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.