# How does infrared spectroscopy identify functional groups?

Jan 23, 2015

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:

$\text{wavenumber" = "frequency"/"speed of light} = \frac{f}{c}$

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.