Polarized measurements can be very useful for the analysis of oriented minerals with IR spectroscopy. Polarized radiation is produced with a polarizer consisting of a fine grating with parallel metal wires. In oriented crystals certain dipole moment changes lie in a fixed direction during the molecular vibration and can be excited when the IR radiation is also polarized in this direction.

In a molecule with a center of symmetry it is seen that vibrations that are Raman active are IR inactive and vice-versa, this is called the Principle of mutual exclusion (eg, as in CO2). In molecules with different elements of symmetry, certain bands may be active in IR, Raman, both or neither. For a complex molecule that has no symmetry except identity element, all of the normal modes are active in both IR and Raman. This does not however mean that they can be observed. In both types the neighbouring strong bands may obscure weak bands, while others may be intrinsically too weak to be observed even if they are theoretically “allowed”.

In general the strong bands in the IR spectrum of a compound corresponds to weak bands in the Raman and vice versa. This complimentary nature is due to the electrical characteristic of the vibration. If a bond is strongly polarised, a small change in its length such as that occurs during a vibration, will have only a small additional effect on polarisation. Vibrations involving polar bonds ( C-O , N-O , O-H ) are therefore, comparatively weak Raman scatterers. Such polarised bonds, however, carry their charges during the vibrational motion, ( unless neutralised by symmetry factors), which results in a large net dipole moment change and produce strong IR absorption band. Conversely, relatively neutral bonds ( C-C , C-H , C=C ,) suffer large changes in polarisability during a vibration, though this is less easy to visualise. But the dipole moment is not similarly affected and vibrations that predominantly involve this type of bond are strong Raman scatterers but weak in the IR.

IR Active and IR Inactive Vibrations

Not all molecular vibrations absorb IR radiation.

To understand which ones do (and which do not) absorb we need to consider how an electromagnetic field interacts with the chemical bond.

Important points are:

(1) A chemical bond which is polarized has a dipole moment (i.e. charge separation).

(2) An electric field will interact with the polar bond, causing it either to stretch or compress.

(3) An electromagnetic wave has a rapidly reversing electric field, so this radiation rapidly compresses and stretches the polar bond.

(4) If the frequency of the stretching and compression is at the frequency of the molecule's natural rate of vibration, then energy may be absorbed.

Vibrations of bonds with dipole moments normally result in the absorption of IR radiation, and are said to be IR active.

However, if a bond is symmetrical, and has zero dipole moment, the electric field does not interact with this bond.

If the symmetrical bond is stretched, the dipole moment still remains zero, the vibration produces no change in the dipole moment, there is no absorption of energy.

This vibration is said to be IR inactive.

To recap:

In general, if a bond has a dipole moment, its stretching frequency causes an absorption in the IR spectrum.

If a bond is symmetrically substituted and has zero dipole moment, its stretching vibration will (essentially) be absent from the spectrum.