14_Lecture.ppt

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Chapter 14 NMR Spectroscopy
Organic Chemistry 6th Edition 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, 15N, 19F, and
31P, have non-zero value for their spin quantum
number 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 spin states
increases with the strength of the applied
magnetic field
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An NMR Spectrometer
In pulsed Fourier transform (FT) spectrometers,
the magnetic field is held constant, and a radio
frequency (rf) pulse of short duration excites
all the protons simultaneously
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The electrons surrounding a nucleus decrease the
effective magnetic field sensed by the nucleus
Beffective Bo Blocal
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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
The greater the chemical shift, the higher the
frequency
The chemical shift is independent of the
operating frequency of the spectrometer
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Protons in electron-poor environments show
signals at high frequencies
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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Diamagnetic Anisotropy
The unusual chemical shifts associated with
hydrogens bonded to carbons that form p bonds
The p electrons are freer to move than the s
electrons in response to a magnetic field
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The protons show signals at higher frequencies
because they sense a larger effective magnetic
field
benzene
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The alkene and aldehyde protons also show signals
at higher frequencies
alkene
aldehyde
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The alkyne proton shows a signal at a lower
frequency than it would if the p electrons did
not induce a magnetic field
alkyne
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1H NMR spectrum of 1-bromo-2,2-dimethylpropane
The area under each signal is proportional to the
number of protons giving rise to the signal
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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 the
absolute number
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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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It is not the number of protons giving rise to a
signal that determines the multiplicity of the
signal
It is the number of protons bonded to the
immediately adjacent carbons that determines the
multiplicity
a a triplet b a quartet c a singlet
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Equivalent protons do not split each others
signal
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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 no more than three s bonds
Long-range coupling occurs over ?systems, such as
benzene
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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 signals for the Hc, Hd, and He protons
overlap because the electronic effect of an ethyl
substituent is similar to that of a hydrogen
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The signals for the Ha, Hb, and Hc protons do not
overlap because of the strong electron-withdrawing
property of the nitro group
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Coupling Constants
The coupling constant (J) is the distance between
two adjacent peaks of a split NMR signal in hertz
Coupled protons have the same coupling constant
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Summary
1. The number of chemical shifts ? specify the
number of proton environments in the compound
2. The chemical shift ??values specify the nature
of the chemical environment alkyl, alkene, etc.
3. The integration values specify the relative
number of protons
4. The splitting specifies the number of
neighboring protons
5. The coupling constants specify the orientation
of the coupled protons
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A Splitting Diagram for a Doublet of Doublets
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Complex Splitting
JAC JAB Triplet
JAC gt JAB Doublet of doublets
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The trans coupling constant is greater than the
cis coupling constant
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A Splitting Diagram for a Quartet of Triplets
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Why is the signal for Ha a quintet rather than a
triplet of triplet?
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The Difference between a Quartet and a Doublet
of Doublets
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When two different sets of protons split a
signal, the multiplicity of the signal is
determined by using the N 1 rule separately for
each set of the hydrogens, as long as the
coupling constants for the two sets are different
When the coupling constants are similar, the
multiplicity of a signal can be determined by
treating both sets of adjacent hydrogens as
though they were equivalent
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Replacing one of the enantiotopic hydrogens by a
deuterium or any other atom or group other than
CH3 or OH forms a chiral molecule
prochiral carbon
Ha is the pro-R-hydrogen, whereas Hb is the
pro-S-hydrogen and they are chemically
equivalent
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Diastereotopic hydrogens have different chemical
shifts
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Diastereotopic hydrogens are not chemically
equivalent
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The three methyl protons are chemically
equivalent because of rotation about the CC
bond
We see one signal for the methyl group in the 1H
NMR spectrum
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1H NMR spectra of cyclohexane-d11 at various
temperatures
axial
equatorial
equatorial
axial
The rate of chairchair conversion
is temperature dependent
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Protons Bonded to Oxygen and Nitrogen
The greater the extent of the hydrogen bond, the
greater the chemical shift
These protons can undergo proton exchange
They always appear as broad signals
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pure ethanol
ethanol with acid
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A 60-MHz 1H NMR spectrum
A 300-MHz 1H NMR spectrum
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To observe well-defined splitting patterns, the
difference in the chemical shifts (in Hz) must be
10 times the coupling constant values
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13C NMR Spectroscopy

• The number of signals reflects the number of
  different
• kinds of carbons in a compound.

• The overall intensity of a 13C signal is about
  6400 times
• less than the intensity of an 1H signal.

• The chemical shift ranges over 220 ppm.

• The reference compound is TMS.

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Proton-Decoupled 13C NMR of 2-Butanol
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Proton-Coupled 13C NMR of 2-Butanol
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The intensity of a signal is somewhat related to
the number of carbons giving rise to it
Carbons that are not attached to hydrogens give
very small signals
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DEPT 13C NMR distinguishes CH3, CH2, and CH
groups
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The COSY spectrum identifies protons that are
coupled
Cross peaks indicate pairs of protons that are
coupled
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COSY Spectrum of 1-Nitropropane
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The HETCOR spectrum of 2-methyl-3-pentanone indica
tes coupling between protons and the carbon to
which they are attached
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Unknown Identification Using Spectroscopy
Example 1 13C-NMR of C5H9Br
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Example 1 1H-NMR of C5H9Br
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Example 1 IR of C5H9Br
Answer
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Example 2 13C-NMR of C6H10O4
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Example 2 1H-NMR of C6H10O4
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Example 2 IR of C6H10O4
Answer