Molecular and electronic structure of 1,2-disilacyclobutabenzenes. Ab initio molecular orbital and density functional study

The geometric and electronic structure of 1,2-disilacyclobutabenzene and its polyannelated analogues is examined by the MP2(fc)/6-31G* and B3LYP/6-31G* methods. It is found that the bond distances alternate in the benzene fragment, the ipso bond being the longest. This finding is rationalised by the rehybridization at the carbon junction atoms. The average C-C distances are larger than in the free benzene, reflecting an interesting blow-up phenomenon. The latter is interpreted by the pi-electron charge transfer interaction between the highest occupied MOs of the benzene ring and the phi*(SiH2) antibonding fragment orbitals of the SiH2 groups. The destabilisation energy of these systems was examined by the homodesmic reactions. It is shown that the total strain energy is additive to a good approximation, depending on the number of annelated four-membered rings.