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FT - IR

Near IR

12500

IR

4000

Far IR

667

50

PRINCIPLE:
The molecules absorption of infra red radiation to excitation
of lower to higher vibrational level
...
So, infrared is called as
vibrational rotational spectra
...

EXAMPLE:
C=O, N-H, -OH Etc
...
Absorption in the IR region is due to the changes in
vibrational and rotational levels
...
Operating frequency is less than 100cm-1 are absorbed
molecules containing rotational level only
...

3
...
These
vibrational spectra appear as vibrational-rotational bands
...
Vibrational energy is depends upon
(i)
(ii)
(iii)

Mass of the atom present in a molecule
...

Arrangement of the atoms within the
molecules
...
Two types of stretching are there,
1
...
Asymmetrical stretching
Symmetrical stretching:
Movement of the atoms with respect to a particular atom in
the same direction
...


Bond length
Decreases

Bond length
Increases

Bond length

Bond length

Increases

Decreases

TYPES OF BENDING VIBRATIONS: (DEFORMATIONS):
1
...

3
...


Scissoring
Rocking
Wagging
Twisting

Scissoring:
Two atoms are approaches each other
...


Or

Wagging:
Two atoms are move up (forward) and down (backward) to
the plane with respect to the central atom
...


Bending vibrations are required lesser energy (higher
wavelength) than stretching vibrations
...
Some fundamental vibrations
are IR active and some are inactive
...

Example:

A–C–A

The molecules has non-centrosymmetric, its IR active
and Raman Inactive
...
The calculated frequency is never exactly equal to the
experimental value
...

i
...


Symmetric

Asymmetric

Asymmetric stretching vibrations always higher
wavenumber than symmetrical vibrations
...
Similarly, the
coupled vibrations of -CH3 group takes place at different frequency
compared to -CH2 group
...
This can be explained due to the
symmetrical and asymmetrical stretching
...
It’s called as Fermi
resonance
...
, In this effect change the force constant or bond strength, its
absorption frequency is shifts from a normal frequency value
...
e
...


HCHO
𝜈 = 1750 cm-1

CH3CHO
𝜈 =1745 cm-1

CH3COCH3
𝜈 = 1715 cm-1

From left to right,





+I effect increases
Bond length increases
Bond strength decreases
Force constant decreases

Absorption is Decreases
...
e
...


CH3COCH3

CH3COCH2Cl

CH3COCHCl2

ClCH2COCHCl2

𝜈 = 725 cm-1

𝜈 = 1740 cm-1

𝜈 =1750 cm-1

𝜈 =1715 cm-1

From left to right,
 -I effect increases
 Bond length decreases
 Bond strength increases
 Force constant increases

Absorption is Increases
iv) Hydrogen bonding:
Hydrogen bond is present in this molecule it affects the
absorption frequency
...


Intermolecular hydrogen bonding:
Intermolecular hydrogen bonds (between the
molecules) are rise to broad peak
...

Example:

H – F ----------- H – F

Intermolecular hydrogen bond

Intramolecular hydrogen bonding:
Intramolecular (within the molecule) hydrogen bond
is appeared as sharp and well defined
...

F --------- H
O

Example:
N – H stretching 3500 cm-1
Dilute solution 3300 cm-1
H – bonded O-H group 3600
Free O-H group 3350

iv) Bond angle:
Less bond angle compounds are occupied in higher wave
number
...
It’s
heated to 1500° to produce IR radiation
...


Monochromator:
Optical prism, grating, quartz
...

 Sodium chloride is hygroscopic nature
...


WORKING:
Light source is splits into two beams one source is
passed through the sample, it called as a sample beam
...
When a beam is passed through a sample, certain
frequency is missing, it compared with reference beam
...
Intensities are converted to measured as
electrical energies with the help of thermopile (detector)
...

 The chopper M rotates at definite speed reflects the sample and
reference beams to monochromator grating (B)
...

 Amplifier is coupled with motor (E) which drives an optical
wedge (F)
...


FINGERPRINT REGION:
The region between 400 – 1500 cm-1 in IR spectrum is
called finger print region
...
So, the compound is unique
and it can be distinguish between compounds

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