The vibrational modes in molecules have specific energies associated with them. These vibrational energies correspond to the near infrared region of the electromagnetic spectrum. If the vibrations can be made to interact with photons then it would be possible to detect these vibrations and determine their energies.

In the case of known compounds this can be used to identify materials and possibly mixtures of materials. For unknown materials, or materials with unknown structures, this can be used to provide supporting evidence for suggested formulae and structures.

Bands in certain regions can indicate specific features (such as a carbonyl group or an hydroxyl group). Also, for organic (carbon based) molecules the actual arrangement and intensity of bands in a particular part of the spectrum (called the fingerprint region) can be used to at least provisionally identify a molecule. Reference books are available which give the vibrations for standard materials (including the relative intensities of the peaks).

For a possible molecular structure, the number and type of vibrations it should exhibit can be derived. Computer programs exist to calculate the expected frequency of these vibrations. Not all of these vibrations will be detectable as they will not all interact with photons. This is because there has to be particular changes in the electronic field of the molecule during the vibration for it to interact with a photon. No change means no interaction (and if the change is only small then the effect might not be visible against the background noise).

To determine which vibrations will be active it is necessary to look at the symmetry of the molecule and the symmetry of each of the vibrations. The molecule will belong to a particular point group and this will determine which symmetries are active in vibrational spectroscopy.

I am familiar with two types of vibration spectroscopy (it's over ten years since I last did this) and they are:

  • Infrared (IR) spectroscopy: In this case the material is illuminated with a near infrared source and absorption of the light detected. To be active a vibration must cause a change in dipole
  • Raman spectroscopy: The sample is illuminated with a single frequency laser beam. Small spikes are detected on the edge of the peak of the main laser beam indicating photons that have lost or gained energy due to interaction with vibrations. To be active a vibration must cause a change in polarizability
These methods are complementary as though some vibrations are active in both many are active only in IR or Raman.

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