CHEMICAL SHIFT (δ): the shift in ppm of an NMR signal from the signal of TMS
δΑ = ( νA – νTMS) ÷ Operating frequency in MHz
Tetramethyl silane
(TMS)
δ0.9
8
Below are spectra of two C2H4Cl2 isomers.
Match the isomers of dichlorobutane to the spectra
shown below. Justify your answer.
C2H4Cl
3.66
Cl
Cl
3.66
If two adjacent coupled nuclei have the same
electronic environment, they will have the same DE
and will not exhibit coupling (ie won't split one
another's signals)
4
3
2
PPM
1
0
C2H4Cl2
2.12
5.56 Cl
Cl
6
5
4
3
PPM
2
1
0
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Coupling constants J are dependant primarily on
conformation/ through-bond orientation between coupled
signals
They are independent of magnet strength
Coupling
constants do not
change when the
magnet is made
larger, but they
may appear
closer together in
the spectrum
25
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/
26
Coupling is Mutual- the coupling constant J should
be equal for signals from both neighboring nuclei
Ethyl group:
27
Sketch a 1H NMR spectrum of 3-pentanone
1.  Sketch number of signals and relative positions
2. Estimate relative integrals of each signal
3. Determine multiplicity (splitting) of each signal
Triplet => 2 neighbors
Quartet means the nuclei
responsible for this signal
have 3 H s next door
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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:
31
Coupling Revisited
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
32
With 3 neighbors on left and 1 neighbor on right
max multiplicity = (3+1)(1+1)= 8
33
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
34
When HB's neighboring protons are
different from eachother (ie HA and
HC are different), their coupling to
B may have different strengths (ie
JAB≠JAC).
Under those circumstances the
multiplicity follows the rule MULT=
(m+1)(n+1) where m= # neighbors
on left and n= # neighbors on right
If JAB=JAC (as it often
will with free rotation
about all the bonds), The
signals overlap such that
the formula simplifies to
MULT= (n+1) where n=
total number of
neighbors on left and
right
Coupling simplifies to n+1 whenever
coupling constants to right and left
neighbors are equal
35