Infrared Spectroscopy

1
Infrared Spectroscopy

• Gives information about the functional groups in
  a molecule

2
Infrared Spectroscopy

• region of infrared that is most useful lies
  between2.5-16 mm (4000-625 cm-1)
• depends on transitions between vibrational
  energy states
• stretching
• bending

3
Stretching Vibrations of a CH2 Group
Symmetric
Antisymmetric
4
Bending Vibrations of a CH2 Group
In plane
In plane
5
Bending Vibrations of a CH2 Group
Out of plane
Out of plane
6
Example of infrared spectrum
Transmittance ()
100

• Hexane

bending
CH stretching
bending
bending
0
4400 3600 2800 2000 1600 1200 800
Wave number, cm-1
7
Example of infrared spectrum
Transmittance ()
100

• Hexane

no peaks higher than 3000 cm-1
0
4400 3600 2800 2000 1600 1200 800
Wave number, cm-1
8
Infrared spectrum of 1-hexene
Transmittance ()
100
CCH
CC
CH2C
peak higher than 3000 cm-1
0
4400 3600 2800 2000 1600 1200 800
Wave number, cm-1
9
Table 13.4 (p 519)Infrared Absorption Frequencies

• Structural unit Frequency, cm-1
• Stretching vibrations (single bonds)
• sp CH 3310-3320
• sp2 CH 3000-3100
• sp3 CH 2850-2950
• sp2 CO 1200
• sp3 CO 1025-1200

10
Table 13.4 (p 519)Infrared Absorption Frequencies

• Structural unit Frequency, cm-1
• Stretching vibrations (multiple bonds)

1620-1680
2100-2200
2240-2280
11
Table 13.4 (p 519)Infrared Absorption Frequencies

• Structural unit Frequency, cm-1
• Stretching vibrations (carbonyl groups)
• Aldehydes and ketones 1710-1750
• Carboxylic acids 1700-1725
• Acid anhydrides 1800-1850 and 1740-1790
• Esters 1730-1750
• Amides 1680-1700

12
Table 13.4 (p 519)Infrared Absorption Frequencies

• Structural unit Frequency, cm-1
• Bending vibrations of alkenes

910-990
890
665-730
960-980
790-840
13
Infrared spectrum of 1-hexene
Transmittance ()
100
CC stretch
CH2C bend
0
4400 3600 2800 2000 1600 1200 800
Wave number, cm-1
14
Infrared spectrum of tert-butylbenzene
Transmittance ()
100
ArH stretch gt 3000
monosubstituted benzene
0
4400 3600 2800 2000 1600 1200 800
Wave number, cm-1
15
Table 13.4 (p 519)Infrared Absorption Frequencies

• Structural unit Frequency, cm-1
• Stretching vibrations (single bonds)
• OH (alcohols) 3200-3600
• OH (carboxylic acids) 3000-3100
• NH 3350-3500

16
Infrared spectrum of 2-hexanol
Transmittance ()
100
CH stretch
OH stretch
0
4400 3600 2800 2000 1600 1200 800
Wave number, cm-1
17
Infrared spectrum of 2-hexanone
Transmittance ()
100
CH stretch
CO stretch
0
4400 3600 2800 2000 1600 1200 800
Wave number, cm-1