Rule of mutual exclusion

The alternative prohibition is a term used in vibrational spectroscopy. It states that a normal vibration of a molecule with a center of inversion is either infrared or active only Raman active.

At a center of inversion symmetry on the vibration, the vibration in the IR spectrum is prohibited ( that is, it is not observed ) in an anti- symmetrical vibration in the Raman spectrum is forbidden. This follows from the fact that this has to change the dipole moment of the molecule in an IR - active vibration, which it does at a inversions - symmetric vibration. This vibration is Raman active, the polarizability of the detected species must change it. In the character table of the molecule shows a symmetrical vibration as " 1" and an asymmetrical as " -1".

Can be demonstrated to the the molecule in the asymmetric stretching vibration and bending vibration are IR active while the symmetric stretching mode is only Raman active.