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
In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient route...
. In other words, if a molecule has a center of symmetry, transitions within a given set of p
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
The Jahn–Teller effect, sometimes also known as Jahn–Teller distortion, or the Jahn–Teller theorem, describes the geometrical distortion of non-linear molecules under certain situations. This electronic effect is named after Hermann Arthur Jahn and Edward Teller, who proved, using group theory,...
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
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...
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
The word antisymmetric refers to a change to an opposite quantity when another quantity is symmetrically changed. This concept is related to that of Symmetry and Asymmetry. The difference between these three concepts can be simply illustrated with Latin letters. The character "A" is symmetric about...
with respect to the inversion center. The rule originates from a quantum mechanical
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
selection rule that, during an electron transition, parity should be inverted.
The rule is named after Otto Laporte
Otto Laporte was a German-born American physicist who made contributions to quantum mechanics, electromagnetic wave propagation theory, spectroscopy, and fluid dynamics...
. It is relevant, in particular, to the electronic spectroscopy
Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent ranges...
of transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...