William H. Brown
Christopher S. Foote
Brent L. Iverson
Eric Anslyn
http://academic.cengage.com/chemistry/brown
Chapter 12
Infrared Spectroscopy
William H. Brown • Beloit College
12-1
Molecular Spectroscopy
 Molecular
spectroscopy The study of which
frequencies of electromagnetic radiation are
absorbed or emitted by a particular substance
and the correlation of these frequencies with
details of molecular structure.
• we study three types of molecular spectroscopy
Region of the
El ectromagneti c Frequency
Spectrum
(hetz)
Radio frequency 3 x10 7-9x10 8
Infrared
Ultraviol etvisible
1 x10 13 -1x1014
Type of
Spectroscopy
Nuclear magnetic
resonance
Infrared
2.5 x10 14 -1.5x10 15 Ultravioletvisible
Absorpti on of
El ectromagneti c
Radiation Results
in Transition Between
Nuclear spi n states
Vibrational energy level s
El ectronic energy level s
12-2
Molecular vibrations
 Fundamental
stretching and bending vibrations
for a methylene group.
12-3
Molecular Vibrations
 For
a molecule to absorb IR radiation
• the bond undergoing vibration must be polar and
its vibration must cause a periodic change in the bond
dipole moment.
 Covalent
bonds which do not meet these criteria
are said to be IR inactive.
• The C-C double and triple bonds of symmetrically
substituted alkenes and alkynes, for example, are IR
inactive because they are not polar bonds.
H3 C
CH3
C C
H3 C
CH3
H3 C-C C-CH3
2,3-Dimethyl-2-butene
2-Butyne
12-4
Infrared Spectroscopy
 Infrared
spectrum of 3-methyl-2-butanone.
C-H Stretching
Fingerprint Region:
Highly Complex and
Unique for Every Molecule
12-5
Correlation Tables
 Infrared
stretching frequencies of selected
functional groups.
Less Valuable
Almost all organic
molecules have C-H
bonds
C=C often too weak or
encroaching on
fingerprint region
Buried in fingerprint
region
Bond
O-H
N-H
C-H
C=C
C=O
C-O
Stretching
Frequency (cm-1)
3200-3650
3100-3550
2700-3300
1600-1680
1630-1820
1000-1250
More Valuable
Intensity
Potentially difficult to
weak to strong
distinguish between oneanother, but good
medium
indication of heteroatomhydrogen bond
weak to medium
weak to medium
Signature Stretch.
strong
Very strong, very
identifiable
strong
12-6
Carboxylic acids
 Infrared
spectrum of pentanoic acid.
12-7
Amines
 Infrared
spectrum of 1-butanamine, a 1° amine.
12-8
Hydrocarbons-Table 12.5
Difficult to
distinguish
between oneanother
Hydrocarbon
Alkane
C-H
CH2
CH3
C-C
Alkene
C-H
C= C
Alkyne
C-H
C C
Arene
C-H
C= C
C-H
Vibration
Frequency
(cm-1 )
Intensity
Stretching 2850 - 3000
Medium
1450-1475
Medium
Bending
1375 and 1450 Weak to medium
Bending
(Not useful for interpretation - too many bands
Stretching
Stretching
3000 - 3100
1600 - 1680
Weak to medium
Weak to medium
Stretching
Stretching
3300
2100-2250
Medium to strong
Weak
Stretching
Stretching
Bending
3030
1450-1600
690-900
Weak to medium
Medium
Strong
12-9
Aromatics
 Infrared
spectrum of toluene.
12-10
Hydrocarbons-Table 12.5
Hydrocarbon
Alkane
C-H
CH2
CH3
C-C
Alkene
C-H
C= C
Alkyne
C-H
C C
Arene
C-H
C= C
C-H
Vibration
Frequency
(cm-1 )
Intensity
Stretching 2850 - 3000
Medium
1450-1475
Medium
Bending
1375 and 1450 Weak to medium
Bending
(Not useful for interpretation - too many bands
Stretching
Stretching
3000 - 3100
1600 - 1680
Weak to medium
Weak to medium
Stretching
Stretching
3300
2100-2250
Medium to strong
Weak
Stretching
Stretching
Bending
3030
1450-1600
690-900
Weak to medium
Medium
Strong
Can be useful.
In fairly distinct
regions
12-11
Alkynes
 Infrared
spectrum of 1-octyne.
12-12