The breakdown of the Born-Oppenheimer approximation for protonated rare gas ions

For a molecular ion, an expression of the rotational constant including corrections to the Born-Oppenheimer approximation was derived. The parameters for Born-Oppenheimer breakdown effects were related to the molecular charge number and to the parameters characteristic of the electron distribution in the molecule. By using these relations, a physical interpretation was given to experimentally obtained breakdown parameters of protonated rare gas ions. It was found that the nonadiabatic correction for an unbalanced distribution of valence electrons dominates the observed trends of the breakdown parameters.