Introduction toInfrared Spectroscopy

Introduction toInfrared Spectroscopy
The vibrations are quantized, that means they occur only at specific frequencies, which correspond to the frequency of IR light.


During the bombardment of a molecule with IR light, when the frequency of IR light matches the frequency of a particular vibrational mode, the molecule will absorb the IR light , causing the amplitude of the particular bond stretch or bond bend to increase.


By determining the wave numbers of the energy absorbed by a compound, we can know the kinds of bonds it has.
The frequency of the stretching vibration depends on two factors:

1- Masses of the Atoms :
Heavier atoms vibrate more slowly than lighter ones which means frequency decreases with increasing atomic weight.
e.g.
The characteristic frequency of a C-D bond is lower than that of a C-H bond.
2- Stiffness of the Bond: Stronger bonds are generally stiffer, they are requiring more force to stretch or compress them. Thus, stronger bonds usually vibrate faster than weaker bonds (assuming the atoms have similar masses).
In a group of bonds having atoms of similar masses,f requency increases with bond energy.
e.g.
1- O-H bonds are stronger than C-H bonds, and 0-H bonds vibrate at higher frequencies.

2- Triple bonds are stronger than double bonds, so triple bonds vibrate at higher frequencies than double bonds.