Modes of vibrations in molecules - Infra Red Spectroscopy - IR Active-inactive-stretching-bending

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The vibrations in molecules are basically are of two types.... i.e. stretching and bending.
In the case of stretching the bond, length is altered without any change in the bond angle, while in the case of bending there is a change in the bond angle.
However, one should note that, inorder to maintain the center of mass, the bond lengths may be slightly altered during the bending vibrations.
They are again sub-divided into other categories.

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Let us start with a simple vibration of a bond in a homonuclear diatomic molecule. This is otherwise known as stretching mode. During this vibration, the dipole moment of the molecule is not changed. Hence this vibration is IR inactive.

For your kind information, in order to be IR active, there must be a change in dipole moment during vibration. In this molecule, the dipole moment before stretching is zero and there is no change in the dipole moment during the vibrational stretch. Hence it is IR inactive.

Now, look at hetero diatomic molecules. Let us consider the smaller atom is more electronegative than the larger one. Hence there is a dipole moment for this molecule as shown. The negative end is directed towards the more electronegative smaller atom.

Upon stretching, the dipole moment is altered. Hence this stretching vibration is IR active.

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When dealing with triatomic molecules, the stretching vibrations can be symmetric or asymmetric. Let us deal with the symmetrical stretching first.

This is a linear triatomic molecule, with two same atoms connected to the central atom. Each bond is associated with a bond moment. Since the bonds are the same and the bond moments are opposite in direction, this molecule has no dipole moment.

And this is symmetric stretching mode since the two bonds are stretching to the same extent. Moreover, the stretchings occur in opposite directions. Therefore during this vibration, there is no change in net dipole moment of the molecule. Hence this vibration is not IR active.

Let us see symmetrical stretching in nonlinear triatomic molecule. Even though the bonds are the same, as they are making angles with each other, the molecule has net dipole moment. It is shown with green arrow.

Now this is symmetrical stretching of each bond. Both are vibrating at the same time to same extent.

However, the net dipole moment is altered during this vibration. Hence this vibration is IR active.

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Now, the asymmetric stretching.

Here, the stretching vibrations are not occurring at the same time. As a result, there will be a change in the net dipole moment of the molecule during vibrations. Hence this stretching mode is infra red active.

The same is the case with nonlinear molecule also.

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Here is a special case of asymmetric triatomic molecule. In this case, the stretching is asymmetric since the atoms connected to the central atom are different.
Look at this hypothetical molecule, the bond moments are unequal. Hence there is an initial net dipole moment.
The dipole moment is changed during the stretching vibrations. Hence it is IR ACTIVE.
One should note that, even if the bonds are vibrating at the same time, the magnitude is not the same. The bond associated with a smaller atom is subjected to more stretch than the bond with larger atom.
Hence this stretching mode is asymmetric.

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Now, let us deal with bending vibrations.
This is a simple bending vibration in a triatomic linear molecule.
The bond moment is altered during this bending mode. Hence this is infra red active.

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For other nonlinear and more complex molecules, the bending vibrations are more interesting and can be sub-divided into
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