Question

When an organic molecule absorbs infrared radiation, what are some things that can happen to the covalent bonds between the atoms of the molecule?

Answer 1

Organic molecules absorbs infrared radiation  when the frequency of IR radiation  is synchronized with a natural vibration frequency of the molecule. We know that for an organic molecule, covalent bonds behave like a spring. At room temperature organic molecules are always in motion because their bonds stretches , bend and twist. IR radiation causes the excitation of the vibrations of the covalent bonds within that molecule. These vibrations include stretching and bending modes.

a) Stretching vibrations :- The vibrations which changes the bond length are known as stretching vibrations. They are of two types,

1) Symmetric stretching where 2 or more bonds vibrate in and out together.

2) Asymmetric stretching where some bonds elongates and some bonds shortens at the same time.  

b) Bending vibrations :- The vibrations which changes the angle between the bonds are known as bending vibrations. They are of four types,

1) Scissoring , where the atoms move towards and away from each other.

2) Rocking , is like the movement of a pendulum. But an atom is the pendulum and there are two instead of one.

3) Wagging , is like the motion in which you make a 'V' sign with your finger and bend them back and forth from your wrist

4) Twisting , is like the motion as if the atoms where walking on a treadmill.

The Polar covalent bonds are the one which are active towards infrared radiation and these bonds represent the functional group. Hence the most useful information one can obtain from IR spectrum is that what functional group is present in the organic molecule.