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How does infrared spectroscopy identify functional groups?

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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:

#"wavenumber" = "frequency"/"speed of light" = 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.
www.chromatographytechniques.com/sites/chromatographytechniques.com/files/legacyimages/Table_2.jpg

Notice how you can identify the important vibrations in the spectrum of ethyl acetate.

orgchem.colorado.edu

The video below gives a simple explanation of infrared spectroscopy.

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