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.
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