Infrared (IR) spectroscopy is a technique that uses infrared light to study molecular interactions, providing information about functional groups in molecules. The IR spectrum acts as a fingerprint for molecular identification, reliant on proper wavelength absorption and changes in dipole moments. The document explains molecular vibrations, including stretching and bending vibrations, and emphasizes the significance of the middle IR region for analytical applications.

1. Presented by:
Md. Faysal Ah
ID:151-29-760
Sec: B 13th
DIU

2. Spectroscopy
• Method of “Seeing the “unseeable”
• Using electromagnetic radiation to obtain information about atoms
and molecules that are too small to see.
atoms
molecule

3. IR spectroscopy
IR spectroscopy is the spectroscopic technique which uses the Infrared light and
studies its interaction with the molecules. The spectra generated can give a lot of
information about the molecule, especially about the functional groups associated.

4. Basics of IR spectrophotometer
Wave number =
1
Wavelength in centimeters

5. In the IR spectroscopy, the absorbed energy brings about predominant changes in the
vibrational energy which depends upon three things.
i) Masses of the atoms present in a molecule,
ii) Strength of the bonds
iii) The arrangement of atoms within the molecule.

6. Instrumentation of IR spectrophotometer:

7. Instrumentation of IR spectrophotometer:

11. When energy in the form of infrared radiation is applied then it causes the vibration between the
atoms of the molecules and when,
Applied infrared frequency = Natural frequency of vibration
Then, Absorption of IR radiation takes place and a peak is observed.
Different functional groups absorb characteristic frequencies of IR radiation. Hence gives the
characteristic peak value.
Therefore, IR spectrum of a chemical substance is a finger print of a molecule for its identification.

12. Criteria for a compound to absorb IR radiation
1. Correct wavelength of radiation
2. Change in dipole moment
1. Correct wavelength of radiation:
A molecule to absorb IR radiation, the natural frequency of vibrations of some part of
a molecule is the same as the frequency of incident radiation.

13. 2. Change in dipole moment :
• A molecule can only absorb IR radiation when its absorption cause a
change in its electric dipole.
• A molecule is said to have an electric dipole when there is a slight
positive and a slight negative charge on its component of atoms.

14. Molecular vibrations
There are 2 types of vibrations:
1.Stretching vibrations
2.Bending vibrations
1. Stretching vibrations:
Occurs at higher energy: 4000-1250 cm-1
2 types:
a) Symmetrical stretching
b)Asymmetrical stretching

15. 1. Stretching vibrations:

16. 2. Bending vibrations
The positions of the atom change with respect to the original bond axis.
Occurs at low energy: 1400-666 cm-1 .
• There are 2 types:
a) In plane bending: scissoring, rocking
b) Out plane bending: wagging, twisting

17. 2. Bending vibrations

18. • IR region: 0.8 µm (800nm) to 1000 µm (1mm)
Sub divided into
1.Near IR: 0.8-2 µm
2.Middle IR: 2-15 µm
3.Far IR: 15-1000 µm
Most of the analytical applications are confined to the middle IR region because absorption
of organic molecules are high in this region.