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

Abstract

An EPR study was carried out to elucidate the hydrogen atom abstraction from methylsilane (CH₃SiH₃) by a methyl radical, CH₃SiH₃ + ˙CH₃ → 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 radical. In the CH₃SiH₃ system, the CH₃SiH₂˙ radical was the major product immediately after the photolysis, while the ˙CH₃ radical was the major one in the CH₃SiD₃ system. The ˙CH₃ 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 × 10⁻² s⁻¹ and k_(Si–D) = 6.9 × 10⁻⁶ s⁻¹ (k_(Si–H)/k_(Si–D) = 5.2 × 10³) at 77 K; the associated apparent activation energies were E_a(Si–H) = 0.85 kJmol⁻¹ and E_a(Si–D) = 8.9 kJmol⁻¹ (E_a(Si–H)/E_a(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.