Absorption and bond vibration
Infrared spectroscopy measures how molecules absorb infrared radiation. Covalent bonds can vibrate rather than remaining fixed, and different vibrations absorb particular frequencies of infrared radiation.
When radiation has the appropriate frequency for a bond vibration, energy is absorbed and the vibration becomes more energetic. Vibrations can include changes such as stretching and bending.
Infrared absorptions are normally recorded as wavenumbers, with units of . Different types of bond absorb within characteristic wavenumber ranges.
The collection of absorptions produced by a molecule forms its IR spectrum. This provides information about the bonds, and therefore the functional groups, present.