Pulsed pyrolysis, matrix isolation

S-37 (see above) it is also possible to prepare and to matrix-isolate the silicon species 124, 125, and 126, which again exist in a photoequilibrium. Our first entry to 1-silacyclopropenylidene (124) was the pulsed flash pyrolysis of 2-ethynyl-l,l,l-trimethyldisilane (123).71,72 Even though the structure of educt molecule 123 suggests formation of ethynylsilylene (125), the isolated product was 124. Obviously 125 had already thermally isomerized to the most stable isomer 124 before the products were condensed at 10 K. 

All of these problems are overcome if the precursors are pyrolyzed in an inert host gas at high pressure, because under these conditions energy transfer occurs mostly by colhsions with the host gas, and radical recombination is largely suppressed. Of course, a constant stream of hot gas at high pressure is incompatible with the requirements of matrix isolation, so the experiment has to be carried out in a pulsed fashion. Chen and co-workers were the first to propose what they called a hyperthermal nozzle for pulsed pyrolysis at very high temperatures, at that time for gas-phase studies. Several research groups have implemented variants of this design" for work in matrix isolation and have used it successfully for a variety of sffidies. 

The formal substitution of a saturated carbon atom in compounds 8, 13 and 16 by silicon results in precursors 21, 17 and 20, which could all be synthesized. In contrast to the results above, pulsed flash pyrolysis of these oligosilanes gave rise to the formation of only one C2H4Si2 isomer, namely 18. Actually, irradiation of matrix-isolated 2-silylsilacyclopropenylidene (18) led to silylene 19 in analogy to reaction 14—>15. 

Alternatively, flash vacuum pyrolysis with subsequent trapping of the products in low temperature matrices of 4 or other thermal precursors should lead to matrix-isolated lb. Under these conditions lb and the by-products are isolated in separate matrix sites and thus the thermal recombination is prevented. Several attempts for the thermal generation of lb were reported, however, in all cases difluoroacetylene 6 was found as the only C2F2 species. Thus, pulse pyrolysis of dibromodifluoroethylene 7 or the stannyl compound 8 produces 6 as the main product. [32] If CO is added in the gas phase, propadienone 4 is generated as an additional product, which indicates that lb is formed as a reactive intermediated in the gas phase that is long-lived enough to be trapped by CO. 

Investigations on benzyl radical were also undertaken by Misic, Piech and Bally.They used this species to generate the corresponding car-bocation under matrix-isolation conditions. The study extended also to the allyl species (radical and cation), and used X-ray irradiation of the radicals to produce in situ the cations. The radicals were generated by pulsed pyrolysis in the gas phase. 

Pulsed flash pyrolysis of 2-ethynyl-l,l,l-trimethyldisilane yields 1-silacyclopropenyldene 81 isolated in an argon matrix. It can further be isomerized by photolysis into ethynylsilanediyl 82, vinylidenesilanediyl 83, and... 

In the past two years, we have been able to isolate four C2H2Si isomers in an argon matrix after pulsed flash pyrolysis of 2-ethynyl-l,l,l-trimethyldisilane [3]. As was proposed earlier, another access to the C2H2Si hypersurface consists of the reaction of silicon atoms with acetylene [5]. Based on this information about the C2H2Si isomers, it was obvious to take the Si/acetylene system to refine our silicon evaporation techniques. 

Summary A number of isomers of composition CsHiSi and C2H4Si2 have been generated by pulsed flash pyrolysis of appropriate precursors and isolated in an argon matrix. Their photochemical interconversions were studied. Quantum chemical calculations have been performed at the BLYP/6-31G level of theory. They play a decisive role in the identification of the highly reactive intermediates. Although silacyclobutadienes were shown to be minima on their respective energy hypersurfaces, no experimental evidence for the existence of such compounds was found. Instead, silylenes were detected, which undergo a variety of mutual interconversions. 

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