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IR Spectroscopy, 19PH05
ANSARI MARYAM IFTEKHAR - Coggle Diagram
IR Spectroscopy
Principle :
The 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 carbon-carbon bonds in symmetrical alkenes and alkynes are not accompanied by the change in change in dipole-moment and thus, absorb strongly in the Infra-red region. But transitions in dipole-moment and hence do not absorb in the infra-red region.
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Degree 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.
Fundamental vibration :
Bending or deformation vibration : In this type of vibrations, the positions of the atoms change with respect to the original bond axis.It has four sub-types :
- Twisting
- Wagging
- Rocking
- Scissoring
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. It has two types :
- Symmetric stretching
- Asymmetric stretching
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