Abstract
Multiconfigurational self-consistent field (MCSCF) wave functions, augmented by second order perturbation theory to partially recover the dynamic correlation, suggest that the most likely route from silacyclobutane to products ethylene + silene is initial cleavage of a ring CC bond to form a trans ·CH₂SiH₂CH₂CH₂· diradical, followed by rupture of the central SiC bond. This prediction is in agreement with the available experimental results. While this trans diradical is predicted to be a minimum on the MCSCF ground state potential energy surface, the transition state separating this species from products disappears when dynamic 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 initiated by cleavage of a ring SiC bond leads to an analogous trans ·SiH₂CH₂CH₂CH₂· diradical. The transition state leading to this species is the highest point on this minimum energy path and is nearly 6 kcal/mol higher in energy than the transition state that leads to the ·CH₂SiH₂CH₂CH₂· diradical. A transition state for the concerted decomposition has also been found, but this structure is much higher in energy (∼10 kcal/mol) than the highest point on the preferred route. Comparison of the multireference perturbation theory and coupled cluster CCSD(T) results suggests that production of propylsilylene should be both thermodynamically and kinetically competitive with the formation of ethylene + silene. This is consistent with the mechanism proposed by one of us in 1984.