Photochemical mechanistic study of hexafluorobenzene involving the low-lying states.

ABSTRACT

The photochemical isomerization and nonradiative decay processes of hexafluorobenzene (HFB) were investigated theoretically to gain insights into its photochemical mechanism and the perfluoro effect. A complete mechanistic scheme is presented through the characterization of all the possible minima and transition states of the S0, S1, and S2 states at the CASPT2/6-311G**//CAS(6,7)/6-31G* level. On the S0 potential energy surface, HFB could isomerize to three different products [Dewar-HFB (S0-P1), benzvalene-HFB (S0-P2), and fulvene-HFB (S0-P3)]. Following excitation to the S2 state with the perpendicular π → σ* transition, a chair-type minimum with Cs symmetry was found on the S2 potential energy surface. The adjacent S2/S1 conical intersection was immediately accessible from the S2 minimum. The nature of the S1 state was confirmed to have a π → π* character. Both the S2 and S1 photochemistries of HFB yielded Dewar-HFB via the S1/S0 conical intersection. The regeneration of the S0 state from the S1 and T2 states via intersystem crossing or internal conversion was also revealed.