How does infrared spectroscopy identify functional groups?

Question

Maya Allen · Accepted Answer

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.

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

The video below gives a simple explanation of infrared spectroscopy.

Brooklyn Barber · Answer

undefined Infrared spectroscopy identifies functional groups by measuring the absorption of infrared radiation by the chemical bonds within a molecule. Each type of chemical bond absorbs infrared radiation at characteristic frequencies, which correspond to the vibrations of the atoms within the bond. By analyzing the pattern of absorption peaks in the infrared spectrum, chemists can determine the presence of specific functional groups in a molecule.