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.