Kinetic and Mechanistic Studies of the Chlorinated Monosilanes and Methyldichlorosilane
Kinetic and mechanistic studies were carried out on the thermal decomposition of three chlorinated monosilanes, H3SiCl, H2SiCl2, and HSiCl3, using low-pressure pyrolysis and quadrupole mass spectrometry. Reaction temperatures ranged from 806 to 921K. Construction and development of the low-pressure pyrolysis/mass spectrometry system was an initial research goal and reaction product identification and kinetic quantification of the homogeneous primary decomposition process for each chlorosilane was the major research goal. Kinetic data were sought in order to provide both insight into the mechanistic steps of the pyrolyses and to utilize in the determination of high-pressure Arrhenius parameters for these reactions. The Arrhenius parameters of interest were the activation energy Ea, and the frequency factor A, as given in the Arrhenius equation. The Arrhenius parameters resulting from this study were determined by application of Rice Ramsperger Kassel Marcus (RRKM)2 theory to the rate constant data. At reaction temperatures, dichlorosilylene formed in the dichlorosilane and trichlorosilane decompositions unexpectedly reacted with the methane bath gas to form quantifiable amounts of methyldichlorosilane. The thermal decomposition of this product (methyldichlorosilane) was also studied by low-pressure pyrolysis and quadrupole mass spectrometry. These results were used in conjunction with thermochemical kinetic techniques to determine thermodynamic parameters for reaction intermediates (such as chlorinated disilanes and silylenes which cannot be observed directly in this system). Results of these studies showed unimolecular elimination reactions to be the primary decomposition step. These reactions and the associated high-pressure Arrhenius parameters are: 1. H3SiCl == H2 + :SiClH E[subscript a]=60.41 ± 1.5 Kcal A=1014.39 ± -65 [subscript s]−1, 2. H2SiCl2 == H2 + :SiCl2 E[subscript a]=65.62 ±1.7 Kcal A = l013.55 ± .4 [subscript s]−1, 3. HSiCl3 == HC1 + :SiCl2 A=1Q14.52 ±.44 [subscript s] −1, E[subscript a]=71.39 ±1.6 Kcal. 4. MeSiCl2H == CH4 + :SiCl2 A=1013.19 E[subscript a]=65.62 kcal (estimated from rate data at one pressure) The decomposition products for each compound are: H3SiCl: H2, H2SiCl2, HSiCl3 H2SiCl2 : H2, HSiCl3, SiCl4, MeSiHCl2 HSiCl3 : HC1, SiCl4, MeSiHCl2, HSiCl3 MeSiHCl2 : CH4, HSiCl3l MeSiHCl2, SiCl4 Products not shown in reactions 1 - 4 are formed from secondary reaction steps via silylene (:SiX2) insertion reactions into other silanes or the methane bath gas.