Photolysis of tetramethylsilane near the absorption onset: mechanism and photophysics

Mechanism of the thermal decomposition of tetramethylsilane: a flash pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry and density functional theory study

The thermal decomposition of tetramethylsilane (TMS) was studied over the temperature range of 298-1450 K by combining flash pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS) and density functional theory (DFT). The initial step in TMS pyrolysis produced a methyl radical (Me˙) and Me3Si˙. Me3Si˙ underwent subsequent loss of a hydrogen atom to form Me2Si[double bond, length as m-dash]CH2 and loss of a methyl radical to form Me2Si:. Isomerizations via 1,2-shift and H2 eliminations were major secondary decomposition reactions of Me2Si[double bond, length as m-dash]CH2 and Me2Si:. Among the various isomers, silylene species containing Si-H bonds, such as :Si(H)CH2CH2CH3, :Si(H)CH2CH[double bond, length as m-dash]CH2, :Si(H)CH2CH3, and :Si(H)CH[double bond, length as m-dash]CH2, played an important role in H2 elimination reactions. On the other hand, silene species were insignificant in H2 eliminations. Unlike the silylene species, H2 elimination of :Si[double bond, length as m-dash]CH2 was energetically unfavorable.

Ultraviolet photochemistry of trichlorovinylsilane and allyltrichlorosilane: vinyl radical (HCCH2) and allyl radical (H2CCHCH2) production in 193 nm photolysis

The absolute gas phase ultraviolet absorption spectra of trichlorovinylsilane and allyltrichlorosilane have been measured from 191 to 220 nm. Over this region the absorption spectra of both species are broad and relatively featureless, and their cross sections increase with decreasing wavelength. The electronic transitions of trichlorovinylsilane were calculated by ab initio quantum chemical methods and the observed absorption bands assigned to the A(1)A''<-- X[combining tilde](1)A'' transition. The maximum absorption cross section in the region, at 191 nm, is sigma = (8.50 +/- 0.06) x 10(-18) cm(2) for trichlorovinylsilane and sigma = (2.10 +/- 0.02) x 10(-17) cm(2) for allyltrichlorosilane. The vinyl radical and the allyl radical are formed promptly from the 193 nm photolysis of their respective trichlorosilane precursors. By comparison of the transient visible absorption and the 1315 nm I atom absorption from 266 nm photolysis of vinyl iodide and allyl iodide, the absorption cross sections at 404 nm of vinyl radical ((2.9 +/- 0.4) x 10(-19) cm(2)) and allyl radical ((3.6 +/- 0.8) x 10(-19) cm(2)) were derived. These cross sections are in significant disagreement with literature values derived from kinetic modeling of allyl or vinyl radical self-reactions. Using these cross sections, the vinyl radical yield from trichlorovinylsilane was determined to be phi = (0.9 +/- 0.2) per 193 nm photon absorbed, and the allyl radical yield from allyltrichlorosilane phi = (0.7 +/- 0.2) per 193 nm photon absorbed.