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Molecular and electronic structure of some silacyclopropabenzenes-Reversed Mills-Nixon Effect

Molecular and electronic structure of silacyclopropabenzenes are examined by the MP3(fc)/6-31G* theoretical model. The salient feature of these compounds is alternation of bond distances of the aromatic fragment in the reversed Mills-Nixon sense i.e. the annelated CC bonds are shortened, whilst the ortho bonds are stretched. This is in harmony with the pi-electron partial bond localization as evidenced by the pi-bond order analysis, which in turn indicates that the pi-effect dominates over the sigma-electron rehybridization effect. Concentration of the pi-density in the ipso bond(s) seems to be triggered by hyperconjugation with SiH2 group(s). The latter mode of interaction is responsible for a decrease in the strain energy of silacyclopropabenzenes relative to the corresponding cyclopropabenzenes. It is shown that the total strain energy is a simple additive function of the number and types of the fused small rings involving SiH2 group(s).

silacycloproparenes; ab initio calculations; B3LYP calculations; bond alternations; reversed Mills-Nixon effect

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