Modes of vibrations in Infra-red/IR spectroscopy (Stretching and bending vibrations)

This video explains the various modes of infra red spectroscopy. It explain the stretching vibrations and bending vibrations of a linear and non-linear molecule. It also explains how the calculate the number of modes of vibrations for a linear and non-linear molecule.
You will be able to
enumerate the different modes of IR vibrations.
calculate the number of vibration modes for linear and non-linear molecules
explain the stretching and bending vibrations of a linear molecule
discuss the stretching and bending vibrations of a non-linear molecule
Criteria for a mode to be observed in the IR spectrum - changes must occur in the permanent dipole (IR active molecules)
Homonuclear diatomic molecules – no dipole moment (does not absorb IR radiation) e.g. O2 , Cl2 ,N2 etc.
Heteronuclear diatomic molecules – have permanent dipole moment e.g. HCl, CO etc.
Polyatomic molecules - have permanent dipole moment and absorbs IR radiation
Atoms moving in a 3 dimensional space has 3 degrees of freedom – only translational motion
Molecule with N no. of atoms have degrees of freedom is 3N.
Since bonds are present – translational, vibration and rotational motions are possible
Non-linear molecule
3 translational and 3 rotational motions along 3 axes
No. of modes of vibration = 3N – 6
Linear molecule
3 translational and 2 rotational motions along 2 axes
(rotation along its own axis shows no change)
No. of modes of vibration = 3N - 5
Heteronuclear diatomic molecules :
HCl
3N – 5 = (3x2) - 5 = 1 (where N = 2)
Polyatomic molecules
CO2 (linear)
3N – 5 = (3 x 3) – 5 = 9 - 5 = 4 modes of vibration (where N = 3)
H2O (non-linear)
3N –6 = (3 x 3) – 6 = 9 - 6 = 3 modes of vibration (where N = 3)
CO2
Symmetric stretching - Both sides contraction or expansion/elongation
Asymmetric stretching - one side contraction and other side expansion/elongation
Bending vibrations - centre atom move downwards and side atoms upwards and vice-versa
Stretching vibrations – eg. H2O
Symmetric stretching - Both sides contraction or expansion/elongation
Asymmetric stretching - one side contraction and other side expansion/elongation
Bending vibrations – In-plane bending
Scissoring - In-plane bending in the opposite sides
Rocking – In-plane bending on the same side
Bending vibrations – Out of plane bending
Twisting – Out of plane on opposite sides
Wagging – Out of plane on the same side

Видео Modes of vibrations in Infra-red/IR spectroscopy (Stretching and bending vibrations) канала Revathi Purushothaman