With two neighboring protons Ha is split into three peaks
Ha is 1:2:1 triplet
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Pascals Triangle predicts pattern
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Using all features together can lead to elucidation of molecular
structure
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Below are spectra of two C4H10 isomers.
Match the isomers of butane to the spectra shown below. Justify your
answer.
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Coupling constants are dependant primarily on
conformation/ through-bond orientation between coupled
signals
They are independent of magnet strength
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At lower field, a 10 Hz doublet appears more broad because the
coupling constant is a larger fraction of 1 ppm http://sdbs.db.aist.go.jp/
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Coupling is Mutual- the coupling constant J should be equal for signals
from both neighboring nuclei
Ethyl group:
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Putting the features together allows for structure elucidation.
The following spectrum is for C2H4Cl2
The Area of signal a is 3 times larger than b (CH3?)
It is a doublet (ie it has only one neighbor)
Therefore a is a methyl next to a methine: CH3-CH
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Features of an NMR spectrum
Which Isomer of C5H10O gives rise to this spectrum?
Triplet => 2 neighbors
Quartet means the nuclei
responsible for this signal
have 3 H’s “next door”
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Draw the structure of C3H7I consistent with this
1H NMR spectrum
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Coupling Revisited:
When neighbors n and m are themselves unique (eg left
and right below), the multiplicity may follow the rule
mult= (n+1)(m+1)
Hb signal:
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