Pseudo jahn‐teller origin of bending distortions in renner‐teller molecules and its spectroscopic implications

In an important reverse of understanding we show that the Renner‐Teller effect (RTE) which has been considered as the driving force of bending instability of linear molecules in degenerate states, is in fact producing just a splitting of the degenerate term and softening the ground state, but not bending instability; all instabilities and distortions of linear molecules are due to, and only to the mixing with appropriate excited states in the pseudo Jahn‐Teller effect (PJTE). The proof of this statement is given analytically in a general way, and it is supported by previously reported ab initio calculations showing that the contribution of the PJTE to the bending is overwhelmingly larger than the RTE softening. We show also that this finding, in addition to rationalizing the origin of instability of linear molecules, has significant implications in the interpretation of their properties, in particular, dipole moments, rotational level positions, spectroscopic selection rules, and line intensities.