Infrared
Spectroscopy
Unit 15
15-1
Electromagnetic Radiation
 Electromagnetic
radiation: light and other forms
of radiant energy
 Wavelength (): the distance between
consecutive peaks on a wave
 Frequency (): the number of full cycles of a
wave that pass a given point in a second
 Hertz (Hz): the unit in which radiation frequency
is reported; s-1 (read “per second”)
15-2
Electromagnetic Radiation
 Common
units used to express wavelength
Un it
Meter (m)
Millimeter (mm)
Micrometer (m)
N anometer (nm)
An gs trom (Å )
Relation
to Meter
---1 mm = 10-3 m
1 m = 10-6 m
1 nm = 10-9 m
1 Å = 10-10 m

15-3
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
Absorption of
Region of the
Electromagnetic
Electromagnetic Type of
Radiation Results
Spectrum
Spectroscopy
in Transition Between
Radio fequency Nu clear magnetic Nuclear spin states
resonan ce
Infrared
Vibrational energy levels
Infrared
UltravioletElectronic energy levels
Ultravioletvisible
visib le
15-4
Infrared Spectroscopy
vibrational IR extends from 2.5 x 10-6 m (2.5
m) to 2.5 x 10-5 m (25 m)
 The
• the frequency of IR radiation is commonly expressed
in wavenumbers
- : the number of waves per centimeter,
• wavenumber()
with units cm-1 (read reciprocal centimeters)
• expressed in wavenumbers, the vibrational IR extends
from 4000 cm-1 to 400 cm -1
-2
-1
10
m•cm
 =
= 4000 cm-1
2.5 x 10-6 m
 =
10-2 m•cm -1
2.5 x 10-5 m
= 400 cm-1
15-5
Infrared Spectroscopy
 IR
spectrum of 3-methyl-2-butanone
15-6
Molecular Vibrations
• atoms joined by covalent bonds undergo continual
vibrations relative to each other
• the energies associated with these vibrations are
quantized; within a molecule, only specific vibrational
energy levels are allowed
• the energies associated with transitions between
vibrational energy levels correspond to frequencies in
the infrared region, 4000 to 400 cm-1
15-7
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
15-8
Molecular Vibrations
 For
a nonlinear molecule containing n atoms,
there are 3n - 6 allowed fundamental vibrations
 For even a relatively small molecule, a large
number of vibrational energy levels exist and
patterns of IR absorption can be very complex
 The simplest vibrational motions are bending
and stretching
15-9
Molecular vibrations
 Fundamental
stretching and bending vibrations
for a methylene group
15-10
Molecular Vibrations
 Consider
two covalently bonded atoms as two
vibrating masses connected by a spring
• the total energy is proportional to the frequency of
vibration
• the frequency of a stretching vibration is given by an
equation derived from Hooke’s law for a vibrating
spring

= 4.12
K

K = a force constant, which is a measure of the bonds’
strength; force constants for single, double, and triple
bonds are approximately 5, 10, and 15 x 105 dynes/cm
 = reduced mass of the two atoms, (m1m2)/(m1 + m2),
15-11
where m is the mass of the atoms in grams
Molecular Vibrations

= 4.12
K

 From
this equation, we see that the position of a
stretching vibration

• is proportional to the strength of the vibrating bond
• is inversely proportional the masses of the atoms
connected by the bond
The intensity of absorption depends primarily on the
polarity of the vibrating bond
15-12
Correlation Tables
 Characteristic
IR absorptions for the types of
bonds and functional groups we deal with most
often
Bon d
O-H
N-H
C-H
C=C
C=O
C-O
Stretching
Frequ ency (cm -1)
3200-3650
3100-3550
2700-3300
1600-1680
1630-1820
1000-1250
Intens ity
w eak to s trong
mediu m
w eak to medium
w eak to medium
strong
strong
15-13
Hydrocarbons
Hydrocarbon
Alk ane
C-H
CH3
C-C
Alk ene
C-H
C=C
Alk yn e
C-H
C C
Arene
C-H
C=C
C-H
Vib ration
Stretchin g
Bend ing
Bend ing
(N ot useful
Frequen cy
(cm-1 )
Intens ity
2850 - 3000
Mediu m
1450-1475
Mediu m
1375 and 1450 Weak to medium
for interpretation - too man y b ands
Stretchin g
Stretchin g
3000 - 3100
1600 - 1680
Weak to medium
Weak to medium
Stretchin g
Stretchin g
3300
2100-2250
Mediu m to stron g
Weak
Stretchin g
Stretchin g
Bend ing
3030
1450-1600
690-900
Weak to medium
Mediu m
Strong
15-14
Alkanes
 IR
spectrum of decane
15-15
Alkenes
 IR
spectrum of cyclohexene
15-16
Alkynes
 IR
spectrum of 1-octyne
15-17
Aromatics
 IR
spectrum of toluene
15-18
Alcohols
Bond
Frequency, cm-1
O-H (free)
3600-3650
O-H (H b ond ed)
C-O
3200 - 3500
1000 - 1250
Inten sity
Weak
Medium, broad
Medium
• IR spectrum of 1-hexanol
15-19
Ethers
 IR
spectrum of dibutyl ether
15-20
Ethers
 IR
spectrum of anisole
15-21
Amines
 IR
spectrum of 1-butanamine
15-22
IR of Molecules with C=O Groups
Vibration
Frequen cy
(cm-1 )
Inten sity
Ketones
C=O
Stretchin g
1630-1820
Strong
Aldeh yd es
C=O
C-H
Stretching
Stretching
1630-1820
2720
Strong
Weak
Carboxylic acids
C=O
Stretching
O H
Stretching
1700-1725
2500-3300
Strong
Strong (broad)
Carbonyl Group
O
RCR'
O
RCH
O
RCOH
15-23
IR of Molecules with C=O Groups
O
RCNH2
Amides
C=O
N H
(1° amides h ave
(2° amides h ave
O
RCOR'
Carboxylic esters
C=O
Stretchin g
2
sp C O
Stretchin g
sp3 C O
Stretchin g
O O
RCOCR
Acid anhydrides
C=O
Stretchin g
RC N
Stretchin g
1630-1680
Stretchin g
3200, 3400
tw o N -H stretches )
one N -H stretch )
Strong
Mediu m
1735-1800
1200-1250
1000-1100
Strong
Strong
Strong
Strong
Mediu m
C O
Stretchin g
1740-1760 and
1800-1850
900-1300
Nitriles
CN
Stretchin g
2200-2250
Strong
15-24
Aldehydes and Ketones
 IR
spectrum of menthone
15-25
Carbonyl groups
 The
position of C=O stretching vibration is
sensitive to its molecular environment
• as ring size decreases and angle strain increases,
absorption shifts to a higher frequency
O
O
O
O
1715 cm-1
1745 cm-1
1780 cm-1
1850 cm-1
• conjugation shifts the C=O absorption to lower
frequency
O
O
O
H
-1
1717 cm
-1
1690 cm
-1
1700 cm
15-26
Carboxylic acids
 IR
spectrum of pentanoic acid
15-27
Esters
 IR
of ethyl butanoate
15-28