Symmetry forbidden transition is a term commonly used in the context of electronic spectroscopy. This term refers to the electronic transitions that occur from one quantum to another.
The Laporte rule is a spectroscopic selection criterion that is applied to molecules that have an inversion centre and atoms. The Laporte rule is named after Otto Laporte, a German born American physicist.
According to the Laporte rule, the electronic transitions that conserve parity, symmetric or asymmetric with respect to the inversion centre, are forbidden. This rule is put to action when determining the molar absorptivity.
In other words, electronic transitions that involve a change in the number of unpaired spins are forbidden and, hence, of low intensity.
In the electron absorption spectra, electronic transitions are symmetry forbidden in complexes having a center of symmetry (octahedral), but not in complexes that do not have a centre of symmetry (tetrahedral). Such forbidden transitions do not occur in the absorption spectrum. Even if they do appear, they do so as weak bands.
This rule originates from the quantum mechanical selection rule, according to which it is necessary for parity inversion during an electron transition.
In some exceptional cases, forbidden transitions are allowed if the centre of symmetry is disrupted. The centre of symmetry may be disrupted due to asymmetric vibrations, or the Jahn-Teller effect.