How to Read IR Spectrums

By Michael D
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An infrared (IR) spectrum shows what functional groups are present in an organic molecule. In IR spectroscopy, a molecule is irradiated with electromagnetic radiation. The molecule absorbs energy if the frequency of the radiation matches the frequency of the vibrations of bonds within the molecule. Each bond type absorbs energy of a specific frequency. Therefore, you can determine the bond types in an element by measuring its IR spectrum. However, IR spectrum is limited in scope to relatively small molecules because little can be determined from IR spectroscopy of large molecules which have dozens of absorptions.

Determine the X-axis and the Y-axis of the spectrum. The X-axis of an IR spectrum is labeled as "Wavenumber" and ranges in number from 400 on the far right to 4,000 on the far left. The X-axis provides the absorption number. The Y-axis is labeled as "Percent Transmittance" and ranges in number from 0 on the bottom and 100 at the top.

Determine the characteristic peaks in the IR spectrum. All IR spectra contain many peaks. However, determine the large peaks on the spectrum because they will provide the data necessary to read the spectrum.

Determine the regions of the spectrum in which the characteristic peaks exist. The IR spectrum can be segregated into four regions. The first region ranges from 4,000 to 2,500. The second region ranges from 2,500 to 2,000. The third region ranges from 2,000 to 1,500. The fourth region ranges from 1,500 to 400.

Determine the functional groups absorbed in the first region. If the spectrum has a characteristic peak in the range of 4,000 to 2,500, the peak corresponds to absorption caused by N-H, C-H and O-H single bonds.

Determine the functional groups absorbed in the second region. If the spectrum has a characteristic peak in the range of 2,500 to 2,000, the peak corresponds to absorption caused by triple bonds.

Determine the functional groups absorbed in the third region. If the spectrum has a characteristic peak in the range of 2,000 to 1,500, the peak corresponds to absorption caused by double bonds such as C=O, C=N and C=C.

Compare the peaks in the fourth region to the peaks in the fourth region of another IR spectrum. The fourth is known as the fingerprint region of the IR spectrum and contains a large number of absorption peaks that account for a large variety of single bonds. If all the peaks in an IR spectrum, including those in the fourth region, are identical to the peaks of another spectrum, then you can be assured that the two compounds are identical.