Term: Jahn–Teller effect
https://doi.org/10.1351/goldbook.J03361

Definition:
Molecular distortions due to an electronically degenerate ground 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 unstable 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@.

Related Terms:
1) renner–teller effect (http://doi.org/10.1351/goldbook.R05290).
2) ground (http://doi.org/10.1351/goldbook.G02704-1).
3) excited state (http://doi.org/10.1351/goldbook.E02257).
4) fluxional (http://doi.org/10.1351/goldbook.F02463).
5) surface crossing (http://doi.org/10.1351/goldbook.S06166).
6) conical intersection (http://doi.org/10.1351/goldbook.CT07347).
7) avoided crossing (<em>of potential-energy surfaces</em>) (http://doi.org/10.1351/goldbook.A00544).
8) ground electronic state (<em>of potential-energy surfaces</em>) (http://doi.org/10.1351/goldbook.G02704-2).
9) stereochemical non-rigidity (http://doi.org/10.1351/goldbook.ST07107).
10) unstable (<em>of potential-energy surfaces</em>) (http://doi.org/10.1351/goldbook.U06569).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 360 (https://doi.org/10.1351/pac200779030293)

Citation: 'Jahn–Teller effect' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.J03361

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