Jahn–Teller effect

https://doi.org/10.1351/goldbook.J03361
Molecular distortions due to an electronically degenerate @G02704-1@ state. For non-linear molecular entities in a geometry described by a point symmetry group possessing degenerate irreducible representations there always exists at least one non-totally symmetric vibration that makes electronically degenerate states @U06569@ at this geometry. The nuclei are displaced to new equilibrium positions of lower symmetry causing a splitting of the originally degenerate states (first-order Jahn–Teller effect).
Note:
Effect due to the odd terms in the vibronic perturbation expansion. In the case of molecules with a non-degenerate @G02704-2@ but with a low-lying degenerate @E02257@, distortions of proper symmetry arise that result in mixing of the ground and excited states, thereby lowering the ground-state energy (pseudo Jahn–Teller effect). The pseudo Jahn–Teller effect manifests itself in @F02463@ behaviour (see @F02463@) and @ST07107@ of molecules and ions. The Jahn–Teller effect generates a @S06166@ (e.g., a @CT07347@) whereas a pseudo Jahn–Teller effect generates an @A00544@.
See also:
Renner–Teller effect
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 360 [Terms] [Paper]