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Infra-Red (IR) Spectroscopy

Infra red (IR) spectroscopy deals with the interaction between a molecule and radiation from the IR region of the EM spectrum (IR region = 4000 - 400 cm-1). The cm-1 unit is the wave number scale and is given by 1 / (wavelength in cm).

IR radiation causes the excitation of the vibrations of covalent bonds within that molecule. These vibrations include the stretching and bending modes.

An IR spectrum show the energy absorptions as one 'scans' the IR region of the EM spectrum.  As an example, the IR spectrum of butanal is shown below.

In general terms it is convienient to split an IR spectrum into two approximate regions:

functional group / fingerprint regions of a typical infra red spectrum


When analysing an IR spectrum avoid the temptation to try to assign every peak.
The fingerprint region, however, can be useful for helping to confirm a structure by direct comparison with a known spectra.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary