Preferred Label : Jahn–Teller effect;
IUPAC 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).;
Scope note : effect due to the odd terms in the vibronic perturbation expansion. in the case of
molecules with a non-degenerate ground electronic state but with a low-lying degenerate
excited state, 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 fluxional behaviour (see fluxional
molecules) and stereochemical non-rigidity of molecules and ions. the jahn–teller
effect generates a surface crossing (e.g., a conical intersection) whereas a pseudo
jahn–teller effect generates an avoided crossing.;
Origin ID : J03361;
See also
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).
