A Summarized Look into

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A Summarized Look into NMR Spectroscopy
From Organic Chemistry Paula Yurkanis Bruice
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Nuclear Magnetic Resonance (NMR) Spectroscopy
Identify the carbonhydrogen framework of an
organic compound
Certain nuclei such as 1H, 13C, 19F, and 31P
have allowed spin states of 1/2 and 1/2 this
property allows them to be studied by NMR
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The spin state of a nucleus is affected by an
applied magnetic field
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The energy difference between the two spin
states depends on the strength of the magnetic
field
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An NMR Spectrometer
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The electrons surrounding a nucleus affect the
effective magnetic field sensed by the nucleus
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Chemically equivalent protons protons in the
same chemical environment
Each set of chemically equivalent protons in a
compound gives rise to a signal in an 1H NMR
spectrum of that compound
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The Chemical Shift
The reference point of an NMR spectrum is defined
by the position of TMS (zero ppm)
The chemical shift is a measure of how far the
signal is from the reference signal
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1H NMR spectrum of 1-bromo-2,2-dimethylpropane
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The chemical shift is independent of the
operating frequency of the spectrometer
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Electron withdrawal causes NMR signals to appear
at higher frequency (at larger d values)
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Characteristic Values of Chemical Shifts
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1H NMR spectrum of 1-bromo-2,2-dimethylpropane
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Integration Line
The area under each signal is proportional to the
number of protons that give rise to that signal
The height of each integration step is
proportional to the area under a specific signal
The integration tells us the relative number of
protons that give rise to each signal, not
absolute number
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Diamagnetic Anisotropy
The p electrons are less tightly held by the
nuclei than are s electrons they are more free
to move in response to a magnetic field
Causes unusual chemical shifts for hydrogen
bonded to carbons that form p bonds
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Splitting of the Signals

• An 1H NMR signal is split into N 1 peaks,
  where N is
• the number of equivalent protons bonded to
  adjacent
• carbons

• Coupled protons split each others signal

• The number of peaks in a signal is called the
  multiplicity
• of the signal

• The splitting of signals, caused by spinspin
  coupling,
• occurs when different kinds of protons are
  close to one
• another

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1H NMR Spectrum of 1,1-Dichloroethane
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The ways in which the magnetic fields of three
protons can be aligned
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Splitting is observed if the protons are
separated by more than three s bonds
Long-range coupling occurs when the protons
are separated by more than three bonds and one of
the bonds is a double or a triple bond
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More Examples of 1H NMR Spectra
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The three vinylic protons are at relatively high
frequency because of diamagnetic anisotropy
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The Difference between a Quartet and a Doublet
of Doublets
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The signals for the Hc, Hd, and He protons overlap
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The signals for the Ha, Hb, and Hc protons do not
overlap