Mechanistic Study of Thermal Decomposition of Hexamethyldisilane by Flash Pyrolysis Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometry and Density Functional Theory

Abstract

Thermal decomposition of hexamethyldisilane (HMDS) was studied from room temperature to 1310 K using flash pyrolysis vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). Decomposition pathways of HMDS and initial reaction intermediates were also investigated using density functional theory (DFT) at the B3LYP/6-311++G­(d,p) level. Unimolecular decomposition reactions of HMDS involving Si–Si and Si–C bond cleavage, as well as decomposition producing Me4Si and :SiMe2 via a three-centered elimination, were determined as the initiation reactions. Me3SiSi­(Me)2 •, Me4Si, Me3Si•, and :SiMe2 were major products of the initiation reactions. These initial products were apt to decompose by homolytic reactions. Me2SiCH2, :SiMe2, and other silene/silylene intermediates preferred decomposing through molecular eliminations. Both homolytic and molecular elimination reactions are important in the pyrolysis of HMDS.

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

“…The overall energy thresholds for these two channels were determined to be 45.5 and 44.1 kcal/mol relative to 1,1,2,2-tetramethyldisilene, respectively. Me 2 SiCH 2 then undergoes a series of secondary reactions which leads to the formation of the m / z = 68 species (reaction ), as studied previously. ,,,− …”

“…Some previous studies have shown that: SiCH 2 could also be produced by a methane elimination from dimethylsilylene (:SiMe 2 ) with an energy barrier of 56.6 kcal/mol. , The m / z = 40 peak did not show up until the temperature reached 1250 K, and it could correspond to SiC which was evolved from an H 2 elimination from: SiCH 2 . The sequential H 2 loss from propane ( m / z = 44) which was produced from the propane elimination of :Si­(H)Pr could also contribute to the signal from m / z = 40–44. ,, The observations of the m / z = 28 peak at 1130 K indicated the formation of Si atoms, and they could be produced from the further reaction of SiC. , …”

“…Theoretical calculations on the geometries and energetics of corresponding transition states, intermediates, products, and reactants were performed using the Gaussian 09 program . All geometries including transition states were optimized using the UB3LYP density functional theory (DFT) method , under 6-311++G­(d,p) basis sets . Energies and vibrational frequencies for each species were obtained at the same level of theory, and zero-point energy corrections were made for all species involved.…”

“…31 All geometries including transition states were optimized using the UB3LYP density functional theory (DFT) method 32,33 under 6-311++G(d,p) basis sets. 34 Energies and vibrational frequencies for each species were obtained at the same level of theory, and zero-point energy corrections were made for all species involved. Transition states with only one imaginary frequency were tested using IRC calculations under the same level.…”

“…The sequential H 2 loss from propane (m/z = 44) which was produced from the propane elimination of :Si(H)Pr could also contribute to the signal from m/z = 40−44. 34,37,39 The observations of the m/z = 28 peak at 1130 K indicated the formation of Si atoms, and they could be produced from the further reaction of SiC. 37,39 ■ CONCLUSIONS…”