1. (From A. Zewail’s Nobel Lecture 1999)

2. Jahn-Teller Effect
If a molecule has a degenerate electronic state (E or T), a normal coordinate exists that lifts the degeneracy by lowering the symmetry of the electronic state.
Example: Hexa-aqua-Cu(II) complex has D4h symmetry instead of Oh, since its ground state in Oh symmetry would be degenerate (Eg).
Jahn-Teller Theorem: “A nonlinear polyatomic system in a spatially degenerate electronic state distorts spontaneously in such a way that the degeneracy is lifted and a new equilibrium structure of lower symmetry is attained.”
From G. Herzberg, Molecular Spectra and Molecular Structure, Vol. III
From Wikipedia Commons

3. Molecular Spectra and Molecular Structure,
From G. Herzberg,
Molecular Spectra and Molecular Structure,
Vol. III

4. Roald Hoffmann, Angew. Chem. Int. Ed. 26 (1987) 846-878