HETERO-PI-SYSTEMS - 28 - REACTION OF SILICON ATOMS WITH ACETYLENE ANDETHYLENE - GENERATION AND MATRIX-SPECTROSCOPIC IDENTIFICATION OF C2H2SI AND C2H4SI ISOMERS

Evaporation of silicon and consecutive cocondensation of the generated atoms with acetylene or ethylene in an argon matrix turnes out to be a new access to C2H2Si and C2H4Si isomers, respectively. In both cases the silicon atoms rather add to the pi system than insert into C-H bo nds, leading to cyclic silylenes as primary reaction products, in the Si/acetylene system the primary product is silacyclopropenylidene (2), which shows the already known photochemical interconversion into addi tional C2H2Si species. On the other hand, cocondensation of silicon at oms with ethylene and subsequent irradiation lead to the matrix-isolat ion of three new C2H4Si isomers [apart from the known silacyclopropene (7) and silylacetylene (8)], namely silacyclopropylidene (10), s-tran s-vinylsilylene (11), and the unsubstituted 1-silaallene (13). These s pecies can be identified by comparison of their experimental IR spectr a and those obtained by ab initio calculations. In a second reaction p ath, SiH2, which is formed as a byproduct from atomic silicon and hydr ogen impurities, adds to the pi system of acetylene, yielding silacycl opropene (7). Silacyclopropane (16), the analogous product of the reac tion of SiH2 with ethylene, could not be observed.