Disilanes, decomposition

Roenigk, K. F., Jensen, K. F., and Carr, R. W., Rice-Rampsperger-Kassel-Marcus theoretical prediction of high-pressure Arrhenius parameters by nonlinear regression Application to silane and disilane decomposition, J. Phys. Chem. 91, 5732 (1987). 

The reported values of quantum yields for each of the above processes are 0.61, 0.18, 0.21, respectively. Of course these quantum yields are not necessarily applicable if excitation energy is different. For example, according to the recent theoretical thermochemical data given by Ho and co-workers (14,15) the reaction enthalpy for the formation of a silylsilylene and two hydrogen atoms from disilane (decomposition (b)) is about 166 kcal/mol, which is equivalent to the photon energy at 172 nm. On the other hand, the dissociative excitation of disilane can be effected with even longer wavelength of up to... 

The gas-phase pyrolysis of disilane and silane is also likely to involve this rearrangement process. The original proposal60, that the thermal elimination of H2 from disilane is a three-center process leading to silylsilylene, was subsequently modified67 in response to the results of chemically activated disilane decomposition. It was concluded that both the... 

In contrast to the monosilane system, the decompositions of disilanes to give silylenes are well-established processes. They can also be employed for synthetic purposes. The general process involved in the disilane decomposition can be represented by the following equation ... 

The reagent was rapidly (less than in 1 ps) heated to 1,200 K using a special experimental methodic. This initiated disilane decomposition. Concentration of SiH2 was measured as a function of time from the initiation (f = 0) to f = 100 ps. 

The thermal decomposition of silanes in the presence of hydrogen into siUcon for production of ultrapure, semiconductor-grade siUcon has become an important art, known as the Siemens process (13). A variety of process parameters, which usually include the introduction of hydrogen, have been studied. Silane can be used to deposit siUcon at temperatures below 1000°C (14). Dichlorosilane deposits siUcon at 1000—1150°C (15,16). Ttichlorosilane has been reported as a source for siUcon deposition at >1150° C (17). Tribromosilane is ordinarily a source for siUcon deposition at 600—800°C (18). Thin-film deposition of siUcon metal from silane and disilane takes place at temperatures as low as 640°C, but results in amorphous hydrogenated siUcon (19). 

Mass-spectrometric research on silane decomposition kinetics has been performed for flowing [298, 302-306] and static discharges [197, 307]. In a dc discharge of silane it is found that the reaction rate for the depletion of silane is a linear function of the dc current in the discharge, which allows one to determine a first-order reaction mechanism in electron density and temperature [302, 304]. For an RF discharge, similar results are found [303, 305]. Also, the depletion and production rates were found to be temperature-dependent [306]. Further, the depletion of silane and the production of disilane and trisilane are found to depend on the dwell time in the reactor [298]. The increase of di- and trisilane concentration at short dwell times (<0.5 s) corresponds to the decrease of silane concentration. At long dwell times, the decomposition of di- and trisilane produces... 

Ab initio molecular orbital calculations have been carried out by Ignacio and Schlegel on the thermal decomposition of disilane and the fluorinated disilanes Si2H F6 17. Both 1,1-elimination of H2 or HF and silylene extrusion by migration of H and F atoms concerted with Si—Si bond cleavage were considered. The transition states for the extrusion reactions all involved movement of the migrating atom toward the empty p-orbital of the extruded silylene in the insertion which is the retro-extrusion (equation 5). 

Ring, M. A. et al., AIP Conf. Proc., 1988, 166 (Photovolt. Safety), 175—182 Very pure silane does not immediately explode with oxygen, but the decomposition products may ignite after a delay [1]. Mixtures of silane with 30% oxygen are metastable and potentially explosive under all pressure conditions studied, and become explosive at above 80°C. Addition of a few % of disilane renders the mixtures much more explosive, and mechanisms are proposed [2],... 

The easy decomposition of the mercury compounds can be used in direct synthesis of disilanes ... 

The photolysis of phenylethynylpentamethyldisilane (56) takes place simultaneously by at least two different processes. The main route proceeds through a silacyclopropene and the other involves transient formation of a silapropadiene. Irradiation of a benzene solution of 56 in the presence of acetone gives four products, 2,2,5,5-tetrameth-yl-3-trimethylsilyl-4-phenyl-l-oxa-2-silacyclo-3-pentene (57), 2,2,5,5-tetra-methyl-3-phenyl-4-trimethylsilyl- l-oxa-2-silacyclo-3-pentene (58), phen-yltrimethylsilylacetylene, and 1-pheny 1-1 -trimethylsily 1-3-methyl-1,2-butadiene (59), in 51,2,10, and 5% yield, respectively, with 81% conversion, of the starting disilane. The formation of 57 and 58 can be explained by insertion of acetone into the silacyclopropene. Liberation of dimethylsilyl-ene species from either direct photolysis of 56 or decomposition of the silacyclopropene results in the formation of PhC=CSiMe3. Product 59... 

Explosive reaction with sodium -I-methanol or sodium methoxide + methanol. Mixtures with sodium or potassium are impact-sensitive explosives. Reacts violently with acetone + alkah (e.g., sodium hydroxide, potassium hydroxide, or calcium hydroxide), Al, disilane, Li, Mg, methanol + alkah, nitrogen tetroxide, perchloric acid + phosphorus pentoxide, potassium-tert-butoxide, sodium methylate, NaK. Incompatible with dinitrogen tetraoxide, fluorine, metals, or trhsopropylphosphine. Nonflammable. When heated to decomposition it emits toxic fumes of CT. 

DOT CLASSIFICATION 2.2 Label Nonflammable Gas SAFETY PROFILE This material is chemically inert in the pure state and is considered to be physiologically inert as well. However, as it is ordinarily obtainable, it can contain variable quantities of the low-sulfur fluorides. Some of these are toxic, very reacdve chemically, and corrosive in nature. These materials can hydrolyze on contact with water to yield hydrogen fluoride, which is highly toxic and very corrosive. In high concentrations and when pure it may act as a simple asphjudant. Incompatible with disilane. Vigorous reaction with disilane. May explode. When heated to decomposition emits highly toxic fumes of F" and SOx. 

Skinner " - 2 estimated (Si - Si) from the activation energy for the thermal decomposition of disilane, but this is probably not valid. Pauling3S2 takes (81-Si) equal to half the heat of atomization of the element, whence (Si - Si) -45 kcal. 

Thermal Properties, Silanes have less thermal stabflity than hydrocarbon analogues. The C—H bond eneigy in methane is 414 kj / mol (98.9 kcal/mol) the Si—H bond energy in silane is 3781 /mol (90.3 kcal/mol) (10). Silane, however, is one of the most thermally stable inoiganic silanes. Decomposition occurs at 500 0 in the absence of catalytic surfaces, at 300°C in glass vessels, and at 180°C in the presence of charcoal (11). Disilanes and other members of the binary series are less stable. Halogen-substituted silanes are subject to disproportionation reactions at higher temperatures (12). 

We would like to suggest, that an attack of a chlorine takes place first at one of the silicon atoms of the tri- or tetrasilane under discussion to form a pentacoordinated silicon. In a second step, a silylene is generated which inserts into a silicon-chlorine bond of the disilane while the silicon skeleton of the oligosilane is shortened by one silicon atom. The source of the attacking chlorine is not yet known An intramolecular reaction with a chlorine of a chloromethylsilyl-sidechain and the central silicon might occur, or remaining traces of the aluminium trichloride, used for the chlorination, might induce the reaction. This is possible since only catalytic amounts would be necessary to cause the decomposition. 

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