Silabenzene calculations

UV irradiation (A>320 nm) of matrix-isolated silabenzene [137a] led to the disappearance of its absorptions and the appearance of bands of other unstable molecules, which were assigned to Dewar silabenzene [138]. This assumption is reasonable because sp hybridization of the silicon atom is preferable to sp hybridization and favourable to the stability of [138]. Besides, according to calculations the energy differences between mono-cyclic [137a] and bicyclic [138] structures decrease from CeHe (314 kJ moP ) to silabenzene CsSiHe (125 kJ moP ). 

The geometries of silabenzene and four of its isomers were optimized using ab initio calculations at the STO-3G level (isomers 3-5 and 1)(60,193) or using semi-empirical MNDO calculations (isomer 6)(194). 

The heat of formation (13.7 kcal/mol) and the first ionization potential (8.6 eV) of silabenzene have also been calculated using the MINDO/3 method (177). 

An estimate of the aromatic stabilization energy of silabenzene, based on the calculation of the ISE (STO-3G basis set) (78JA6499), concludes that its value is 2/3 of the benzene stabilization energy. Possible ap-... 

The stability of phosphabenzene and of arsabenzene in the absence of air and the isolation of the silicon-carbon and silicon-silicon double bonds might suggest that silabenzene, appropriately substituted, could be stable enough to be isolable. Indeed, calculations suggest that it would have a it-resonance stabilization energy about two-thirds that of benzene (78JA6499). 

The photoelectron spectra of silabenzene was measured and assigned on the basis of a double-zeta ab initio SCF calculation44. The lowest ionization energy of 8 is 8.11 eV (2/ii state, calculated to be 7.8 eV at HF/DZ44), by ca 1 eV lower than in benzene. The authors conclude that silabenzene is best considered as a symmetry-distorted (due to the presence of the Si atom) cyclic 67r-electron system. 

Schleyer and coworkers40 studied in detail the energetic relationships between silabenzene 8 and four of its isomers, 13-16. However, as this study is quite old, the calculated relative energies of 8 and 13-16, which have been calculated only at HF/3-21G( )//HF/STO-3G, are probably only approximate and higher level calculations are required for a more reliable estimate of these relative energies. 

In summary, all the calculations find silabenzene to be highly aromatic. However, in view of the relatively basic level of theory which was used in most of the available studies, it is highly desirable to repeat the calculations for this fundamentally important molecule using more reliable levels of theory. 

TABLE 2. Calculated substituent effects on the thermodynamic stability, AE (in kcalmol-1), of (pso-substituted silabenzenes and of substituted benzenes (HF/3-21G)"... 

Although the resonance energy of silabenzene is calculated to be about three-fourths of that of benzene, silabenzenes are extremely reactive and, thus, have been examined only in frozen matrices and in the gas phase. By far the simplest route to silabenzenes is pyrolysis of the 1,4-dihydro derivatives. 

TABLE 30. Calculated substituent effects on the thermodynamic stabilities of substituted benzenes and of ipso-substituted silabenzenes (3-21G AE in kcalmol - lf... 

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