© www.chemsheets.co.uk
AS 062
12-Jul-12
• 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.
THE ELECTROMAGNETIC SPECTRUM
 Atoms, molecules and ions can absorb (or emit)
electromagnetic radiation of specific frequencies, and this
can be used to identify them.
Electromagnetic
radiation absorbed
What the energy is
used for
Spectroscopy
technique
Ultra-violet / visible
Movement of electrons to
higher energy levels
Ultra-violet / visible
spectroscopy
Infra-red
To vibrate bonds
Infra-red
spectroscopy
Microwaves
To rotate molecules
Microwave
spectroscopy
Radio waves
To change nuclear spin
NMR spectroscopy
© www.chemsheets.co.uk
AS 062
12-Jul-12
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.
INFRA-RED SPECTROSCOPY
• If IR light is passed through the compound, it will absorb some
or all of the light at the frequencies at which its bonds vibrate.
• Wavenumbers (cm-1) are used as a measure of the wavelength
or frequency of the absorption.
Wavenumber =
1
wavelength (cm)
• 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.
© www.chemsheets.co.uk
AS 062
12-Jul-12
BELOW 1500 cm-1 – “Fingerprinting”
•
Complicated and contains many signals – picking out
functional group signals difficult.
•
This part of the spectrum is unique for every compound, and
so can be used as a "fingerprint".
•
This region can also be used to check if a compound is pure.
© www.chemsheets.co.uk
AS 062
12-Jul-12
CH2
C
CH2
CH3
CH3
CH3
C
CH3
CH
CH3
cyclohexane
C–H
cyclohexene
C–H
O
butanal
CH3
CH2
CH2
C
C–H
H
O
ethanoic acid
CH3
C
O
O–H
H
ethanol
CH3
CH2
O
H
O–H
butanal
O
CH3
CH2
CH2
C
H
C=O
O
propanone
CH3
C
CH3
C=O
O
ethanoic acid
CH3
C
O
C=O
H
O
methyl ethanoate
CH3
C
C=O
O
CH3
CH3
CH2
O
H
Exercise 1
Match the following eight compounds to the
following eight IR spectra.
hex-2-ene
butanal
pentane
butanoic acid
methylpropan-1-ol
propyl ethanoate
2-methylpentan-3-one
nitrobenzene

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
       
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
       

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
       
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
       

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

O
propyl ethanoate
CH3
C
O
CH2
CH2
CH3
95
90
85
80
75
70
65
%T
60
55
50
45
40
C=O
35
30
C-O
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

2-methylpentan-3-one
CH3
CH3
O
CH
C
CH2
CH3
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
C=O
30
25
20
15
10
4 000
3 500
3 000
       
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
       

CH3
methylpropan-1-ol
CH3
CH
CH2
OH
95
90
85
80
O-H
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

nitrobenzene
NO2
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
C-H
25
20
15
10
4 000
3 500
3 000
       
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
       

pentane
CH3
CH2
CH2
CH2
CH3
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
C-H
25
20
15
10
4 000
3 500
3 000
       
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
       

O
butanal
CH3
CH2
CH2
C
H
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
C-H
25
20
C=O
15
10
4 000
3 500
3 000
2 500
2 000
1 500
1 000
5 00
W ave num bers (c m-1 )
       
       

O
butanoic acid
CH3
CH2
CH2
C
O
H
95
90
85
80
75
70
65
O-H
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
       
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
       

hex-2-ene
CH3
CH
CH
CH2
CH2
CH3
95
90
85
80
75
70
65
%T
60
55
50
45
C-H
C-H
40
35
C=C
30
25
20
15
10
4 000
3 500
3 000
       
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
       
F
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
G
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
9
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
10
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
11
95
90
85
80
75
70
65
%T
60
55
50
45
40
35
30
25
20
15
10
4 000
3 500
3 000
2 500
2 000
W ave num bers (c m-1 )
1 500
1 000
5 00
P
Q
R
S
T
U