2. ContentS
• Introduction
• History
• Basic principle
• Theory of molecular
vibrations
• Instrumentation
• Finger Print region
• Applications
• Conclusion
• References
3. • It is the study of absorption of infra red radiation which results in
the vibrational transitions.
• IR spectrum is an important records which gives sufficient
information about the structure of a compound and also
determine the functional groups.
• For a molecule to be IR active the vibration should give rise to a
net change in dipole moment.
• The range of IR radiation is from 2.5 μ – 15 μ.
Introduction
0.8 2.5 15 200
μ
IR region Far IR regionNear IR
region
12500. 4000. 667. 50
cm-1
5. Basic principle
• The principle of IR spectroscopy is related to the vibrational and
rotational energy of a molecule.
• Absorption of IR radiation causes an excitation from a lower to the
higher vibrational level.
• Each vibrational level is associated with a number of closely placed
rotational levels. Therefore the IR spectroscopy is also refer as
vibrational-rotational spectroscopy.
• All the bonds in a molecule are not capable of absorbing IR energy
but those bond which are accompanied by a change in dipole
moment will absorb in the IR region and such transition are called IR
active transitions.
6. Theory of molecular vibrations
Any change in shape of the molecule :- stretching of bonds, bending
of bonds and internal rotation around single bonds.
Molecular vibrations is of two types :-
1.Fundamental Vibration
2.Non-fundamentall vibration
Fundamental vibration is also divided into 2 types
1. Stretching vibrations :-
Vibration involves a continuous
change in inter atomic distance
along the same axis of bond
between two atoms.
2. Bending vibration :-
It involves change in bond angle
between two atoms.
8. Instrumentation
The main parts of IR
Spectrophotometer are :-
1. Radiation source
2. Sample cell and
sampling of substance
3. Monochromator
4. Detector
5. Recorder
9. Finger print region
• In IR, the region between 1500 – 667 cm-1 is rich in
many absorption bands and the region is known as
Finger Print region.
• In this region the number of bending vibrations is
usually more than the number of stretching vibrations.
• Many compounds shows unique absorption bands in
this region and which is very useful for the
identification of the compounds.
• Finger print region can be sub-divided into 3 regions.
1500 – 1300 cm-1 :- presence of tertiary butyl
groups.
1300 – 1000 cm-1 :- presence of group like alcohol, ester,
lactones, acid anhydride due to presence of c---o stretching
1000 - 667 cm-1 :- distinguished between cis-trans alkenes
and mono & di substituted at ortho, meta, para position in
benzene.
10. Applications
• Identification of an organic compounds.
• Quantitative analysis of functional groups.
• Distinction between hydrogen bonds.
• Study of chemical reaction.
• Determination of molecular structure of organic
compounds.
11. Reference
1. Elementary Organic Spectroscopy
By- Y. R. Sharma, S.Chand Publisher, Page no- 69 – 150
2. Spectroscopyy
By- H. Kaur, Pragati Publisher, Page no- 166 – 301
3. Instrumentalal Methods of Chemical
Analysis
By – Gurdeep R. Chatwal , Sham K. Anand, Himalaya Publisher,
page no – 2.29 – 2.82
4. Spectroscopy of Organic Compounds
By – P.S Kalsi, Page no- 50 – 164
5. Fundamentals of Molecular Spectroscopy
. By Dr. C.N Banwell, E.M. McCASH (Mc graw Hill), Page no – 55 - 99
