The Renner-Teller effect is a phenomenon in molecular spectroscopy where a pair of electronic states that become degenerate at linearity are coupled by rovibrational motion.
As part of the Renner–Teller effect, the rovibronic levels of such a pair of states will be strongly Coriolis coupled by the rotational kinetic energy operator causing a breakdown of the Born–Oppenheimer approximation.
In its original formulation, the Renner–Teller effect was discussed for a triatomic molecule in an electronic state that is a linear Π-state at equilibrium.
The 1934 article by Rudolf Renner[1] was one of the first that considered dynamic effects that go beyond the Born–Oppenheimer approximation, in which the nuclear and electronic motions in a molecule are uncoupled.
Renner chose an electronically excited state of the carbon dioxide molecule (CO2) that is a linear Π-state at equilibrium for his studies.