Modelling extended systems containing siloxane building blocks

Systematic ab initio computations have been carried out to establish the utility and limitations of clusters in modelling extended siloxane based systems. Using isodesmic reactions the effect of chain length, substituent groups as well as ring formation and spirocyclization were examined. The results were also tested at different levels of the theory. It has been shown that both in SiOSi and OSiO units three center interactions contribute to the stability of the siloxanes. These effects however, do not extend beyond the next Si or O atom, thus clusters can be used in the description of siloxane systems. Large (eight membered and onwards) ring systems do not suffer from ring strain. The strain in the six membered rings is small, while in four membered rings it is significant, in spite of in plane orbital interactions compensating somewhat the destabilization in cyclodisiloxanes.