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
1
Top spectrum taken at 90 MHz
 ≈ 0.3x90 = 2.7 Hz apart
Bottom spectrum taken at 500
MHz
 ≈ 0.3x500 = 15.0 Hz apart
2
Complications:
OH groups are typically singlets (broad)
Frequently protons attached to hydroxy- or aminofunctional groups do not show coupling and can
appear broad.
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For Hydroxyl protons, the lack of splitting is the result of “fast proton
exchange.”
Proton exchange may be slowed or stopped by removal of traces of water
or acid or by cooling. This allows observation of coupling in accordance
with the n+1 rule.
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Non-first-order Spectra are common at
lower Magnetic Fields
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NON-H Nuclei:
The proton nucleus possesses a greater magnetic moment and
magnetic resonance frequency than do other spin-active nuclei.
Below is a sketch of resonant frequencies for some common nuclei.
This is not a real spectrum, since the NMR instrument is tuned to
acquire a signal from just one nucleus at a time.
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Fourier Transform NMR excites just one nucleus at a time
(ie 1H nucleus @ 300,000,000 ± 3000 Hz)
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Carbon-13 Nuclear Magnetic Resonance
•13C resonate at lower E (they are less sensitive to H0) In a 7.05 T
magnet where 1H’s resonate at 300 MHz, 13C’s resonate at just 75 MHz
•13C NMR spectra have much lower S/N than 1H NMR spectra
•13C NMR spectra typically lack the coupling information found in 1H
NMR spectra
•13C NMR spectra have a larger chemical shift range than 1H NMR
spectra Is the range larger in Hz?
Typical 13C spectra are analyzed for chemical shift and # signals
only! (Integrals and Multiplicity are not reliable/ available)
Range in Hz:
(13C: 200 ppm = 200x75 = 1500 Hz);
1H: 12 ppm = 12x300 = 360 Hz)
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Chemical Shifts in
13C
NMR
13C signals are typically well resolved from one another
The chemical shifts of carbon atoms in 13C NMR depend on the
same effects as the chemical shifts of protons in 1H NMR.
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Attached protons couple to 13C nuclei and complicate
spectra
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The splitting is removed through an
electronic method called “broad-band
decoupling”:
In broad-band decoupling a pulse is
applied to the proton range causing
rapid - flips of hydrogen nuclei, and
effectively averaging their local
magnetic field contributions.
Using this technique simplifies the
spectra of bromoethane to two single
lines.
12
Recall coupling is mutual. Why don’t we see proton
signals split by their attached carbons?
Carbon-carbon coupling is not visible in 1H NMR spectra due to the
very low probability of two 13C nuclei being adjacent to each other
in a single molecule (.0111 x .0111 ~ .0001).
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Decoupling enhances some 13C signals more than
others, and so areas no longer correspond to the
number of nuclei present.
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Why is the S/N so low for 13C NMR?
•Because of the low abundance of
13C
•It has a weaker inherent magnetic resonance (1/6000 as strong
as 1H)
•Its lower E means fewer nuclei showing net absorption.
When the energy
difference is small,
so is the population
difference between
the two spin states.
The smaller the E,
the weaker the
signal.


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
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
Weak H0
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Strong H0
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Diastereotopic Protons
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Diastereotopic Nuclei
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Problems
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Problems
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