Transcript CHAPTER_13_NMR_2

Features of an NMR spectrum: SHAPE
Spin Coupling: Neighboring nuclei “split” NMR signals
Usually n neighbors splits the signal into n+1 peaks
Multiplicity = n+1
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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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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Pascals Triangle predicts pattern
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Multiplets have predictable shapes
Doublets: 1:1
Triplets: 1:2:1
Quartets: 1:3:3:1
Quintet: 1:4:6:4:1
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The extent to which neighboring nuclei effect the E changes
depends on the geometry of the molecule. It does not depend on
the magnet strength.
Coupling is MUTUAL, so extent of splitting is equal for both
signals.
The distance between the individual peaks making up the doublet
is called the “coupling constant” (J). Here J is 7 Hz.
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Coupling constants are dependant primarily on
conformation/ orientation between coupled signals
They are independent of magnet strength
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Generally coupling is only observed when neighbors are
≥ 3 bonds away from eachother
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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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It is important to note that non-equivalent nuclei split each other.
A split in one requires a split in the other. In addition, the
coupling constants will be the same for each type of nuclei.
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Draw the structure of C3H7I consistent with this
1H NMR spectrum
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Complications in 1H NMR
Hb is split into more than 5 peaks.
Coupling to “non-equivalent” neighbors may modify the simple n+1 rule: Max splitting
is (n+1)(m+1)
where n and m are the numbers of neighbors on each side
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With 3 neighbors on left and 1 neighbor on right
max multiplicity = (3+1)(1+1)= 8
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Despite this complication, the splitting may still simplify
to n+1 even with non-equivalent neighbors
Multiplicity = 6
(predicted as
(3+1)(2+1) = 12
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Complications in 1H NMR:
Signal distortion in non-first-order spectra
Signals from coupled protons that are close together (in Hz) can
show distorted patterns.
When ν >> J, the spectra is said to be first-order.
Non-first-order spectra assume more complex shapes than Pascal’s
triangle predicts and can only be analyzed with the help of computers.
J
>>J

>J
≈J
<<J
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