Infrared spectroscopy involves the interaction of a molecule with electro magnetic radiation. When an organic molecule is irradiated with infrared energy, certain frequencies are absorbed by the molecule. The frequencies absorbed correspond to the amounts of energy needed to increase the amplitude of specific molecular vibrations such as bond stretchings and bendings. Since every functional group has a characteristic combination of bonds, every functional group has a characteristic set of infrared absorptions.
John McMurry,Organic Chemistry 8th ed. (2012), Ch. 12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy
