Editing Infrared (section)

=== Spectroscopy ===

[[Infrared spectroscopy|Infrared vibrational spectroscopy]] (see also [[near-infrared spectroscopy]]) is a technique that can be used to identify molecules by analysis of their constituent bonds. Each chemical bond in a molecule vibrates at a frequency characteristic of that bond. A group of atoms in a molecule (e.g., CH<sub>2</sub>) may have multiple modes of oscillation caused by the stretching and bending motions of the group as a whole. If an oscillation leads to a change in [[dipole]] in the molecule then it will absorb a [[photon]] that has the same frequency. The vibrational frequencies of most molecules correspond to the frequencies of infrared light. Typically, the technique is used to study [[organic compound]]s using light radiation from the mid-infrared, 4,000–400&nbsp;cm<sup>−1</sup>. A spectrum of all the frequencies of absorption in a sample is recorded. This can be used to gain information about the sample composition in terms of chemical groups present and also its purity (for example, a wet sample will show a broad O-H absorption around 3200&nbsp;cm<sup>−1</sup>). The unit for expressing radiation in this application, cm<sup>−1</sup>, is the spectroscopic [[wavenumber]]. It is the frequency divided by the speed of light in vacuum.