Molecular and electronic structure of some silacyclopropabenzenes: the reversed Mills–Nixon effect

Abstract

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 π-electron partial bond localization as evidenced by the π-bond order analysis, which in turn indicates that the π-effect dominates over the σ-electron rehybridization effect. Concentration of the π-density in the ipso bond(s) seems to be triggered by hyperconjugation with SiH 2 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 SiH 2 group(s).