Nuclear Magnetic Resonance Spectroscopy

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Nuclear Magnetic Resonance Spectroscopy

• (NMR)

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Introduction
NMR spectroscopy dates from the 1950 - 1960s and
in the space of some 40 years has revolutionized
organic chemistry as tool for structural
elucidation, kinetic studies, and quality control.
If you have attended an Edmonton Eskimo game or
read reports on injured Oilers, you will have
heard of magnetic-resonance imaging, a technique
which monitors the 1H magnetic resonance signals
from water in the body. This application of NMR
science enables physicians to image body organs
without recourse to X-rays.
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Introduction
We have seen that IR spectroscopy is a vital tool
for the identification of the functional groups
present in organic molecules.
NMR provides an image of the molecules
hydrocarbon skeleton. NMR spectra result from the
absorption of radio frequency radiation when
certain atomic nuclei are placed in magnetic
fields. Why?
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Nuclear magnetic resonance
The nuclei of the elements fall into two
categories those which have a spin and those
which do not.
The 1H, 13C, 19F nuclei and many others posses a
spin. As they are positively charged, the
spinning nucleus generates a tiny magnetic moment
- it behaves like a small bar magnet.
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The effect of the magnetic field

• In the absence of an applied magnetic field,
  these little magnets are randomly oriented.

• When placed between the poles of a strong
  magnet, they are either aligned with (parallel)
  or against (antiparallel) the field.
• The parallel spin state is slightly more stable
  than the antiparallel state. For every 1,000,000
  protons, some 500,005 - 500,010 have the parallel
  state! Not a large difference but it is the
  basis of NMR!

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The effect of the magnetic field

• When a molecule is placed in a magnetic field
  and is subjected to electromagnetic radiation
  (radio frequency), the nuclear spin can flip from
  the parallel to the antiparallel state - the
  nuclei are said to be in resonance.

h?
spin aligned with the applied field
antiparallel spin
magnetic field
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The effect of the magnetic field

• The difference in energy, DE, between the two
  spin states depends on the strength of the
  applied field, H0. The stronger the field, the
  larger the value of DE

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Schematic representation of the NMR spectrometer
sample
N
S
radio frequency detector
radio wave generator
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1H NMR spectrometers

• The sample is placed in a glass tube which is
  then placed between the two poles of a very
  strong magnet.

• It is then irradiated using radiation of a
  constant low frequency.
• The magnetic field is varied. When it reaches
  the correct strength, the nuclei absorb energy
  and resonance occurs.
• The absorption causes a tiny electrical current
  to flow in a receiver coil surrounding the sample
  which is displayed as a peak.

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Fourier Transform NMR Spectrometers

• The instrument irradiates the sample with a short
  pulse of RF radiation for about 10-5 s.

• This pulse excites all the nuclei at once.
• A computer performs a mathematical calculation
  known as a Fourier transform and a spectrum is
  produced.

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CH3CH2OH
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Homotopic hydrogen atoms
Such atoms can be identified by replacing each in
turn with some other atom such as a bromine. If
one gets the same compound, the atoms being
replaced are chemically equivalent or homotopic.
These protons are not homotopic!
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Homotopic hydrogen atoms
Such atoms can be identified by replacing each in
turn with some other atom such as a bromine. If
one gets the same compound, the atoms being
replaced are chemically equivalent or homotopic.
These protons are homotopic!
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Identify the number of groups of peaks in the 1H
NMR spectra of the following
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Shielding effects
The position of an NMR peak is controlled by the
shielding or deshielding of the nucleus (by
electrons).
The free proton is a nucleus free of any
influence by exterior, electronic
factors. However organic molecules contain
covalently bonded nuclei, not free protons......
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Shielding effects
Protons in organic molecules are surrounded by
electrons.

• The electron density about the protons varies
  according to several factors
• bond polarity
• the hybridization state of the atom to which a
  given hydrogen is bonded
• the presence of electron attracting or donating
  groups

