Bond-Stretch Isomerism in Tetrasilabicyclo[1.1.0]butane Derivatives

The existence of bond-stretch isomers (isomers which differ primarily in the length of the bridgehead bond) of tetrasilabicyclobutane, Si 4 H 6, recently has been predicted using ab initio electronic structure methods of quantum chemistry. At the GVB/3-21G* level of theory, the “short bond” isomer is predicted to be less stable than the “long bond” isomer. Although the parent compound has not yet been experimentally observed, the X-ray crystal structure of a heavily-substituted derivative is known. This derivative compound apparently does not exist in two bond-stretch forms; rather, only the short bond isomer is observed. The present work is an attempt to resolve the differences between the theoretical predictions for the parent Si 4 H 6 molecule and the experimental observations of the derivative compound. In particular, a systematic study of how methyl and tert -butyl substituents affect the relative stabilities of the two bond-stretch isomers is presented. A detailed analysis of the relative energies of the bond-stretch isomers and isomerization barriers for the parent compuonds (Si 4 H 6 ) as a function of basis set and theoretical method is presented.