The well-established relation r0 > re holds always?

Abstract The relation r 0 > r e is a well established wisdom in molecular spectroscopy. However, when we compare r e with the r 0 , eff , derived from the experimentally determined rotational constant B 0 , eff , this relation does not necessary hold. For example, in the cases of benzene and such linear molecules as HCN, CsOH, and C3, r 0 , eff r e . This happens because the r 0 , eff derived from a set of B 0 values is not the actual ro-vibrationally averaged bond length r 0 for the ro-vibrational ground state, but its projection onto the principal axes of the moment of inertia. We show here even in such anomalous cases of benzene and these linear molecules, the r 0 bond-lengths calculated properly and literally according to its definition are longer than r e : the relation r 0 > r e holds.