RING-OPENING OF SILACYCLOBUTANE

Multiconfigurational self-consistent field (MCSCF) wave functions, aug mented by second order perturbation theory to partially recover the dy namic correlation, suggest that the most likely route from silacyclobu tane to products ethylene + silene is initial cleavage of a ring CC bo nd to form a trans . CH2SiH2CH2CH2 . diradical, followed by rupture of the central SiC bond. This prediction is in agreement with the availa ble experimental results. While this trans diradical is predicted to b e a minimum on the MCSCF ground state potential energy surface, the tr ansition state separating this species from products disappears when d ynamic correlation is added. Therefore, the bottleneck on this part of the potential energy surface is likely to be the transition state for the initial CC bond cleavage. The alternative mechanism that is initi ated by cleavage of a ring SiC bond leads to an analogous trans . SiH2 CH2CH2CH2 . diradical. The transition state leading to this species is the highest point on this minimum energy path and is nearly 6 kcal/mo l higher in energy than the transition state that leads to the . CH2Si H2CH2CH2 . diradical. A transition state for the concerted decompositi on has also been found, but this structure is much higher in energy (s imilar to 10 kcal/mol) than the highest point on the preferred route. Comparison of the multireference perturbation theory and coupled clust er CCSD(T) results suggests that production of propylsilylene should b e both thermodynamically and kinetically competitive with the formatio n of ethylene + silene. This is consistent with the mechanism proposed by one of us in 1984.