Linear and bent triatomic molecules are not qualitatively different!

I, and other authors, have discussed in several recent publications that “linear” triatomic molecules (defined as having linear equilibrium structures) are necessarily observed as being bent on ro-vibrational average. We have demonstrated this theoretically by calculations of the rotation–vibration expectation values, [Formula: see text], where [Formula: see text] is the bond angle supplement, [Formula: see text] being the instantaneous value of the bond angle of the triatomic molecule A–B–C. Direct experimental evidence of bent average structures has been obtained by other authors in Coulomb explosion imaging experiments, and indirect evidence from re-interpretation of experimentally derived rotational constant values. In spite of a rather significant amount of evidence in support of the bent average structures, the idea has been heavily criticized. In the present work I discuss in more detail some of the arguments for the bent average structures put forward in previous papers, and I hope to correct and clarify some of the misunderstandings leading to the criticisms. Part of the criticism originates in a widespread, but fallacious, belief among spectroscopists that linear and bent chain molecules have qualitatively different energy-level and spectral intensity patterns. This is not true. One can view the linear-molecule energy level and spectral patterns as limiting cases of the bent-molecule ones.