Alternative ban
The alternative ban is a term from vibration spectroscopy . It says that a normal oscillation of a molecule with an inversion center is either only infrared active or only Raman active.
In the case of an oscillation symmetrical to the center of inversion, the oscillation in the IR spectrum is prohibited (i.e. it is not observed), in the case of an anti-symmetrical oscillation in the Raman spectrum is forbidden. This follows from the fact that the dipole moment of the molecule has to change with an IR-active oscillation , which it does not with an inversion-symmetrical oscillation. For an oscillation to be Raman-active, however, the polarizability of the detected species must change. In the character table of the molecule, a symmetrical oscillation appears as “+1” and an unsymmetrical one as “-1”.
This can be illustrated in the case of the molecule , in which the asymmetrical stretching vibration and bending vibration are only IR-active, while the symmetrical stretching vibration is only Raman-active.
