Ab Initio Molecular Orbital Investigation of the Unimolecular Decomposition of CH3SiH2+

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. Disciplines Chemistry Comments Reprinted (adapted) with permission from Journal of Physical Chemistry 99 (1995): 148, doi:10.1021/ j100001a026. Copyright 1995 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/chem_pubs/260 148 J. Phys. Chern. 1995, 99, 148-153 Ab Initio Molecular Orbital Investigation of the Unimolecular Decomposition of CH3SiH2 + M.S. Gordon* and L.A. Pedersont Department of Cheff!istry, Iowa State University and Ames Research Laboratory, Ames, Iowa 50011-3020 R. Bakhtiar* and D. B. Jacobson* Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105-5516 Reoeived: June 28, 1994; In Final Form: October 19, 1994° The potential energy surface for the decomposition of CH3SiHz + was studied by ab initio electronic structure theory. At the MP2/6-31G(d,p) level of theory, CH3SiHz+ is the only minimum energy structure on the SiCHs + potential energy surface. Lower levels of theory reported that +cHzSiH3 was also a local minimum, about 40 kcaVmol higher in energy with only a small (ca. 1-2 kcaVmol) barrier for conversion back to CH3SiHz+. 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, !-dehydrogenation from silicon, 1 ,2-dehydrogenation, 1, !-dehydrogenation from carbon, and demethanation were calculated to be 30.2, 69.1, 107.3, and 45.3 kcaVmol, 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 !,!-dehydrogenation from silicon, 1,2-dehydrogenation, 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 1,1-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 l,l-H2 elimination from carbon proceeds monotonically uphill. The closed-shell structures for the products of the above reactions (CH3Si+, CHzSiH+, and CHSiHz+) were calculated at the MP2/6-31G(p,d) level of theory. Finally, triplet products were also examined.