Silenes calculated

Direct spectroscopic studies and calculations of cyclic silylene-to-silene and germylene-to-germene interconversions 99IZV2027. 

A number of relatively stable silenes with bulky substituents are known at present, but stable silanones have not been isolated till now. Their instability, like that of silenes, is caused by a kinetic factor, according to various calculations. Thus, cyclooligomerization of silanones should proceed with zero activation barrier (Kudo and Nagase, 1985). 

For R = H the silole 51 had a at 278 nm while the silylene 48 absorbed at 250 and 480 nm. The isomeric silenes 49 and 50 absorbed at 2% and 270 nm, respectively. The UV absorptions for these species have been calculated and assigned, and their IR spectra have also been obtained. When R = Me, there was little change in the A.max, the four species absorbing at 280, 255 and 480, 312, and 274 nm, respectively. 

Considerable information is available about the infrared absorption of silenes. This information was obtained in large part from experimental studies of simple species such as H2Si=CH2 and Me2Si=CH2 in argon matrices at about 10 K or from calculations, and in small part from room-temperature studies of a few relatively stable silenes in solution. 

Bands in the region 950-1100 cm"1 have been attributed to the Si=C stretching frequency of the simple silenes, the frequency reported depending on the experimental method, the methodology employed in the calculations, and the substituents present on the ends of the silicon-carbon double bonds there is generally good agreement between these observations and the appropriate calculations. The material has been well summarized.6,153... 

Several further calculations of various properties of the silicon-carbon double bond have been reported. As in the previous decade the strength of the silene bond and the reasons for it have continued to attract attention. 

The mechanisms by which silenes dimerize, both head-to-tail and head-to-head, have been the subject of intense interest to theoreticians for as long as the dimerization process has been known. Siedl, Grev, and Schaefer167 calculated that the barrier between the parent H2Si=CH2 68 and its head-to-tail dimer 69 was 5.2 kcal mol-1, with the product head-to-tail dimer 69 being 79 kcal mol-1 more stable than reactants. The head-to-head dimer 70 was found to be 19.8 kcal mol-1 less stable than 69. 

This review describes the current status of silenes (silaethylenes, silaethenes), molecules which contain a silicon-carbon double bond. The heart of the material is derived from a computer-based search of the literature which we believe reports all silenes that have been described to date, either as isolated species, chemically trapped species, proposed intermediates (in reactions where some experimental evidence has been provided), or as the result of molecular orbital calculations. Ionized species... 

Silene IR method temperature (K) IR bands observed0 (cm-1) Calculated Si=C stretching frequency (cm-1) Reference for observed spectrum Reference for calculated spectrum... 

A large number of calculations have been done to determine the vibrational frequencies of some simple silenes. The parent silene and its deuterated analogs have been investigated (4,9,188) as well as 1,1-dimethylsilene, its deuterated derivatives (38,181), and 1,1-dichlorosilene (4). For the undeuterated compounds a Si=C stretching vibration of approximately 1000 cm-1 was calculated. Deuteration usually results in a decrease of this value. 

The first ionization potentials of several silenes have been predicted using MINDO/3 calculations (177). The ionization potentials found for the parent silene, 8.95 0.1 eV, (174) and 1,1-dimethylsilene, 7.7 eV (184), 7.5 0.3 eV (191), and 8.14 eV (176), have also been estimated from ab initio (174,176) or thermochemical (184,191) calculations. 

Despite the differing levels of calculations, the same general conclusions were reached. The silicon-carbon double bonds in 1-silaallene (1.69 A) and 2-silaallene (1.70 A) are shorter than in isolated silenes at the same level of theory. This trend is also observed in the analogous carbon series. 1-Silaallene is thermodynamically more stable than 2-silaallene by 21 kcal/mol (22). Intuitively, this is what would have been expected, realizing the low ability of silicon to participate in multiple bonds. As may be expected from simpler systems (i.e., H2Si=CH2)(i97), silylene isomers (for example, structures 8 and 9) are considerably more stable (approximately IS kcal/mol) than their silaallene counterparts. 

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