How do I Predict Peaks in IR Spectra?

Written by jack brubaker
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The infrared spectrometer (IR) represents one method used by chemists to elucidate the structure of organic compounds. An IR passes infrared radiation through a thin film of the organic sample. Different types of bonds in the compound absorb different frequencies of radiation as the bonds stretch and bend. The instrument then produces a spectrum that plots the amount of infrared radiation absorbed by the sample as a function of frequency, usually expressed as wavenumbers or reciprocal centimetres. Chemists then use correlation tables to determine the various types of bonds present in the compound. If the chemist already knows or at least suspects the molecule's structure, then she can work backwards and predict the spectrum before running the scan. This is also a common exercise given to students to help them learn the fundamentals of IR spectroscopy.

Skill level:

Things you need

  • Infrared spectroscopy correlation table (see Resources)

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  1. 1

    Draw the molecular structure of the compound whose IR peaks you intend to predict. If you do not know the molecular structure, numerous websites provide this information (see Resources). As an example, ethylenediamine exhibits a structural formula of NH2-CH2-CH2-NH2.

  2. 2

    Examine the structure and identify the various bond types present in the molecule. Note that IR distinguishes between single and double bonds. Thus, a C-O bond will show an absorption peak at a different wavelength than a C=O bond. It also differentiates between bonds involving atoms attached to a benzene ring, called "aromatic," and those not attached to a benzene ring, called "aliphatic." In the case of the molecule from Step 1, ethylenediamine possesses four different bond types: N-H, C-N, C-H and C-C -- all of which are aliphatic.

  3. 3

    Locate the various bond types in an IR correlation table, such as the one provided in the Resources section from the University of Colorado, and make a list of the various absorbance frequencies. An aliphatic N-H bond absorbs at 3400-3250 1/cm, 1650-1580 1/cm and 910-655 1/cm. An aliphatic C-N bond absorbs at 1250-1020. An aliphatic C-H bond absorbs at 3000-2850 1/cm, 1470-1450 1/cm and 725-720 1/cm. An aliphatic C-C bond does not absorb in the infrared region. Each of these absorbances will produce a peak in the compound's IR spectrum.

Tips and warnings

  • Verify your results by looking up the compound's actual IR spectrum in an online database, such as the one provided in the Resources section from the National Institute of Advanced Industrial Science and Technology.

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