Chemistry, Ames Laboratory
Journal or Book Title
Journal of Physical Chemistry
The potential energy surface for the decomposition of CH3SiH2+ was studied by ub initio electronic structure theory. At the MP2/6-31G(d,p) level of theory, CH3SiH2+ is the only minimum energy structure on the SiCH5+ potential energy surface. Lower levels of theory reported that +CH2SiH3 was also a local minimum, about 40 kcal/mol higher in energy with only a small (ca. 1-2 kcdmol) banier for conversion back to CH3SiH2+. However, at higher levels of theory, the C, structure of +CHzSiH3 has an imaginary frequency, indicating that it is a saddle 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,l -dehydrogenation from carbon, and demethanation were calculated to be 30.2,69.1, 107.3, and 45.3 kcdmol, respectively. Activation energies (0 K) were calculated at the MP4/6-311++G(2df,2pd) level of theory with the classical barriers subsequently adjusted for zero-point vibrational energies. The 0 K activation energies for 1,l-dehydrogenation from silicon, l,Zdehydrogenation, and demethanation are predicted to be 66.6, 72.7, and 73.0 kcavmol, respectively. All attempts to locate a transition state for the insertion of the carbene-like species, CHSiHZ+, into Hz (reverse of the 1,l-dehydrogenation from carbon) were unsuccessful. This is not surprising since analogous carbene insertions are known to occur without a barrier. Thus, we conclude that this 1,l-H2 elimination from carbon proceeds monotonically uphill. The closed-shell structures for the products of the above reactions (CH3Si+, CH2SiH+, and CHSiH2+) were calculated at the MP2/6-31G(p,d) level of theory. Finally, triplet products were also examined.
American Chemical Society
Gordon, Mark S.; Pederson, L. A.; Bakhtiar, R.; and Jacobson, D. B., "Ab Initio Molecular Orbital Investigation of the Unimolecular Decomposition of CH3SiH2+" (1995). Chemistry Publications. 260.