S(H)2 reaction vs hydrogen abstraction/expulsion in methyl radical-methylsilane reactions: effects of prereactive complex formation.
A quantum chemical study has been undertaken to elucidate the cause of the recently observed S(H)2 reaction between the deuterated methyl radical (*CD3) and methylsilane (SiD3CH3) following the photolysis of CD3I. [Komaguchi, K.; Norberg, D.; Nakazawa, N.; Shiotani, M.; Persson, P.; Lunell, S. Chem. Phys. Lett. 2005, 410, 1-5.] It is found that the backside S(H)2 mechanism may proceed favorably for C-Si-C angles deviating with up to 40 degrees from linearity. The competitive hydrogen abstraction reaction is predicted to be active in the range of 90 degrees <or= C-Si-C <or= 135 degrees. For steeper attack angles, the frontside S(H)2 mechanism is activated. However, high barriers along the corresponding reaction paths probably make the frontside mechanism less important for the present S(H)2 reaction. A number of bound SiH3CH3/CH3I complexes have been located with the MP2 method. At the CCSD(T) level, a complex corresponding to the collinear arrangement where the methyl moiety of methyl iodide points toward the silicon, which is the most favorable conformation for the subsequent SH2 reaction with the backside mechanism, is found to be the most stable linear conformer. A complex with similar energy is found where the methyl moiety of methyl iodide points approximately toward an Si-H bond. However, because C-Si-C = 69.4 degrees in this complex, subsequent photolysis of methyl iodide would probably not lead to hydrogen abstraction with full efficiency. These findings could provide an explanation for the observed S(H)2 reaction.
Norberg
Daniel
D
Department of Quantum Chemistry, Uppsala University, Box 518, SE-751 20 Uppsala, Sweden.
Shiotani
Masaru
M
Lunell
Sten
S