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H NMR SPECTROSCOPY
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H NMR or proton magnetic resonance
Records signals from hydrogen nuclei in
different structural environments
Provides information of the hydrocarbon partthe compound : relative numbers of different
kinds of hydrogen in the compound;nature of
the carbons bonded to them; and which
hydrogen nearby
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Proton NMR vs. 13C NMR
1H is the major isotope of hydrogen (99.985% natural abundance),
while 13C is only a minor isotope (1.1%)
1H NMR is quantitative: the area under the peak tells us the number
of hydrogen nuclei, while 13C NMR may give strong or weak peaksfrom the same number of 13C nuclei
Protons interact magnetically (couple) to reveal the connectivity ofthe structure, while 13C is too rare for coupling between 13C nucleito be seen
1H NMR shifts give a more reliable indication of the local chemistry
than that given by 13C spectra
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1st Possibility
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2nd Possibility
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3rd Possibility
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4th Possibility
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Structural Information
1. Number of Signals
Each group of chemically equivalent protons gives
rise to a signal
Chemically Equivalent Protons
- protons that are in the same environment identical in
every way
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Structural Information
2. Position of Signals based on how far they are from the signal of the reference compound
tells us the kind of proton or protons that are responsible for the signal
and the kinds of neighbouring substituents depends on the chemical shift
Chemical Shift
- a measure of how far the signal produced from the proton is from the
reference signal
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The greater the chemical shift, the higher is the frequency.
Also, chemical shift is independent of the operating frequency
of the NMR spectrometer.
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Right hand side of an
NMR spectrum
Left hand side of an
NMR spectrum
Generalization:
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Distance from the electron-withdrawing group
protons closer to electron-withdrawing group
less shielded
higher frequency
protons farther to electron-withdrawing group
more shielded
lower frequency
Electron withdrawal causes NMR signals to appear at higher
frequencies (at larger chemical shift values)
Electronegativity
More electronegative
Higher frequency
Less electronegative
Lower frequency
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In a similar environment, the signal for methyl protons occurs at a lower
frequency than the signal for methylene protons, which in turn occurs at a
lower frequency than the signal for a methine proton.
3 signals
a protons lowest frequency (farthest)
b protons lower than the frequency of c
c protons highest frequency
B and C have the same distance but b is methyl protons appear
to be lower at frequency than methylene protons.
3 signals
a protons lowest frequency (farthest)
b protons lower than the frequency of c
c protons highest frequency
B and C have the same distance but b is methyl protons
appear to be lower at frequency than methine protons.
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Structural Information
3. Integration of NMR Signals
Tells the relative number of protons responsible for
the signal, not the absolute number
Integration
- area under each signal
- proportional to the number of protons that gives rise to signal
Beers Law amount of energy absorbed or transmitted is proportional to a certain
number of moles present
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1.6 : 7 = 1: 4.4 x 2 = 2 : 9
The numbers do not always correspond
to the exact or absolute number of
protons. Instead, it tells us the relative
number or ratio of the amount of
equivalent protons.
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Diamagnetic Anisotropy
Magnetic field created by pi electrons or rings
Describes an environment where different magnetic fields are found at
different points in space
Pi electrons are held less strongly than sigma electrons they are more
able to move in response to the magnetic field
Magnetic field applied to compound with pi electrons - electrons move in
a circular path - causes an induced magnetic field which affects the
chemical shift of a proton depends on the direction of the induced
magnetic field relative to the direction of the applied magnetic field
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Effect ofDiamagnetic Anisotropy
The unusual chemical shifts
associated with hydrogen
bonded to carbons that
form bonds are due to
diamagnetic anisotropy
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Structural Information
4. Splitting of the signals
caused by (and therefore tells us the number of)
protons bonded to adjacent carbons
N + 1 rule- number of splitting of the signal that occurs
- where N is the number of equivalent protons adjacent to C but not
equivalent to the proton giving rise the signal
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Multiplicity: The number of peaks in a signal
Coupled Protons: They are protons that split each others signalSinglet: 1 peak
Doublet: 2 peaks
Triplet: 3 peaks
Quartet: 1 peak
Quintet: 5 peaks
Multiplet: more complex than the other peaks
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Spin Spin Coupling
Different kinds of protons are close enough for their magnetic fields to
influence on another. The spin of one nucleus influences the spin of
another nucleus.
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The relative intensities of the peaks in a signal reflect the number of ways the
neighbouring protons can be aligned relative to the applied magnetic field.
1:3:3:1
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Long-range coupling:
occurs when the protons are separated by more than three
bonds and one of the bonds is a double or triple bond. A smallsplitting is observed.
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Spin Spin Coupling
Rules for proton-proton spin-spin coupling.
1. Only non equivalent protons couple. Equivalent protons are in the same environment,
and their signals overlap, so only non equivalent protons can split signals.
2. Protons that are separated by more than three single bonds usually do not couple
because they are not close enough to each other to be influenced by each other magnetic
fields. Pi bonds do not count toward this, but the coupling constants may be too small to
distinguish. Benzene rings have a coupling club in which they only couple with each
other. Nonequivalent benzene ring protons can couple with each other, but the coupling
constants may be too small to be significant. A benzene ring blocks coupling with other
protons outside of the benzene ring. This usually causes complicated splitting patterns.
3. Signals for O-H and N-H protons are usually singlets.
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