Selectivity and enormous H/D isotope effects on H atom abstraction by CH3 radicals in solid methylsilane at 3.0 K-115 K

An EPR study was carried out to elucidate the hydrogen atom abstraction from methylsilane (CH3SiH3) by a methyl radical, CH3SiH3 + (CH3)-C-. --> CH3SiH2. + CH4, in a solid solution of CH3SiH3 containing 1 mol% CH3I at the low temperatures of 3 K-115 K. The EPR spectra observed after UV-photolysis of the CH3I at 77 K were attributed to a mixture of the CH3SiH2. radical and the (CH3)-C-. radical. In the CH3SiH3 system, the CH3SiH2. radical was the major product immediately after the photolysis, while the (CH3)-C-. radical was the major one in the CH3SiD3 system. The (CH3)-C-. radicals decayed following first order kinetics in the dark in both systems. The decay rate constants for the reaction were experimentally determined to be k((Si-H)) = 3.6 x 10(-2) s(-1) and k((Si-D)) = 6.9 x 10(-6) s(-1) (k((Si-H))/k((Si-D)) = 5.2 x 10(3)) at 77 K; the associated apparent activation energies were Ea(Si-H) = 0.85 kJ mol(-1) and Ea(Si-D) = 8.9 kJ mol(-1) (Ea(Si-H)/Ea(Si-D) = 1/10) above 20 K. A non-linear Arrhenius plot was obtained for the rate constant, k((Si-H)), and the rate became almost independent of the temperature below 20 K. These results suggest that the quantum mechanical tunneling effect contributes significantly to the H atom abstraction from the -SiH3 group.