Multi-Mode Jahn-Teller and Pseudo-Jahn-Teller Effects in Benzenoid Cations

The multi-state multi-mode vibronic interactions in the benzene radical cation and some of its fluorinated derivatives are surveyed from a theoretical point of view. While the parent system is a prototypical example for the multi-mode dynamical Jahn-Teller effect, partial fluorination leads to a reduction of symmetry and a 'disappearance' of the Jahn-Teller effect. Nevertheless, strong vibronic interactions prevail also there and lead to marked effects in the spectral intensity distributions and to an ultrafast electronic population dynamics. These phenomena have been analyzed theoretically in our group by means of a well-established vibronic coupling scheme, combined with an ab initio quantum dynamical approach (namely, ab initio coupled cluster calculations for the underlying potential energy surfaces and coupling constants, and the so-called MCTDH wavepacket propagation technique for the nuclear motion). The results are presented and discussed, putting emphasis on their dependence on the respective system, especially the degree of fluorination. They shed new light on the substitutional effects on vibronic interactions and demonstrate the degree of sophistication that can be achieved nowadays in their theoretical treatment.