Laporte rule

The Laporte rule is a spectroscopic selection rule. It states that electronic transitions that conserve either symmetry or asymmetry with respect to an inversion center — i.e., g (gerade) → g, or u (ungerade) → u respectively—are forbidden. In other words, if a molecule has a center of symmetry, 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.

The rule basically says that having inversion symmetry "forbids" transitions. However, this isn't the case — forbidden transitions still can be observed experimentally. For various reasons, such as the Jahn-Teller effect and asymmetric vibrations, complexes are not perfectly symmetric all the time. With the loss of symmetry, the Laporte rule no longer applies. Transitions that would theoretically be forbidden, such as a d → d transition are now possible.

A designation of g means there is symmetry with respect to an inversion center. That is, if all the atoms (or orbitals) are inverted across the inversion center, the resulting compound would look exactly the way it did before having inversion applied to it. (This includes same orientation in space). A designation of u means the molecule is antisymmetric with respect to the inversion center. The rule originates from a quantum mechanical selection rule that, during an electron transition, parity should be inverted.

The rule is named after Otto Laporte. It is relevant, in particular, to the electronic spectroscopy of transition metals.

See also

• Ligand field theory
• Tanabe-Sugano diagram