Transcript NMR Spectroscopy

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• Protons and neutrons can be regarded as spinning about
their axis.
• In many atoms these spins are paired against each other
and so the nucleus has no overall spin (e.g. 12C).
• In some atoms (e.g. 1H and 13C) the nucleus has an
overall spin.
• A nucleus that spins generates a magnetic
field.
• The direction of the magnetic field depends
which way the nucleus spins.
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• Usually the two possible spin states of the nucleus have
the same amount of energy.
• However, in a magnetic field, the two spin states have
different energies.
Energy
Magnetic field
opposed to
applied field
Applied
magnetic
field
Energy gap corresponds to
frequency of radiowaves
Magnetic field in
same direction as
applied field
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butanone
8
7
6
5
4
3
2
1
0
 chemical shift
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•
•
In a spectrum, there is one signal for each set of
equivalent H atoms.
The intensity of each signal being proportional to the
number of equivalent H atoms it represents.
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CH3
CH2
2 sets of equivalent H’s: ratio 6:2 (3:1)
CH3
Br
CH3
CH
CH2
4 sets of equivalent H’s: ratio 3:1:2:3
CH3
Br
CH3
CH
CH2
CH2
CH3
5 sets of equivalent H’s: ratio 3:1:2:2:3
CH3
CH3
C
OH
CH2
CH3
4 sets of equivalent H’s: ratio 6:1:2:3
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For each of the following compounds, predict the
number of signals and the relative intensity of the
signals.
a)
b)
c)
d)
e)
methylpropene
propene
2-chloropropane
propanone
methylamine
f) ethyl propanoate
g) 1,2-dibromopropane
h) dimethylethyl
propanoate
i) but-2-ene
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CH3
CH2
C
O
CH3
CH3
2 signals: ratio 6:2 (3:1)
CH2
CH
C
CH3
1 signal
CH3
CH3
NH2
3 signals: ratio 2:1:3
2 signals: ratio 3:2
Cl
O
CH3
CH
CH3
2 signals: ratio 6:1
CH3
CH2
C
O
CH2
CH3
4 signals: ratio 3:2:2:3
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Br
Br
CH2
CH
O
CH3
CH3
CH2
C
CH3
O
3 signals: ratio 3:2:9
3 signals: ratio 2:1:3
CH3
CH
CH
C
CH3
CH3
CH3
2 signals: ratio 6:2 (3:1)
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•
Samples are dissolved in solvents free of 1H atoms, e.g.
CCl4, CDCl3.
CH3
•
A small amount of TMS
(tetramethylsilane) is added to calibrate
the spectrum.
•
CH3
Si
CH3
It is used because:
•
•
•
•
•
CH3
its signal is away from all the others
it only gives one signal
it is non-toxic
it is inert
it has a low boiling point so is easy to remove
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8
7
6
5
4
3
2
1
0
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 chemical shift
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8
7
6
5
4
3
2
1
0
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 chemical shift
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•
The spectra are recorded on a scale known as the
chemical shift (), which is how much the field is shifted
from the field for TMS compared to the field for TMS.
 = Field for TMS - Field measured x 106
Field for TMS
• The  is a measure in parts per million (ppm) of how far the
magnetic field required for absorption is shifted away from
that for TMS.
10
9
8
7
6
5
4
3
2
1
0
 chemical shift (ppm)
• The  depends on what other atoms/groups are near the H
– more electronegative groups gives a greater shift.
