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IR SPECTROSCOPY
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Light is one form of electromagnetic radiation. Light is only a very small part of the electromagnetic spectrum. Electromagnetic waves consist of electric and magnetic fields
which are perpendicular to each other and to the direction of travel of the wave.
The electric and magnetic fields vibrate at the same frequency
as each other.
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THE ELECTROMAGNETIC SPECTRUM
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Atoms, molecules and ions can absorb (or emit)electromagnetic radiation of specific frequencies, and this
can be used to identify them.
Electromagneticradiation absorbed
What the energy isused for
Spectroscopytechnique
Ultra-violet / visible Movement of electrons tohigher energy levels
Ultra-violet / visiblespectroscopy
Infra-red To vibrate bondsInfra-red
spectroscopy
Microwaves To rotate molecules Microwavespectroscopy
Radio waves To change nuclear spin NMR spectroscopy
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INFRA-RED SPECTROSCOPY
All bonds vibrate at a characteristic frequency. There are different types of vibration.
Symmetric stretch Assymmetric stretch Bending
The frequency depends on the mass of the atoms in the bond,
the bond strength, and the type of vibration. The frequencies at which they vibrate are in the infra-red region
of the electromagnetic spectrum.
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INFRA-RED SPECTROSCOPY
Wavenumbers (cm -1) are used as a measure of the wavelengthor frequency of the absorption.
Wavenumber = 1wavelength (cm)
If IR light is passed through the compound, it will absorb someor all of the light at the frequencies at which its bonds vibrate.
IR light absorbed is in the range 4000 400 cm -1.
Above 1500 cm -1 is used to identify functional groups.
Below 1500 cm -1 is used for fingerprinting.
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BELOW 1500 cm -1 Fingerprinting
Complicated and contains many signals picking outfunctional group signals difficult.
This part of the spectrum is unique for every compound, andso can be used as a "fingerprint".
This region can also be used to check if a compound is pure.
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CH2 C
CH3
CH2 CH3
CH3 C CH
CH3
CH3
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cyclohexane
C H
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cyclohexene
C H
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butanal
C H
CH2 CH2 C
O
HCH3
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ethanoic acid
O H
CH3 C
O
O H
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propan-1-ol
O H
CH 3 CH 2 CH 2 OH
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butanal
C=O
CH2 CH2 C
O
HCH3
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propanone
C=O
CH3 C
O
CH3
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ethanoic acid
C=O
CH3 C
O
O H
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methyl ethanoate
C=O
CH3 C
O
O CH 3
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10
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25
30
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45
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55
60
65
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% T
5001000150020002500300035004000
Wavenumbers (cm-1)
hex-1-ene
C=C
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2-methylbut-2-ene
C=C
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Exercise 1
Match the following eight compounds to thefollowing eight IR spectra.
hex-2-ene
pentane
methylpropan-1-ol
2-methylpentan-3-one
butanal
butanoic acid
propyl ethanoate
nitrobenzene
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10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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10
15
20
25
30
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40
45
50
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60
65
70
75
80
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90
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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10
15
20
25
30
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40
45
50
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60
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
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C-O
10
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30
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
C=O
propyl ethanoate CH3 CO
O CH 2 CH2 CH3
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C=O
10
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
2-methylpentan-3-one CH3 CH CO
CH2 CH3
CH3
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methylpropan-1-ol
O-H
10
15
20
25
30
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40
45
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60
65
70
75
80
85
90
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
CH3 CH CH 2 OH
CH3
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C-H
10
15
20
25
30
35
40
45
50
55
60
65
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75
80
85
90
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
nitrobenzene NO2
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C-H
10
15
20
25
30
35
40
45
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
pentane CH3 CH2 CH2 CH2 CH3
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C-H 10
15
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30
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
C=O
butanal CH3 CH2 CH2 CO
H
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O-H
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20
25
30
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
butanoic acid CH3 CH2 CH2 CO
O H
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C=C
10
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% T
5001000150020 0025 003000350 0400 0
Wavenumbers (cm-1)
C-H C-H
hex-2-ene CH3 CH CH CH 2 CH2 CH3
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9
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
% T
5001000150020002500300035004000
Wavenumbers (cm-1)
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10
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
% T
5001000150020002500300035004000
Wavenumbers (cm-1)
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11
10
15
20
25
30
35
40
45
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55
60
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75
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85
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% T
5001000150020002500300035004000
Wavenumbers (cm-1)
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% T
5001000150020002500300035004000
Wavenumbers (cm-1)
F
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G
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% T
5001000150020002500300035004000
Wavenumbers (cm-1)
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P
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Q
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R
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S
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T
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U