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IR SPECTROSCOPY
images, Ayesha karimji
19PH34 - Coggle Diagram
IR SPECTROSCOPY
Principal
he absorption of Infra-red radiations causes an excitation of
molecule from a lower to the higher vibrational level.
We know that each vibrational level is associated with a number of
closely spaced rotational levels. Clearly, the Infra-red spectra is
considered as vibrational-rotational spectra.
All the bonds in a molecule are not capable of absorbing infra-red
energy but only those bonds which are accompanied by a change
in dipole moment will absorb in the infra-red region.
For example, vibrational transitions of C = O, N—H, O—H etc. bands are accompanied by a
change in dipole-moment and thus, absorb strongly in the Infra-red region. But transitions in
Carbon-Carbon bonds in symmetrical alkenes and alkynes are not accompanied by the change in
dipole-moment and hence do not absorb in the infra-red region.
Fundamental vibration
Stretching vibration:-
In this type of vibrations, the distance between the two atoms increases or decreases
but the atoms remain in the same bond axis.
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Bending or deformation vibration
n this type of vibrations, the positions of the atoms change with respect to the original
bond axis.
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Degrees of freedom
Polyatomic molecules may exhibit more than one fundamental
vibrational absorption bands.
The number of these fundamental bands is related to the degrees
of freedom in a molecule.
The number of degrees of freedom is equal to the sum of the co-
ordinates necessary to locate all the atoms of a molecule in space.
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