Kinetics of the gas-phase pyrolysis of pentamethyldisilane. A re-investigation

Abstract

Pentamethyldisilane pyrolysis results in the formation of trimethylsilane and heptamethyltrisilane as the major products. Detailed analytical data at a variety of times, supplemented by runs in the presence of ethanol and buta-1,3-diene at temperatures in the range 591–639 K support the occurrence of the reaction Me₃SiSiMe₂H → Me₃SiH + SiMe₂(1), followed by a variety of scavenging steps of SiMe₂. Kinetic tests show that reaction (1) is unimolecular and homogeneous and fits the Arrhenius equation: log(k₁/s^–1)=(12.83 ± 0.18)–(195.1 ± 2.0 kJ mol^–1)/RT ln 10, where the error limits are two standard deviations. This equation is in good agreement with previous experimental estimates. The A factor, unusually low for a dissociation process, is interpreted as support for a two-stage mechanism for step (1), which proceeds via an intermediate complex. Kinetic modelling of the full analytical data up to > 70% conversion reveals that in the temperature range 615 ± 24 K, SiMe₂ reacts 11 times faster with Si₂Me₅H than with Me₃SiH, but 60 times slower with ethanol than with Me₃SiH.