How does infrared spectroscopy identify functional groups?
Vibrating bonds in functional groups absorb energy at a frequency that corresponds to the vibrational frequency of the bond.
In organic chemistry, this corresponds to frequencies of 15 to 120 THz.
These frequencies are expressed as wavenumbers:
The wavenumbers range from 500 to 4000 cm⁻¹.
If the frequency of the radiation matches the vibrational frequency, the bond will absorb the radiation. The amplitude of the vibration will increase.
Within a narrow range, each type of bond vibrates at a characteristic wavenumber. This makes infrared spectroscopy useful for identifying functional groups in a molecule.
Here’s a short table of common absorption frequencies.
Notice how you can identify the important vibrations in the spectrum of ethyl acetate.
The video below gives a simple explanation of infrared spectroscopy.