Knowledge Bank

New very high-resolution data on phosphorous $pentafluoride1$ has prompted a renewed interest in the effects of the Berry pseudorotation on the energy levels of this molecule. Although these effects have already been thoroughly investigated by $Dalton,2$ the interaction of the tunneling motion with the overall rotation of the molecule has not been examined yet. In the present paper, the non-rigid energy levels of $PF5$ are reinvestigated using an IAM-like $treatment.3$ The energy level pattern is first derived and compared with the one obtained by $Dalton.2$ The interaction of the overall rotation of the molecule with the tunneling motion is afterward investigated. This interaction gives rise to a fast rotational dependence of the tunneling splitting, i.e., a variation of the tunneling pattern with the two symmetric top rotational quantum numbers J and K. Unexpectedly enough, the parameters involved in the characterization of this rotational variation are independent on the geometry of the equilibrium configuration of the molecule and on the intermediate configuration of the tunneling path. These results will be discussed along with the fine structure expected for rovibronic transitions.