IR Spectra Analysis

Once the IR spectrum is complete, it is available for analysis. Each compound has a distinct set of strong absorptions, or peaks, which the chemist can use to identify the compound present. Each functional group within the molecule has a distinct absorption. For a complete list see the functional group absorptions.

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IR Spectra

The peak is the name given to the position in the spectrum at which an absorption occurs. The horizontal scale is measured in cm-1 where cm-1 is known as wavenumber (1/wavelength), which is equivalent to frequency. For instance, in the acetophenone spectrum above, the carbonyl group has an absorption peak at 1700 cm-1. This peak has a minimum point at about 5 percent transmittance on the vertical scale, which means the carbonyl group absorbed almost all the infrared radiation at 1700 cm-1. The carbonyl group bond vibrations contain a very large oscillating dipole moment which can absorb IR radiation of the same frequency.