Mechanism of the Thermal Decomposition of Ethylsilane

Abstract

Products and kinetics of the thermal decomposition of ethylsilane, under static system conditions, are reported. Both neat and propylene-inhibited reactions follow silylene chain mechanisms. Modeling fits to product yield and reactant loss data establish approximate values for the rate constants of the ethylsilylene decomposition to ethylene and silylene (kz * 2 x lo6 s-I) and the ethylsilylene isomerization to vinylsilane (ks 5 x lo2 s-l), both at 723 K. RRKh4 and thermochemical analyses of these results yield values for the high-pressure activation energy of the ethylsilylene decomposition (EtSiH -C2& + SiH2) of E2 = 25.0 kcal/mol and a heat of formation for ethylsilylene of AHdEtSiH) = 47.7 f 2.5 kcal/mol. An enthalpy group additivity for the methylene group in organosilanes and organosilylenes of AHf[C-(H2)(C)(Si)] = -1.3 f 2 kcal/mol is deduced as well as the activation energy of the isomerization of ethylsilylene to vinylsilane (EtSiH -.CHz-CHSiH3) of E8 = 35.6 f 2 kcal/mol. 2 8

Kinetics of the Thermal Decomposition of Ethylsilane: Shock-Tube and Modeling Study

“…In this work, a new reaction mechanism is developed to describe the thermal decomposition of EtSiH 3 and the product formation in the gas phase at high temperatures. According to refs − , the three-center elimination of H 2 is the primary unimolecular decomposition step of EtSiH 3 : The silylene intermediate HSiC 2 H 5 then further decomposes to C 2 H 4 and SiH 2 : …”

“…In a theory study, Skancke et al analyzed possible pathways of silacyclopropane (cyclo-C 2 H 4 SiH 2 ) isomerization by performing CASSCF/6-31G­(d) and CASPT2 N /6-31G­(d) computations. Starting from HSiC 2 H 5 on the cyclo-C 2 H 4 SiH 2 potential energy surface computed by Skancke et al, cyclo-C 2 H 4 SiH 2 will be the preferred product, and then cyclo-C 2 H 4 SiH 2 can decompose by another consecutive reaction, which has no well-defined transition state (TS) structure, to C 2 H 4 and SiH 2 : On the basis of their product compositions, Jardine et al mentioned the possibility that HSiC 2 H 5 can isomerize to vinylsilane (H 3 SiC 2 H 3 ): …”

“…On the basis of their product compositions, Jardine et al 22 mentioned the possibility that HSiC 2 H 5 can isomerize to vinylsilane (H 3 SiC 2 H 3 ):…”

“…Therefore, reaction 8 is regarded to be the only relevant bimolecular process between SiH 2 and EtSiH 3 . Jardine et al 22 estimated the rate constant to be 5 × 10 10 cm 3 mol −1 s −1 , i.e., this reaction was assumed to have no activation energy. In the experiments conducted without the SiH 2 scavenger (C 2 H 2 ), EtSiH 3 is already consumed at slightly lower temperatures, because without adding C 2 H 2 , reaction 8 takes place and can therefore contribute to the consumption of EtSiH 3 .…”

“…The thermal decomposition is initiated by a 1,1elimination of H 2 and leads to the formation of ethylsilylene (HSiC 2 H 5 ), which is supposed to decompose to C 2 H 4 and SiH 4 after a sequence of reactions. Jardine et al 22 reinvestigated the thermal decomposition of EtSiH 3 at reaction times of up to 1200 s in a static system in a temperature range of 695−748 K and a pressure range of 150−707 mbar. They used the rate constant data of Rickborn et al 20 and observed that HSiC 2 H 5 can also isomerize to vinyl silane (H 3 SiC 2 H 3 ), but as in the previous study, the main products were found to be C 2 H 4 and SiH 4 .…”