Silylenes calculated

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

Does a Silylene-Complex exist This rhetorical question is the title of a theoretical paper published in 1983 [84], As a result of an ab-initio calculation, the authors came to the conclusion that a moderately positive answer can be given. However, silylene complexes are thermodynamically less stable than carbene complexes (the MSi bond energy for the hypothetical complex (OC)5Cr = Si(OH)H is 29.6 kcal/mol, the bond energy of the MC bond in (OC)5Cr = C(OH)H is 44.4 kcal/mol) [85], and therefore silylene complexes should be difficult to isolate. 

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

Maier and co-workers condensed formaldehyde and elemental silicon at 12 K in an argon matrix and photolyzed the mixture to form silaketene H2SiCO, which is similar in structure to the silylene-CO adduct mentioned above. The reactants first form siloxiranylidene 49 (which equilibrates with an unknown species postulated as the planar/linear silaketene 50 when exposed to 313-nm-wavelength light) and then forms complex 51 when photolyzed at 366 nm (Scheme 15). This species could also be formed by photolyzing diazidosilane 52 in the presence of CO, and complex 51 equilibrates with SiCO (53) and H2. The CO infrared shift for this bent structure was calculated at 2129 cm , which is shifted —80cm from the calculated value of free CO, at 2210 cm. The experimentally observed value was reported at 2038-2047 cm at 12 K. 

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. 

Ab initio (3-21G( )//STO-3G) calculations by Chandrasekhar and Schleyer163 on 1,4-disilabenzene 58, its Dewar benzene isomer 59, and a silylene isomer 60 showed that all three species exhibited approximately similar stabilities, the silylene 60 being 9.9 kcal mol-1 more stable than the planar aromatic form 58, which was 5.9 kcal mol-1 more stable than the Dewar benzene form 59. 

It has become common to classify all molecular compounds, which fulfill the above characteristics, as carbene analogs 9,13>. As a consequence, compounds of divalent silicon, germanium, tin, and lead may be regarded as carbene-like and are therefore called silylenes, germylenes, stannylenes, and plumbylenes. In contrast to carbenes they have one property in common the energetically most favorable electronic state is the singlet 1a2 found by experiments and calculations 9). 

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. 

However, another study concluded that the changes of the hydrogen-bond stability may be important in biological processes. For these, the influence of local electric fields created by Li+, Na+, and Mg2+ ions on the properties and reactivity of hydrogen bonds in HF and HC1 dimer has been carried out by means of ab initio self-consistent field (SCF) method [33]. A few years later, the effect of intensity and vector direction of the external electric field on activation barriers of unimole-cular reactions were studied using the semiempirical MINDO/3 method [34]. However, both semiempirical and ab initio calculations were performed to study the multiplicity change for carbene-like systems in external electric fields of different configurations (carbene and silylene) and the factor that determines the multiplicity and hence the reactivity of carbene-like structures is the nonuniformity of the field [35]. 

This indicates that the deviations are due to systematic errors, for example deficiencies of the applied methods and basis sets. DFT-based methods, such as GIAO/DFT calculations are known to overestimate paramagnetic contributions to the chemical shielding. This results, for critical cases with small HOMO/LUMO separations, in overly deshielded competed chemical shifts. Notorious examples for these deficiencies are 29Si or 13C NMR chemical shift computations of silylenes, silylium ions or dienyl cation .(5/-54) Taking into account the deficiencies of the applied method, and bearing in mind very reasonable correlations shown in Figures 4 and 5, the computational results do support the structural characterization of the synthesized vinyl cations. 

MO calculations have been carried out for the reactions of disilacyclopropane (248) with oxygen to give dioxadisilacyclopentane (251) (Scheme 27) <89TL6705, 930M1514,90JA7804) and for the pyrolytic transformation of bisTMS silylene (236) to trisilacyclopentane (238) <84JOM(272)ll>. 

Again dismissing semi-empirical and STO-3G models, calculations and experiment are in close accord with regard to bond angles in silylene (near to 90°) and its difluoro and dichloro analogues (near to 100°). 

Theoretical calculations suggest that these reactions are viable routes for the preparation of compounds with Si=0, Si=S, Si=Se double bonds via the barrierless formation of an encounter complex between a silylene and an oxirane or its heavier... 

Theoretical calculations suggested three steps for the insertion of silylene into a Si—H bond, that is (a) the formation of the complex with the interaction of the hydrogen atom of a silane and the empty orbital of a silylene (b) the formation... 

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