AQA Chem4: Infra-red Spectroscopy

Revision notes on infra-red spectroscopy from AQA Chem4

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  • Created by: anna
  • Created on: 09-06-13 14:44

Infra-red Spectroscopy

  • A pair of atoms joined by a chemical bond are always vibrating
  • Stronger bonds vibrate faster at a higher frequency and heavier atoms make the bond vibrate more slowly at a lower frequency
  • When we shine infra-red radiation through a sample, the bonds absorb energy from the radiation and vibrate more
  • Particular bonds can only absorb radiation that has the same frequency as the natural frequency of the bond
  • Therefore the radiation that emerges from the sample will be missing the frequencies that correspond to the bonds in the sample
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  • the peaks in an infra-red spectrum represent particular bonds
  • you identify the type of bond using the infra-red absorption data
  • in infra-red spectrums the region below 1500cm-1 is known as the fingerprint region, it is due to the vibrations of the whole molecule
  • some bonds have broad peaks for example, the O-H alcohol group with an absorption of 3220-3550cm-1 while some bonds such as C=O have sharp peaks at 1680-1750cm-1
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Spectra of the Carbonyls (C=O)

Ketones and Aldehydes:

  • the peak produced by ketones and aldehydes is usually strong and sharp
  • in aldehydes it is between 1740 and 1720 cm-1
  • in ketones it is between 1725 and 1700 cm-1

Carboxylic Acids:

  • have two important peaks
  • one at around 3100 cm-1 due to an O-H peak
  • one at around 1700 cm-1 due to a C=O peak


  • a strong C=O peak at around 1750 cm-1
  • two peaks for the C-O bonds as the stretching frequency of each C-O bonds is different due to the different atoms around them 
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