Infrared (IR) spectroscopy
One may see the shimmering of air above hot metal and refer to it as "heat". Actually, the wavelengths that most contribute to simple heating have a longer wavelength than the visible spectrum, and hence cannot actually be seen. This is the infrared region, from 2500-250 nanometers; the range is usually quoted as 4000-400 cm-1, which is a type of frequency measurement. Alternatively, this corresponds to 12-120,000,000,000,000,000 Hz.
IR spectroscopy is quite a bit less sensitive than UV-vis, although it is easily possible to obtain spectra on 1 mg of compound. Spectra can be obtained both by transmitted and by reflected light, so opaque samples and surfaces can be analyzed. It is fairly easy and convenient for quantitative measurements, using spectrometers that presently cost in the range of US$10,000-50,000. Most samples are solutions, thin films, gases, or solids ground and pressed into windows of potassium bromide (an easy process that take 2-5 minutes per sample).
An IR spectrometer
IR spectra show many more peaks than UV-vis spectra (most IR spectra by convention show absorption as going down, rather than up). Thus IR spectra have much more information, but are harder to interpret.
Long experience has shown that the IR spectral region gives information about molecular vibrations, hence about specific pieces of the molecule (functional groups). Certain types of functional groups always give IR absorption in the same region. When we see such an absorption, we know that the associated functional group is present in the molecule.
Many functional groups have been correlated with specific absorption wavelengths, and can be identified. However, a molecule with N atoms can have up to 3N-5 different molecular vibrations, hence complex molecules usually have so many vibrations that it is practically impossible to identify more than a few of them.
An example spectrum and molecular structure for acetic
acid are shown to the left, with functional group assignments
where possible. Note that energy decreases from left to right.
Still, it is very helpful to know the structure of parts of the molecule. The table below shows some functional groups that have well-established IR absorption peaks, and the ranges at which most are found.
Functional Group |
IR Absorption Frequency (cm-1) |
>C=O (carbonyl) |
1700-1730 |
-CN (cyanide) |
2100-2200 |
-OH (hydroxyl) |
3300-3600 (broad) |
Overall, IR spectra give information about whether or not specific functional group fragments are present in a molecule. As a result, a partial analysis of the molecular structure can be built up from IR spectroscopy, even for a completely unknown substance.