How to Interpret Infrared Spectroscopy

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How to Interpret Infrared Spectroscopy
Infrared spectroscopy aids in the structural determination of organic molecules. (the molecule of tryptophan image by Stanislav Pepeliaev from

Many chemical bonds absorb electromagnetic radiation in the infrared (IR) portion of the electromagnetic spectrum, which exhibits wavelengths ranging from 2500 to 16,000 nanometres. An instrument called an infrared spectrometer, or IR spectrometer, measures the amount and frequency of IR radiation absorbed by the various chemical bonds in compounds. The instrument records this information as per cent transmittance, abbreviated %T, of the IR radiation as a function of frequency. Per cent transmittance represents the percentage of IR radiation passing through the sample without being absorbed. Thus, 100 per cent transmittance represents no absorption and 0 per cent transmittance represents complete absorption. By convention, chemists use frequency units of wavenumbers, also known as reciprocal centimetres and written 1/cm, in IR spectrometry.

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  1. 1

    Label the absorption bands in the IR spectrum. Some IR instruments perform this function automatically. Spectra from older instruments require the user to manually label the bands by approximating the wavenumber of each band's maximum absorption from the units printed on the x-axis.

  2. 2

    Categorise each absorption band as "narrow" or "broad," depending on whether the band forms a "point" or assumes a broad "U" shape. Further categorise each band as either "strong," "medium" or "weak," depending on the relative intensity of the absorption. A band that reaches almost 0 per cent transmittance, for example, would be strong.

  3. 3

    Correlate each absorbance band to a specific functional group by comparing the position, width and intensity of each band to the information provided in an IR correlation table. Some bands can be assigned unambiguously. The narrow, strong band near 1700 1/cm, for example, indicates the presence of a carbonyl, or C=O, group. A narrow, medium band near 1200 1/cm, however, could indicate either a C-O or a C-N bond.

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