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Shielding effects
When a nucleus is placed in a magnetic field, the
electrons surrounding it are in motion about the
nucleus and create a small, localized magnetic
field which opposes the applied field.
Thus the electrons generate a small magnetic
field that shields the proton from the external
field.
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Shielding effects
The result is that one observes a decrease in the
intensity of the total field near to the nucleus.
The nucleus is said to be shielded. Thus the
position of an NMR absorption depends on the
electron density about the hydrogen.
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Chemical shifts
The position of an NMR absorption is called the
chemical shift.
An internal standard is added and the positions
of the peaks are measured relative to that of
the internal standard, tetramethylsilane
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Tetramethylsilane
The protons in TMS are shielded and resonate at
a position far removed from the spectral zone in
which one usually finds proton absorption.
The chemical shifts are measured (in Hz) relative
to this reference.
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Chemical shifts
The absorptions are measured in relation to their
distance from the TMS peak.
These distances vary according to the strength of
the applied magnetic field. Peaks separated by
54Hz at 60MHz are separated by 72Hz at 80MHz,
270Hz at 300MHz and by 540Hz at 600MHz.
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Chemical shifts
This complication is eliminated by dividing the
distance between the peak and that of TMS (in Hz)
by the radio frequency of the spectrometer (in
MHz)
distance between peak and that of (CH3)4Si in Hz
?
ppm
frequency of the spectrometer in MHz
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Typical chemical shifts
Type of hydrogen Chemical shift, ? (ppm) primary
alkyl RCH3 0.9 secondary alkyl RCH2R 1.3 tertia
ry alkyl R3CH 1.5 allylic CC-CH3 1.7
benzylic ArCH2R 2.2
- 3
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Typical chemical shifts
Type of hydrogen Chemical shift, ? (ppm)
chloroalkane H-C-Cl 3 - 4 bromoalkane H-C-Br 2
.5 - 4 iodoalkane H-C-I 2 - 4 ether RCH2OR
3.3 - 4 alcohol RCH2OH 3.4 - 4 ketone CH3-CO
2 2.7
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Typical chemical shifts
Type of hydrogen Chemical shift, ? (ppm)
aldehyde H-CO 9 10 alkene CCH 4.6 -
5.9 aromatic ArH 6.0 - 8.5 alkyne RC?CH 2 -
3 alcoholic OH ROH 0.5 - 5.0
(variable) amine RNH2 0.5 - 5.0
(variable) carboxylic RCO2H 10.5 - 12
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Peak areas
64 mm
43 mm
21 mm
The area under an NMR peak is directly
proportional to the number of protons giving rise
to the peak.
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Peak areas C2H6O
64 mm
43 mm
43/21 2 protons
21 mm
21/21 1 proton
64/21 3 protons
43 21 64 128 mm
128 mm/6 protons 21 mm/proton
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A problem C11H16
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Spin - spin coupling CH3CH2I
There are two principal absorptions. These are
divided into three and four component peaks which
are equally spaced.
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Spin - spin coupling
Lets examine a simple molecule
Consider the absorption by the CH2Br protons in
the absence of the other proton of the -CBr2H
a single peak
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The CH2Br protons
The magnetic field experienced by these protons
at any given moment is either slightly increased
or reduced due to the spin of the CHBr2 proton.
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The CH2Br protons
The magnetic field is increased if this proton is
aligned with the applied field.
Therefore a lower external field is necessary to
maintain resonance and the peak is situated at
lower applied field
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The CH2Br protons
The magnetic field is reduced if this proton is
aligned against this applied field.
The exterior field must be increased to maintain
resonance and so we observe a peak at higher
field
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Spin - spin coupling
The signal due to the CH2Br protons is divided
into two peaks-
A doublet of peaks of equal size.
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Spin - spin coupling
Now lets look at the absorption due to the
-CHBr2 group. It is affected by the spin of the
neighboring protons. There are 4 possible spin
combinations which are equally possible
1 2 1 a triplet
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Spin-spin coupling constants
J
J
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Spin - spin coupling CH3CH2I
There are two principal absorptions. These are
divided into three and four component peaks which
are equally spaced.
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CH3CH2CH3
Predict its 1H NMR spectrum.....
TMS
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CH3CH2CH3
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Spin - spin coupling
n equivalent neighbouring protons divide an NMR
signal into n 1 peaks.
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Pascals triangle
The relative peak areas are given by Pascals
triangle
1
1 1
1 2 1
1 3 3 1
1 4 6 4
1 1 5 10
10 5 1 1 6
15 20 15 6
1
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Coupling constants
J 2-6 Hz
J 0-7 Hz
J 2-13 Hz
J 5-14 Hz
cis - J 2-15 Hz trans - J 10-21 Hz
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Proton exchange
Protons bonded to electronegative atoms undergo
rapid exchange and do not show spin - spin
coupling with neighboring protons. A
time-averaged spectrum is observed, a singlet. If
such a proton is suspected, add a few drops of
D2O. The rapid exchange of H - D causes the OH
peak to disappear.
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C8H9Br
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Degree of unsaturation
Degree of unsaturation (2NC - NX NN NH
2)/2 NC number of carbons NX number of
halogens NN number of nitrogens NH number of
hydrogens
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C6H14O
SDBSWeb http//www.aist.go.jp/RIODB/SDBS/
24/7/02
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C6H14O
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C8H8O2
SDBSWeb http//www.aist.go.jp/RIODB/SDBS/
25/7/02
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C8H8O2
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C9H12O
SDBSWeb http//www.aist.go.jp/RIODB/SDBS/
25/7/02
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C9H12O
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C8H11NO
SDBSWeb http//www.aist.go.jp/RIODB/SDBS/
25/7/02
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C8H11NO
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Sketch a spectrum
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Propose a structure for the compound that fits
the following data
C10H14 ? 0.88 ppm (6H, doublet) ? 1.86
ppm (1H, multiplet) ? 2.45 ppm (2H,
doublet) ? 7.12 ppm (5H, singlet)