AB-INITIO MOLECULAR-ORBITAL INVESTIGATION OF THE UNIMOLECULAR DECOMPOSITION OF CH3SIH2+

The potential energy surface for the decomposition of CH3SiH2+ was stu died by ab initio electronic structure theory. At the MP2/6-31G(d,p) l evel of theory, CH3SiH2+ is the only minimum energy structure on the S iCH5+ potential energy surface. Lower levels of theory reported that ( CH2SiH3)-C-+ was also a local minimum, about 40 kcal/mol higher in ene rgy with only a small (ca. 1-2 kcal/mol) barrier for conversion back t o CH3SiH2+. However, at higher levels of theory, the C-s, structure of (CH2SiH3)-C-+ has an imaginary frequency, indicating that it is a sad dle point rather than a local minimum on the potential energy surface. The 0 K reaction enthalpies for 1,1-dehydrogenation from silicon, 1,2 -dehydrogenation, 1,1-dehydrogenation from carbon, and demethanation w ere calculated to be 30.2, 69.1, 107.3, and 45.3 kcal/mol, respectivel y. Activation energies (0 K) were calculated at the MP4/6-311++G(2df,2 pd) level of theory with the classical barriers subsequently adjusted for zero-point vibrational energies. The 0 K activation energies for 1 ,1-dehydrogenation from silicon, 1,2-dehydrogenation, and demethanatio n are predicted to be 66.6, 72.7, and 73.0 kcal/mol, respectively. All attempts to locate a transition state for the insertion of the carben e-like species, CHSiH2+, into H-2 (reverse of the 1,1-dehydrogenation from carbon) were unsuccessful. This is not surprising since analogous carbene insertions are known to occur without- a barrier. Thus, we co nclude that this 1,1-H-2 elimination from carbon proceeds monotonicall y uphill. The closed-shell structures for the products of the above re actions (CH3Si+, CH2SiH+, and CHSiH2+) were calculated at the MP2/6-31 G(p,d) level of theory. Finally, triplet products were also examined.