Silyl radicals structure

Structures that come to mind for assignment to die observed spectra are those that would be expected from an initial disproportionation of two primary silyl radicals to a silene and a hydrosilane, followed by an addition of another silyl radical to the silene, producing a / silylated silyl radical. Repetition of the process would eventually lead to a highly sterically encumbered and undoubtedly persistent silicon-based radical carrying only silicons in its / positions ... 

Hybrids of the type sp3 are unjustified for disilane. An important conclusion from the above hybridization statement No. 4 is concerned with the contrasting structures of the radicals SiH3 and CH3. The planar geometry of the methyl radical can readily be explained by the (bond-strengthening) sp2-hy-bridization, while the pyramidal silyl radical is thought to be stabilized (with respect to the planar arrangement) through the s-admixture to the lone electron orbital. 

The pyramidal structure of the silyl radical provides the key to understanding that silicon is unlikely to appear in a trigonal planar form in silicon hydrides. 

The formation of silyl radicals in the exhaustive photolysis of the silane polymers was indicated by the isolation of disilanes of general structure (HSiRR SiRR H) as shown in Table 1. These materials accumulate in the photolysate and are photostable as they absorb only weakly at the irradiation wavelength (254 nm). Longer chain silanes are presumably continuously degraded under the conditions of the exhaustive irradiation. 

Unlike silyl radicals and silylium ions that have been elusive for a long time, silyl anions (silanides) are well known since the pioneering work of Gilman etal. in the 1960s. Two literature reviews on silyl anions by Lickiss and Smith as well as Tamao appeared in 1995. Many structural elucidations and reactivity studies of silyl anions have been... 

Scheme 19)." Homoallyloxysilanes gave a mixture of five- and six-membered rings, but the intermediate silyl radical underwent predominantly 6-endo cyclization. Pentenyloxysilane gave the 1-endo product only. The stereochemistry of these reactions was found to be determined by steric effects, even in the presence of chiral thiol catalysts. The structures of the radical intermediates were studied by EPR. 

For a long time, this knowledge on carbon-centred radicals has driven the analysis of spectroscopic data obtained for silicon-centred (or silyl) radicals, often erroneously. The principal difference between carbon-centred and silyl radicals arises from the fact that the former can use only 2s and 2p atomic orbitals to accommodate the valence electrons, whereas silyl radicals can use 3s, 3p and 3d. The topic of this section deals mainly with the shape of silyl radicals, which are normally considered to be strongly bent out of the plane (a-type structure 2) [1]. In recent years, it has been shown that a-substituents have had a profound influence on the geometry of silyl radicals and the rationalization of the experimental data is not at all an extrapolation of the knowledge on alkyl radicals. Structural information may be deduced by using chemical, physical or theoretical methods. For better comprehension, this section is divided in subsections describing the results of these methods. 

Structural information on silyl radicals has also been obtained from the isomerization of 9,10-dihydro-9,10-disilaanthracene derivatives 6 and 7 [26,27]. 

The most persistent trialkyl-substituted silyl radical is [(Me3Si)2CH]3Si , which at 20 °C follows a first-order decay with a half-life of 480 s [36]. An exceptionally stable diradical was isolated by reaction of l,l-dilithio-2,3,4,5-tetraphenylsilole with l,l-dichloro-2,3-diphenylcyclopropene, for which the structure 13 was suggested on the basis of EPR data and theoretical calculations... 

The crystal structures of two isolable silyl radicals have recently been reported. The bulky substituted (t-Bu2MeSi)3Si radical was isolated as air-sensitive yellow needles [13], whereas the conjugated and bulky substituted cyclotetrasi-lenyl radical 15 was obtained as red purple crystals [51]. 

The optimized structural parameters of the a-trisubstituted silyl radical (XsSi, where X = H, CH3, NH2OH, F, SiHs, PH2, SH and Cl) were performed at the UMP2/DZP level of theory [33]. As expected, the bond lengths decrease... 

The addition of silyl radicals to thiocarbonyl derivatives is a facile process leading to a-silylthio adducts (Reaction 5.37). This elementary reaction is the initial step of the radical chain deoxygenation of alcohols or Barton McCombie reaction (see Section 4.3.3 for more details). However, rate constants for the formation of these adducts are limited to the value for the reaction of (TMS)3Si radical with the xanthate c-C6HuOC(S)SMe (Table 5.3), a reaction that is also found to be reversible [15]. Structural information on the a-silylthio adducts as well as some kinetic data for the decay reactions of these species have been obtained by EPR spectroscopy [9,72]. 

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