Ab initio study of the photochemistry of c-C₂H₂Si

The photochemical reaction pathway of silacyclopropenylidene (c-C ₂H₂Si) has been investigated using the eight electrons in eight orbitals complete active space with the 6-311++G (3df,3pd) basis sets. The mechanism of drastic structural change in the reaction of c-C ₂H₂Si and the difference from the reaction of cyclyopropenylidene (c-C₃H₂) are elucidated. The photochemically active relaxation path of c-C₂H₂Si on the S₁ excited-state potential surface leads to an S₁/S ₀ conical intersection where the photoexcited system decays nonradiatively to S₀. The relaxation of c-C₂H ₂Si on the S₁ surface causes the cleavage of the Si-C single bond, while that of c-C₃H₃ causes the cleavage of the C-C double bond. The difference in photochemical cleavage sites is well explained by the difference in the electronic nature of the S₁ excited state. In the dark reaction following the relaxation on the S ₁ surface, hydrogen migration from carbon to silicon occurs together with ring opening at the Si-C bond.