Renner-Teller effect

In molecular spectroscopy, the Renner–Teller effect or Renner effect is an effect of molecular rotations and  vibrations on the electronic spectra of linear molecules in degenerate electronic (Π, Δ, ..., etc.) states.

In his original paper Renner (1934) did not consider rotations, but only the  coupling between  electrons and  nuclear vibrations in triatomic linear molecules. This article was the first that considered dynamic effects that go beyond the Born-Oppenheimer (BO) approximation (which neglects altogether the effect of the nuclear motions on the electrons in a molecule). The BO approximation is good when the electronic energies are well separated. However, in linear molecules many of the electronic states are two-fold degenerate due to C∞v or D∞h point group symmetry, and the BO 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  molecule as a prototype model for his studies. The products of purely electronic and purely nuclear vibrational states serve as the zeroth-order (no coupling) wave functions in Renner's study. A simple model for vibronic coupling (coupling between vibrations and electrons) acts as a perturbation.

Because Renner is the only author of the 1934 paper that first described the Renner-Teller effect, it was long called the Renner effect. However, as Herzberg's prestigious book refers to it as the Renner–Teller effect (after Edward Teller), it is now more common to use the names of both physicists.

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 molecules larger than three-atoms in mind. Usually one considers now effects not only due to the coupling of electrons with vibrations, but also electronic couplings with rotations (rovibronic coupling).