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
ChemActivity 16: Spectroscopy
‹ Split
quickly in groups of 4
‹ Assign one role to each person: 1) Manager, 2)
Recorder, 3) Time-keeper, 4) Spokesperson
‹ Go to the purple book to page 137 (Chem Activity
16, Part A)
‹ Work on the critical thinking questions (1-9)
‹ You have 15-20 min
‹ We will discuss the questions in 5 min
12-2
Electromagnetic Radiation
‹ Electromagnetic
radiation: Light and other forms
of radiant energy.
‹ Wavelength (λ): The distance between
consecutive peaks on a wave.
‹ Frequency (ν): The number of full cycles of a
wave that pass a given point in a second and is
reported in hertz, which has the units s-1.
‹ Hertz (Hz): The unit in which radiation frequency
is reported: s-1 (read “per second”).
12-3
Electromagnetic Radiation
‹ Common
units used to express wavelength(λ)
Un it
Meter (m)
Millimeter (mm)
Micrometer (μm)
N anometer (nm)
An gs trom (Å )
Relation
to Meter
---1 mm = 10-3 m
1 μm = 10-6 m
1 nm = 10-9 m
1 Å = 10-10 m
λ
12-4
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
Electromagnetic Frequency
Spectrum
(hetz)
Radio frequency 3 x10 7-9x10 8
Infrared
Ultravioletvisible
1 x1013 -1x10 14
Type of
Spectroscopy
Nuclear magnetic
resonance
Infrared
2.5 x10 14 -1.5x10 15 Ultravioletvisible
Absorption of
Electromagnetic
Radiation Results
in Transition Between
Nuclear spin states
Vibrational energy levels
Electronic energy levels
12-5
Infrared Spectroscopy
vibrational IR extends from 2.5 x 10-6 m (2.5
μm) to 2.5 x 10-5 m (25 μm).
‹ The
• The frequency of IR radiation is commonly expressed in
wavenumbers. (ν)
• Wavenumber: The number of waves per centimeter,
with units cm-1 (read reciprocal centimeters).
• Expressed in wavenumbers, the vibrational IR extends
from 4000 cm-1 to 400 cm -1.
-2
-1
ν = 10 m•cm
2.5 x 10-6 m
= 4000 cm -1
ν =
10-2 m•cm
-1
2.5 x 10-5 m
= 400 cm -1
12-6
Infrared Spectroscopy
‹ Infrared
spectrum of 3-methyl-2-butanone.
12-7
Molecular Vibrations
• Atoms joined by covalent bonds undergo continual
vibrations relative to each other.
• The frequencies of rather than 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.
12-8
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
2,3-Dimethyl-2-butene
H3 C-C C-CH3
2-Butyne
12-9
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.
12-10
Molecular vibrations
‹ Fundamental
stretching and bending vibrations
for a methylene group.
12-11
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; E = hν where h is Planck’s constant.
• The frequency of a stretching vibration is given by an
equation derived from Hooke’s law for a vibrating
spring.
ν
= 4.12
K
μ
K = a force constant, which is a measure of bond
strength; force constants for single, double, and triple
bonds are approximately 5, 10, and 15 x 105 dynes/cm.
μ = reduced mass of the two atoms, (m1m2)/(m1 + m2),
12-12
where m is the mass of the atoms in amu.
Molecular Vibrations
ν
= 4.12
K
μ
‹ From
this equation, we see that the peak 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.
12-13
Correlation Tables
‹ Infrared
stretching frequencies of selected
functional groups.
Bon d
O-H
N-H
C-H
C=C
C=O
C-O
Stretching
Frequ ency (cm -1) Intens ity
3200-3650
w eak to s trong
mediu m
3100-3550
2700-3300
w eak to medium
1600-1680
w eak to medium
1630-1820
strong
1000-1250
strong
12-14
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
2850 - 3000
Stretching
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
12-15
Alkanes
‹ Infrared
spectrum of decane.
12-16
Alkenes
‹ Infrared
spectrum of cyclohexene.
12-17
Alkynes
‹ Infrared
spectrum of 1-octyne.
12-18
Aromatics
‹ Infrared
spectrum of toluene.
12-19
Alcohols
Bond
O- H (free)
O- H (H bond ed)
C-O
Frequency , cm -1
3600-3650
3200 - 3500
1000 - 1250
Intensity
Weak
Medium, broad
Medium
• The free O-H is not intrinsically weak. It’s intensity depends on
concentration and the hydrogen bonding character of the solvent
• Infrared spectrum of 1-hexanol
12-20
Ethers
‹ Infrared
spectrum of dibutyl ether.
12-21
Ethers
‹ Infrared
spectrum of anisole.
12-22
Amines
‹ Infrared
spectrum of 1-butanamine, a 1° amine.
12-23
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
Stretching
O H
1700-1725
2500-3300
Strong
Strong (broad)
Carbonyl Group
O
RCR'
O
RCH
O
RCOH
12-24
IR of Molecules with C=O Groups
O
RCNH2
Amides
C=O
Stretchin g
1630-1680
Stretchin g
3200, 3400
N H
(1° amides h ave tw o N -H stretches )
(2° amides h ave one N -H stretch )
O
RCOR'
Carboxylic esters
Stretchin g
C=O
2
Stretchin g
sp C O
Stretchin g
sp3 C O
O O
RCOCR
Acid anhydrides
Stretchin g
C=O
RC N
Strong
Mediu m
1735-1800
1200-1250
1000-1100
Strong
Strong
Strong
Strong
Strong
Mediu m
C O
Stretchin g
1740-1760 and
1800-1850
900-1300
Nitriles
C≡N
Stretchin g
2200-2250
12-25
Aldehydes and Ketones
‹ Infrared
spectrum of menthone.
12-26
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
1717 cm -1
1690 cm -1
1700 cm -1
12-27
Carboxylic acids
‹ Infrared
spectrum of pentanoic acid.
12-28
Esters
‹ Infrared
spectrum of ethyl butanoate.
12-29
Infrared
Spectroscopy
End Chapter 12
12-30