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Renner-Teller effect

In its original formulation the Renner-Teller effect is a (so-called vibronic) coupling in triatomic linear molecules between the motions of the electrons and the nuclear vibrations. Historically, the article by Renner (1934) is the first that considered dynamic effects that go beyond the Born-Oppenheimer approximation. Within the Born-Oppenheimer approximation the nuclear and electronic motions in a molecule are uncoupled. This is a good approximation when the electronic energies are well separated. However, in linear molecules many of the electronic states are two-fold degenerate due to $C_{\infty v}$ or $D_{\infty h}$ symmetry and the Born-Oppenheimer approximation breaks down. Since the best-known linear triatomic molecule (CO2) is electronically non-degenerate in its ground state, Renner chose the electronically excited two-fold degenerate Π-state of this well-known molecule as a model for his studies. The products of purely electronic and purely nuclear vibrational states served as the zeroth-order (no vibronic coupling) wave functions in Renner's study. The vibronic coupling acts as a perturbation.

Because Renner is the only author of the 1934 paper that first described the effect, it was long called the Renner effect. However, as Herzberg's prestigious books refer to it as the Renner-Teller effect, it is now more common to affix these two names to the effect.

While Renner's theoretical study concerned the linear triatomic molecule CO2, the first actual observation of the Renner-Teller effect was in the electronic absorption spectrum of NH2 and its isotopologue ND2. In 1959 Dressler and Ramsay found that the first electronically excited states of these triatomic molecules have a linear geometry and they observed in these excited states an unusual type of vibronic structure: the Renner-Teller effect.

Much has been published about the Renner-Teller effect after its first experimental observation in 1959. At present, authors referring to it have usually broader physical phenomena and larger than three-atomic linear molecules in mind. Usually one considers now effects not only due to the coupling of electrons with vibrations, but also electronic couplings with rotations. The coupling of electronic, vibrational and rotational motions is called briefly rovibronic. In short, the Renner-Teller effect is an effect—due to rovibronic couplings—on the spectra of three- or more-atomic linear molecules in degenerate electronic (Π, Δ, ..., etc.) states.