Please enable JavaScript.
Coggle requires JavaScript to display documents.
IR Spectroscopy - Coggle Diagram
IR Spectroscopy
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
-
Only
three co-ordinates are necessary to locate a molecule in space, we say that a molecule has
always three translational degrees of freedom.
-
-
-
-
-
-
-
Fundamental Vibration
Stretching =
In this type of vibrations, the distance between the two atoms increases or decreases
but the atoms remain in the same bond axis.
Symmetric Stretching = movement of the atoms
with respect to a particular atom in a
molecule is in the same direction.
-
Bending =
In this type of vibrations, the positions of the atoms change with respect to the original
bond axis.
-
-
-
Twisting = one of the atoms moves up the plane while the other moves down the
plane with respect to the central atoms.
Vibrational Frequency
-
The value of vibrational frequency or wave number depends upon:
(i) Bond strength, and
(ii) Reduced mass.
-
Absorption of IR radiation can be expressed either in terms of wavelength or wave number.
Wavenumbers = 1/ wavelength (cm)
Overtones = Results from excitation of ground state to higher energy states, which corresponds to
integral multiples of frequency of fundamental.
Combination Band = When two vibrational frequencies in a molecule couple to give rise to a vibration of new
frequency within the molecule and when such molecule is infrared active,
Difference band = it is similar to combination bands. The observed frequency in this case results from
the difference between the two interacting bands.
Fermi Resonance =
When fundamental vibration couples with on overtone or combination band, the coupled vibration
is called Fermi resonance.
It is often observed in carbonyl compounds.