INFARED SPECTROSCOPY

1. Infrared (IR) Spectroscopy
Vibrational Spectroscopy
Dr. J.J.E Munissi

2. • Spectroscopy: is the study of
interaction of matter with
electromagnetic radiation
What it Does
• Infrared (IR) spectroscopy measures the
bond vibration frequencies in a molecule
and is used to determine the functional
group.
• The instrument is called Infrared
Spectrophotometer.

3. Electromagnetic Radiation
• Energy with particle-like and wave-like properties
e.g X-rays, microwaves, radio waves, visible light, IR
and UV.
• A particle of an electromagnetic radiation is a
photon.
• Thus, EMR are photons travelling at the speed of
light. Hence, can be characterized by frequency ()
or wavelength ().
• Frequency = no. of complete wave cycles that pass a
fixed point in a second – (Hz)

4. Electromagnetic radiation
•Frequency and wavelength are inversely proportional.
•Thus, their related by the equation given ;
• Wavelength = distance between
any two peaks (or troughs) of the
wave- μm (10-6 m) or nm (10-9 m)

5. Electromagnetic Radiation
Where h is the Planck’s constant
• Wavenumber (ῡ) is another way to describe frequency of electromagnetic radiation
and is common in infrared spectroscopy.
•It is the number of waves in one centimeter.
•Therefore has units of reciprocal centimeter.
•Thus,

6. Exercise
Problem 1
Problem 2

7. Chapter 12 7
The Spectrum and
Molecular Effects
=>

8. 8
Infra Red Spectroscopy
• Background
– The infrared region of the electromagnetic spectrum
extends from 14,000 cm-1 to 100 cm-1.
– The region of most interest to the organic chemist is
the mid-infrared region (4,000 cm-1 to 400 cm-1)
which corresponds to changes in vibrational energies
within molecules.
– The far infrared region (400 cm-1 to 10 cm-1) is useful
for molecules containing heavy atoms such as
inorganic compounds but requires rather specialized
experimental techniques. Not useful to Organic
chemistry.

9. 9
Principle: Molecular Vibrations
• All covalent bonds in a molecule vibrate, even at a temperature of
absolute zero.
• Bonds behave like springs connecting two atoms.
• Two types of vibrations are involved, stretching and bending motions.
• A stretch is a vibration along the line of the bond that changes the
bond length.
• A bend is a vibration that does not occur along the line of the bond
but changes the bond angle.
• A diatomic molecule like H-Cl can undergo stretching vibrations only
since it has no bond angles.

10. 10
•In general, a polyatomic nonlinear molecule
with ‘n’ atoms has 3n-6 distinct vibrations.
Linear molecules have 3n-5 vibrations.
• These vibrations are either
Symmetric or asymmetric.
• Bending vibrations are usually described as
scissoring, rocking, twisting or wagging.

11. 11
Important Stretching Vibrations
C H
H
H
R C H
H
H
RMethyl
Anhydride O
O O
O
O O
2872 cm-1
2960 cm-1
1760 cm-1
1800 cm-1
Symmetric Stretch Asymmetric stretch

12. 12
Important Stretching Vibrations
N
H
H
R N
H
H
RAmine
N
O
O
R N
O
O
RNitro
3300 cm-1
3400 cm-1
1350 cm-1
1550 cm-1

13. Vibrations
• What is a vibration in a molecule?
– Any change in shape of the molecule-stretching of
bonds, bending of bonds or internal rotation around
single bonds.
• Can a vibration change the dipole moment of a
molecule?
– Asymmetrical stretching/bending and internal
rotation change the dipole moment of a molecule.
Asymmetrical stretching/bending are IR ACTIVE!
– Symmetrical stretching/bending does not change the
dipole moment. NOT IR ACTIVE!

14. 14
Infrared Activity
• Not all possible vibrations result into absorption band.
• For activity: the vibration must result in a change of
dipole moment during vibration.
• Homonuclear molecules: IR inactive because DM = 0. e.g
Cl-Cl or symmetrical molecules such as the triple bond in
dimethyl acetylene
• No need of permanent DM for IR activity.
• CO2 symmetric and asymmetric vibrations
• Asymmetric vibration give rise to IR activity.
C C CH3H3C
C OO
Symmetrical
= 0
C OO
Asymmetrical
= 

15. 15
How Does IR Work?
• Each stretching and bending of a bond occurs at a specific
frequency. When a compound is bombarded with IR
radiation of a frequency that exactly matches the
frequency of one of its vibrations, the molecule will
absorb energy.
• When this frequency is determined experimentally, one
can ascertain the type of bonds a molecule has.
• Light with wavelengths of 25 µm to 2.5 µm is commonly
used in IR analysis.
• IR is therefore used to identify Functional groups.

16. Sketch Diagram of the Infra red
Spectrophotometer
• A beam of Infra-Red light is passed through a
sample comparing it to a reference beam.
• The wavelength of the light is scanned over time.
• The light is collected by a detector and transformed
into an electrical signal with a processor.
• A spectrum with % transmittance vs wavenumber is
obtained.

17. An example of the IR spectrum (4-
hydroxy-4-methyl-2-pentanone)

18. 18
The Functional Group Region and The
Fingerprint Region
• The functional group region is the left hand two-thirds of
an IR spectrum (4000 -1400 cm-1) where most of the
functional groups show absorption bands.
• Complex vibrations: 1400 - 400 cm-1, called the
“fingerprint region.” But the fingerprint region is most
useful in identifying compounds by comparison with
known spectra.
• The region is characteristic of the compound as a whole.
A unique pattern of absorption bands is reflected in this
region.
• No two molecules will give exactly the same IR spectrum
(except enantiomers).

19. The Position of IR Frequencies
• Measured in wavenumbers;
• The frequency of vibrations depends on the atoms at the end of
bond.
• Strength of the bond and the masses of atoms on either end of
the bond determine which wavelength is absorbed.
• Because bonds behave much like springs Hooke’s Law is applied
to molecules to determine the frequency of vibrations of a bond
• Where ѵ =frequency; c=speed of light; k=force constant that
indicates the strength of the bond; µ= reduced mass (a function
of the masses of two atoms involved in the bond)

c
k

m1m2
m1+m2
 =
2
1

20. 20

21. 21
Factors Affecting Frequency
C C C C C C
2100 1650 1200
Bond strength
C H C C C O
3000 1200 1100
Mass of atoms
C I
500
C Br
600
C Cl
750
Hybridization C CH H H
3300 3100 2900
Conjugation
O O O
1715 1675 - 1680

22. 22
IR Frequencies
• 3700 - 2500 cm-1: X-H stretching (X = C, N, O, S)
• 2300 - 2000 cm-1: C≡X stretching (X = C or N)
• 1900 - 1500 cm-1: C=X stretching (X = C, N, O)
• 1300 - 800 cm-1: C-X stretching (X = C, N, O)
NOTE: Because it takes more energy to stretch a bond
than to bend it, absorption bands for stretching
vibrations occur in the functional group region whereas
absorption bands for bending vibrations typically occur
in the fingerprint region.

23. 23
What Information from IR?
• Only IR frequency matching natural frequency (of
functional groups) in molecule lead to absorption.
Condition for absorption = absence of symmetry.
CH3H3CH2C
H3CH2C
Symmetrical Pseudosymmetrical

24. 24
Typical regions of functional
groups

25. 25
IR of Common Functional
Groups

26. 26
Exercise: Assign (cm-1)
OH
O
O
OO
A B C

27. Exercise
Take Home Activity
•What are the different ways of preparing samples/compounds
for determination of infrared spectrum?
•What the factors that affect the intensity of an IR absorption band?