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IR SPECTROSCOPY R W Grime Ripon Grammar School
IR SPECTROSCOPY
R W Grime Ripon Grammar School
• 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
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
• Wavenumbers (cm-1) are used as a measure of the wavelength or frequency of the absorption.
Wavenumber = 1 wavelength (cm)
• 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.
• 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.
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.
CH2 C
CH3
CH2 CH3
CH3 C CH
CH3
CH3
cyclohexane
C–H
cyclohexene
C–H
butanal
C–H
CH2 CH2 C
O
HCH3
ethanoic acid
O–H
CH3 C
O
O H
ethanol
O–H
CH3 CH2 O H
butanal
C=O
CH2 CH2 C
O
HCH3
propanone
C=O
CH3 C
O
CH3
ethanoic acid
C=O
CH3 C
O
O H
methyl ethanoate
C=O
CH3 C
O
O CH3
Exercise 1Match the following eight compounds to the following eight IR spectra.
hex-2-ene
pentane
methylpropan-1-ol
2-methylpentan-3-one
butanal
butanoic acid
propyl ethanoate
nitrobenzene
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
C-O
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
C=O
propyl ethanoate
CH3 C
O
O CH2 CH2 CH3
C=O
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
2-methylpentan-3-one
CH3 CH C
O
CH2 CH3
CH3
methylpropan-1-ol
O-H
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
CH3 CH CH2 OH
CH3
C-H 10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
nitrobenzene NO2
C-H 10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
pentane CH3 CH2 CH2 CH2 CH3
C-H 10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
C=O
butanal CH3 CH2 CH2 C
O
H
O-H
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
butanoic acid CH3 CH2 CH2 C
O
O H
C=C
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
C-HC-H
hex-2-ene CH3 CH CH CH2 CH2 CH3
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
F
G
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
9
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
10
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
11
10 15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
