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

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 S 0 , S 1 , and S 2 states at the CASPT2/6-311G**//CAS(6,7)/6-31G* level. On the S 0 potential energy surface, HFB could isomerize to three different products [Dewar-HFB (S 0 -P 1 ), benzvalene-HFB (S 0 -P 2 ), and fulvene-HFB (S 0 -P 3 )]. Following excitation to the S 2 state with the perpendicular π → σ* transition, a chair-type minimum with C s symmetry was found on the S 2 potential energy surface. The adjacent S 2 /S 1 conical intersection was immediately accessible from the S 2 minimum. The nature of the S 1 state was confirmed to have a π → π* character. Both the S 2 and S 1 photochemistries of HFB yielded Dewar-HFB via the S 1 /S 0 conical intersection. The regeneration of the S 0 state from the S 1 and T 2 states via intersystem crossing or internal conversion was also revealed.