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How to Read Infrared Graphs

Chemists have long known that compounds absorb infrared (IR) light in specific ways. The energy of IR light is just high enough to match the energy of vibrations within a molecule, so IR light shone through a chemical sample is absorbed and changed into molecular motion. The specific frequencies of IR light absorbed depend on the way the molecule is put together, so IR absorption patterns can offer clues to the makeup of an unknown substance. An IR graph is a plot of light absorption over the entire frequency range of the infrared light region and shows distinct patterns for each chemical.

Things You'll Need

Table of functional groups IR absorption frequencies

Instructions

- 1
  Look over the IR graph to find the most prominent absorption maxima. These are sharp downwards peaks which correspond to the wavelengths of IR light which the unknown chemical has absorbed most strongly.
- 2
  Note the frequencies at which the absorption peaks occur. Most IR charts (or "spectra") use units of reciprocal centimeters (cm^-1) --- also known as wave numbers --- to measure wavelength. For example, you may see strong peaks at approximately 1700 cm^-1 and 3000 cm^-1.
- 3
  Look up the absorption wavelengths where you found peaks on a table of IR absorption frequencies. These tables are found in many standard chemistry textbooks and reference books. The table give a list of common absorption frequencies and the corresponding molecular group. In the case of the example, a peak around 1700 cm^-1 is generally due to a C=O group, whereas absorption around 3000 cm^-1 is due to C-H.
- 4
  Use the information from the IR table to make conclusions about the particular chemical which was used to obtain the spectra. At the very least, you now know something about the chemical and portions of its molecule which may be enough to confirm or exclude a particular compound as being your unknown. The example spectra, for instance, could help confirm that your unknown is acetone, since this chemical contains both C=O and C-H groups on its molecule.

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