Crucial role of the S1/T1/S0 intersection point in photochemical reaction of cyclobutanone

The role of the intersection between two potential energy surfaces has been more generally emphasizing on than before. In the present system, the intersections between S-1, T-1 and S-0 surfaces were optimized with the state-averaged CAS(8, 7)/6-31G* method. The comparison of their optimized structures demonstrates that the intersection points of S-1/S-0 and S-1/T-1 locate in the same region, referred to as S-1/T-1/S-0 point. The existence of S-1/T1S0 point and its high possibility of radiationless transition via intersystem crossing or internal conversion, would make the system have the most efficient funnel to generate the ground- and triplet-state products, which is consistent with the experimental facts.