will split into two close-lying nondegenerate states for non-linear configurations. 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.
For nonlinear configurations, the state splits into two adjacent non-degenerate states. The Renner–Teller effect causes strong Coriolis coupling between the rovibronic levels of these states, which disrupts the Born-Oppenheimer approximation.