Vibronic interactions in molecules with a fourfold symmetry axis

Abstract

In molecules with a fourfold symmetry axis the Jahn-Teller active vibrations are nondegenerate. Thus, the dynamical Jahn-Teller effect in such molecules, which is discussed in this paper, is qualitatively different from that in molecules with a threefold axis, which has been discussed extensively in other papers. The degenerate vibrations in molecules with a fourfold symmetry axis are not Jahn-Teller active. Nevertheless, as discussed in this paper, vibronic interactions similar to the Renner effect do affect the positions of the energy levels associated with degenerate vibrational motions in degenerate electronic states. Certain limiting cases of both the Jahn-Teller effect and the Renner effect in molecules of symmetry D 4 h in degenerate electronic states are pointed out, which might be called accidental Born-Oppenheimer cases. It is possible in such limiting situations to write the vibronic wave functions as a single product of an electronic part and a vibrational part, in spite of the fact that a degenerate electronic state and vibrations capable of splitting the electronic degeneracy are involved.