Laporte rule

The Laporte rule is a spectroscopic selection rule that only applies to centrosymmetric molecules (those with an inversion centre) and atoms. It states that electronic transitions that conserve parity, either symmetry or antisymmetry with respect to an inversion centre — i.e., g (gerade = even (German)) → g, or u (ungerade = odd) → u respectively—are forbidden. Allowed transitions in such molecules must involve a change in parity, either g → u or u → g. As a consequence, if a molecule is centrosymmetric, transitions within a given set of p or d orbitals (i.e., those that only involve a redistribution of electrons within a given subshell) are forbidden.