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Data from AQA datasheet
For each of the following compounds, predict the
number of signals and the relative intensity of the
signals.
a) but-1-ene
d) 2-methylpropan-2-ol
b) 1,2-dichloropropane e) pentane
c) propanal
f) ethyl methylpropanoate
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a) but-1-ene
d) 2-methylpropan-2-ol
b) 1,2-dichloropropane e) Pentane
c) propanal
f) ethyl methylpropanoate
OH
CH3
CH2
Cl
Cl
CH2
CH
CH
CH2
CH3
C
CH3
CH3
CH3
CH3
CH2
CH2
CH2
CH3
O
CH3
CH2
C
H
CH3
CH3
O
CH
C
O
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CH2
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CH3
17-Jul-12
• Number of signals
• Relative “size”
a)
• Chemical shift b)
propanal
ethyl methylpropanoate
O
CH3
CH3
CH2
C
CH3
O
CH
C
H
O
CH2
CH3
Data from AQA datasheet
Coupling / Splitting / Multiplicity
8
7
6
5
4
3
2
1
0
 chemical shift
Number of H’s next door +1
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n+1
O
0 H next door
singlet (s)
CH3
1 H next door
doublet (d)
CH2
C
CH3
2 H next door
triplet (t)
3 H next door
quartet (q)
more H next door
multiplet (m)
8
7
6
5
4
3
2
1
0
 chemical shift
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signal
singlet
doublet
triplet
quartet
number of
lines
1
2
3
4
number of H’s
next door
0
1
2
3
1:1
1:2:1
1:3:3:1
appearance
relative size
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CH3
Br
Br
C
CH
CH3
Br
CH3
CH3
CH3
O
CH
C
O
CH2
CH2
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CH3
CH3
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CH3
8
7
CH2
6
5
CH2
4
3
CH3
2
1
0
 chemical shift
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CH3
CH2
CH2
CH2
O
CH3
C
O
CH2
CH2
O
CH3
C
CH3
C
CH3
O
CH2
CH2
C
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CH2 CH3
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CH3
8
7
CH2
6
5
OH
4
3
2
1
0
 chemical shift
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Number of H’s next door +1
But you don’t couple to
• H’s that are equivalent
• H’s on O’s
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Number of signals
how many different sets of
equivalent H atoms there are
Position of signals
information about chemical
environment of H atom
Relative intensities
gives ratio of H atoms for peaks
Splitting
how many H atoms on adjacent C
atoms
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For each of the following compounds, predict the
number of signals, the relative intensity of the
signals, and the multiplicity of each signal.
a)
b)
c)
d)
e)
methylpropene
propene
2-chloropropane
propanone
methylamine
f) ethyl propanoate
g) 1,2-dibromopropane
h) dimethylethyl
propanoate
i) but-2-ene
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CH3
CH2
C
O
CH3
CH3
2 signals: ratio 6 : 2 (3 :1)
s s
CH2
CH
CH3
CH3
NH2
2 signals: ratio 3 : 2
t q
O
Cl
CH
CH3
1 signal
3 signals: ratio 2 : 1 : 3
d m d
CH3
C
CH3
2 signals: ratio 6 : 1
d m
CH3
CH2
C
O
CH2
CH3
4 signals: ratio 3 : 2 : 2 : 3
t q q t
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Br
Br
CH2
CH
O
CH3
CH3
CH2
C
CH3
O
3 signals: ratio 3 : 2 : 9
t q s
3 signals: ratio 2 : 1 : 3
d m d
CH3
CH
CH
C
CH3
CH3
CH3
2 signals: ratio 6 : 2 (3 :1)
d q
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• Number of signals
• Relative “size”
• Chemical shift
• Multiplicity
Cl
Cl
CH2
CH
a) 1,2-dichloropropane
b) but-1-ene
CH3
CH3
CH2
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CH
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CH2
17-Jul-12
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•
13C
NMR spectra are often simpler than 1H NMR spectra.
• They give a lot of valuable information about the chemical
environment of C atoms (e.g. the difference between C
atoms in C=O, C-N, CN, C-C, C=C, etc.).
• There is one signal for each set of equivalent C atoms.
• There is no coupling (unlike 1H NMR).
• The size of signal is not relative to the number of equivalent
C atoms (unlike H atoms in 1H NMR).
• As in 1H NMR, the chemical shift (d) is measured relative to
TMS.
• Although deuterated solvents are usually used, there will be
a signal for any C atoms in the solvent.
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Data from AQA datasheet
Data from AQA datasheet
Data from AQA datasheet