Analytical Techniques

Infra-red Spectroscopy

Infra-red Spectroscopy

You will find covalent bonds in all molecules. These bonds are in a constant state of motion: rotating, bending and stretching at specific frequencies. If these bonds come into contact with infra-red radiation at the same frequency then they will stretch and bend to a greater extent and end up absorbing the radiation of that frequency.

A bond will only absorb one or two specific frequencies. Therefore, when infra-red radiation containing the whole range of frequencies is passed through an organic compound some of the frequencies are absorbed. Others that pass through can be detected and converted into an intra-red spectrum, an example of which is shown below.

The wave number in cm-1, which is related closely to the frequency, is shown on the horizontal axis. The transmittance, which is expressed as a percentage, is shown on the vertical axis. A transmittance under 50% means that a significant amount of the radiation of that particular frequency was absorbed. This is called the peak.

  • A sharp peak is one that occurs at a precise frequency.
  • A broad peak occurs over a range of frequencies.

Due to the fact different wave numbers are absorbed by different bonds, the peaks can be used to figure out which bonds are present within a molecule.

C-H and C-C bonds are found in nearly all compounds and, therefore, are present in the majority of infra-red spectra.

Bond Frequencies absorbed (cm-1) Peak shape
C-HC-C 2650 – 3310720 – 1175 SharpSharp

Other bonds give peaks which are more distinct.

Bond Frequencies absorbed (cm-1) Peak shape
C-OC=CC=OO-H (alcohols)O-H (acids)C-O 1120 – 13101620 – 16901630 – 18153230 – 33502500 – 30001000 – 1300 SharpSharpSharpBroadBroadSharp

Peaks due to C=O and O-H bonds being present are particularly easy to recognise. This is because the absorbance is very high and the transmittance low. A bond which is also part of intermolecular bonding has a very broad peak. Bonds without hydrogen bonding give a sharper peak.