The document provides an overview of infrared (IR) spectroscopy, discussing its theory, modes of vibration, and various instrumentation used for measurements. It explains the electromagnetic spectrum range of IR, the significance of Hooke’s law in molecular vibrations, and details different types of IR spectrometers and their components. Additionally, it covers sample preparation techniques for solids, liquids, and gases, as well as the classification of detectors used in IR spectroscopy.

1. IR spectroscopy
Supriya panda
Lecturer in chemistry
Ramadevi Women’s University

2. CONTENTS
• Theory of IR spectroscopy
• Hooke’s law
• Modes of vibration
• Interpretation of IR frequency of various compounds
• Instrumentation

3. • The term "infra red" covers the range of the electromagnetic
spectrum between 0.78 and 1000 mm. In the context of infra red
spectroscopy, wavelength is measured in "wavenumbers", which
have the units cm-1.
wavenumber = 1 / wavelength in centimeters
• It is useful to divide the infra red region into three sections; near, mid
and far infra red region Wavelength range (mm) Wavenumber range:
Near 0.78 - 2.5 12800 - 4000
Middle 2.5 - 50 4000 – 200
Far 50 -1000 200 - 10
• The most useful I.R. region lies between 4000 - 670cm-1
•

4. Theory of infra red
absorption
IR radiation does not have enough energy to induce electronic
transitions as seen with UV. Absorption of IR is restricted to compounds
with small energy differences in the possible vibrational and rotational
states.For a molecule to absorb IR, the vibrations or rotations within a
molecule must cause a net change in the dipole moment of the
molecule. The alternating electrical field of the radiation (remember that
electromagnetic radation consists of an oscillating electrical field and an
oscillating magnetic field, perpendicular to each other) interacts with
fluctuations in the dipole moment of the molecule. If the frequency of
the radiation matches the vibrational frequency of the molecule then
radiation will be absorbed, causing a change in the amplitude of
molecular vibration.

5. Molecular rotations
Rotational transitions are of little use to the spectroscopist.
Rotational levels are quantized, and absorption of IR by gases
yields line spectra. However, in liquids or solids, these lines
broaden into a continuum due to molecular collision and other
interactions.
Molecular vibrations
The positions of atoms in a molecules are not fixed; they are
subject to a number of different vibrations. Vibrations fall into the
two main catagories :
stretching and bending.

6. Modes of vibration
Stretching: Change in inter-atomic distance along bond axis
1. Symmetric
2. Asymmetric
Bending: Change in angle between two bonds. There are
four types of bending
• Rocking
• Scissoring
• Wagging
• Twisting

8. HOOKE’S LAW
• The Hooke’s Law is a mathematical formula that relates the
vibrational frequency of a spring connected to two spheres to
the stiffness of the spring and to the masses of the spheres.
• Vibrations of a covalent bond is thought to be similar to those
of the above system. Thus, the Hooke’s Law can be applied to
the vibrations of a covalent bond. Given below is the Hooke’s
Law as it applies to a covalent bond.

10. According to the Hooke’s Law,
1. The stronger the bond, the faster the bonds vibrates.
2. The lighter the atoms linked by the bond, the faster the
bond vibrates.

11. Energy of a particle that can be mapped by
simple harmonic oscillation is shown .
• The energy splitting is
either ħω which is
equivalent to hv0.
• The energy splittings
are equal to one
another.
• The lines on either side
are not asymptotic on
the y axis; this means
the particles .

12. Anharmonic oscillator.
• The energy spacing is not equal between the energy levels.
• The potential energy barrier does not cross the y axis as the
nuclei cannot pass through one another.
• The molecule dissociates at the largest separation.
• Comparison shows that the quantum harmonic oscillator is
pulled up on the left hand side.
• right hand side has a tailing off of energy.

13. Alkanes:

14. Alkenes:
Alkynes:

15. Aromatic Rings

16. Alcohols and phenols
Ethers

17. Carbonyl compounds:
Aldehydes :

18. Ketones
Carboxylic Acid

19. Esters :

20. Amides :
Acid chlorides:
Acid anhydride:

21. Instrumentation
There are four types of instruments for infrared absorption
measurements available:
 Dispersive grating spectrophotometers for qualitative measurements
 Nondispersive photometers for quantitative determination of organic
species in the atmosphere
 Reflectance photometers for analysis of solids
 Fourier transform infrared (FT-IR) instruments for both qualitative
and quantitative measurements.

22. The main parts of IR
spectrometer are as follows:
o Radiation source
o sample cells
o Monochromators
o Detectors
o recordered measurements

23. Instruments for measuring infrared absorption all require a source of continuous
infrared radiation. Infrared sources consist of an inert solid that is electrically heated
to a temperature between 1,500 and 2,200 K. The heated material will then emit infra
red radiation.
• The Nernst glower:
The Nernst glower is constructed of rare earth oxides in the form of a
hollow cylinder. Platinum leads at the ends of the cylinder permit the passage of
electricity. Nernst glowers are fragile. They have a large negative temperature
coefficient of electrical resistance and must be preheated to be conductive.
• The globar source
A globar is a rod of silicon carbide (5 mm diameter, 50 mm long) which is
electrically heated to about 1,500 K. Water cooling of the electrical contacts is
needed to prevent arcing. The spectral output is comparable with the Nernst 28
glower, execept at short wavelengths (less than 5 mm) where it's output becomes
larger.
Infrared light sources

24. IR spectroscopy has been used for the characterization of solid, liquid or
gas samples.
• i. Solid - Various techniques are used for preparing solid samples
such as pressed pellet technique, solid run in solution, solid films,
mull technique etc.
• ii. Liquid – samples can be held using a liquid sample cell made of
alkali halides. Aqueous solvents cannot be used as they will dissolve
alkali halides. Only organic solvents like chloroform can be used.
• iii. Gas – sampling of gas is similar to the sampling of liquids.
Sample Cells and sampling of substances

25. Monochramotor
• Various types of monochromators are prism, gratings and
filters. Prisms are made of Potassium bromide, Sodium chloride
or Caesium iodide. Filters are made up of Lithium Fluoride and
Diffraction gratings are made up of alkali halides.
Detector
• The detectors can be classified into three categories, thermal
detectors, pyroelectric detectors and photoconducting detectors.

26. • Thermal detectors :
Thermal detectors can be used over a wide range of wavelengths and they operate at room
temperature. Their main disadvantages are slow response time and lower sensivity relative to
other types of detectors.
 Thermocouple
A thermocouple consists of a pair of junctions of different metals;
for example, two pieces of bismuth fused to either end of a piece of antimony. The potential
difference (voltage) between the junctions changes according to the difference in
temperature between the junctions. Several thermocouples connected in series are called a
thermopile.
 Bolometer
A bolometer functions by changing resistance when heated. It is constructed of strips of
metals such as platinum or nickel or from a semiconductor.
 Pyroelectric detectors
Pyroelectric detectors consists of a pyroelectric material which is an insulator with special
thermal and electric properties. Triglycine sulphate is the most common material for
pyroelectric infrared detectors. Unlike other thermal detectors the pyroelectric effect depends
on the rate of change of the detector temperature rather than on the temperature itself. This
allows the pyroelectric detector to operate with a much faster response time and makes these
detectors the choice for Fourier transform spectrometers where rapid response is essential.