A Mechanistic Study of Thermal Decomposition of 1,1,2,2-Tetramethyldisilane Using Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometry

Abstract

Thermal decomposition of 1,1,2,2-tetramethyldisilane was performed by flash pyrolysis in a SiC microreactor in the temperature range from 295 to 1340 K, followed by molecular beam sampling and vacuum ultraviolet photoionization mass spectrometry analysis. Density functional theory investigations on the energetics of reactants, intermediates, and products were carried out to support the experimental observations. Energetics for 1,1,2,2-tetramethyldisilane initiation decomposition reactions and important secondary reactions were calculated. Dimethylsilane, dimethylsilyl radicals, dimethylsilylene, trimethylsilane, and tetramethyldisilene were determined as the primary reaction products in the initiation thermal decompositions of 1,1,2,2-tetramethyldisilane. Further decomposition reactions of tetramethyldisilene, such as production of dimethylsilene (m/z = 72) and eventually SiC3H4 (m/z = 68) fragments, were examined. Other products from secondary reactions of dimethylsilane and dimethylsilylene such as SiC2H2–6 and SiCH0–4 were also observed. The comprehensive pyrolysis mechanism of 1,1,2,2-tetramethyldisilane was proposed.

Thermal Decomposition Mechanism of Tetraethylsilane by Flash Pyrolysis Vacuum Ultraviolet Photoionization Mass Spectrometry and DFT Calculations: The Competition between β-Hydride Elimination and Bond Homolysis

“…and Zagidullin et al, the residence time within the heated region was estimated to be less than 100 μs in this work. , Based on Zagidullin et al, the average pressure within the heated region in the microreactor was estimated to be 10 torr . The reaction conditions within the microreactor significantly favored unimolecular reactions, while bimolecular reactions and wall reactions were reduced. ,− After exiting the nozzle, the gas mixture, which contained the products, reactive intermediates, and unreacted reactants, supersonically expanded into the main chamber. After being selected by a skimmer, the isolated molecular beam was intercepted by 118 nm VUV radiation in the photoionization region.…”

“…The signal of m / z = 73 (SiC 3 H 9 ) also first appeared at 1140 K, and the origin of this peak was unclear. As previous studies inferred that the ionization cross-section of the m / z = 73 species is large, ,, its barely detectable peak intensity suggested that the m / z = 73 species were negligible in the system. Also, quantum chemistry calculations did not demonstrate any possible unimolecular dissociation channel that could explain the appearance of the m / z = 73 signal. HSi Et 2 → EtHSi= normalC normalH 2 + normalC normalH 3 …”

“…When the temperature further increased, the signal of the m / z = 72 peak remained nearly constant, while the signal intensity of the m / z = 68 peak kept increasing. Earlier studies have established that the detection of the m / z = 68 signals signifies the formation of dimethylsilene (Me 2 SiCH 2 ) ( m / z = 72) and its isomers such as EtHSiCH 2 , as dimethylsilene (Me 2 SiCH 2 ) can readily undergo thermal decomposition to the neutral m / z = 68 species under the comparable reaction conditions. ,,, It is noted that the dimerization of Me 2 SiCH 2 , which could lead to the production of 1,1,3,3-tetramethyl-1,3-disilacyclobutane ( m / z = 144), as reported by Tong et al, could not be confirmed in this study due to the overlap of the resulting signal with that of the parent molecule SiEt 4 ; this dimerization process was unlikely under the current experimental conditions (very short residence time and low concentrations). Furthermore, as the co-product of EtHSiCH 2 , the signal of the methyl radical ( m / z = 15) was also first observed at 1140 K. Therefore, it is argued that the secondary reaction of HSiEt 2 led to the production of EtHSiCH 2 and CH 3 from around 1140 K and on.…”

“…It was possibly caused by further dissociations of the m / z = 44 species. Previous studies have discussed the secondary reactions of the m / z = 44 species in detail; the m / z = 44 species could lose H or H 2 and lead to the production of the SiCH 3 ( m / z = 43) or SiCH 2 ( m / z = 42) species . Also, the m / z = 44–42 signals could correspond to signals of hydrocarbons (C 3 H 8 –C 3 H 6 ) from multiple secondary reactions, and it could be further identified by structural studies in the future.…”

Flash pyrolysis vacuum ultraviolet photoionization mass spectrometry of cycloheptane: A study of the initial decomposition mechanism

“…The overall design has been described previously. [21][22][23][24][25][26][27] The vapor of cycloheptane (98%, Sigma Aldrich) was diluted to around 1% in the He carrier gas. The backing pressure of the gas mixture before the pulse valve was 1050 torr.…”