A MOLECULAR-MODEL FOR AN EXCESS ELECTRON IN SILICON - ELECTRON ADDITION TO HEXATRIMETHYLSILYL DISILANE, A DERIVATIVE WITH 8 SIGMA-BONDED SILICON ATOMS

Exposure of the hexatrimethylsilyl disilane to ionizing radiation at 7 7 K gave ESR (EPR) spectra including Si-29 satellite features that are assigned to the radical anions I-.-. Similar features were obtained f rom dilute solutions in certain solvents known to promote electron cap ture. The results are analysed in terms of extensive delocalization wi thin the seven Si-Si sigma bonds, with a small preference for the cent ral Si-Si bond. There is no case for invoking extensive use of the 3d silicon orbitals, the data being fully accommodated using the normal 3 6 + 3p orbitals of the a framework. Similarly, using Freon (CFCl3) as a solvent that promotes specific electron loss, the parent radical cat ions have been prepared by radiolysis. The ESR spectra are discussed i n terms of distortions involving stretching of one or two specific Si- Si bonds. Thus, for this model compound. with seven Si-Si bonds, an ex cess electron is accommodated within the a framework and is extensivel y delocalized. We suggest that these results lead to the conclusion th at the conduction band for pure silicon is also based on the a framewo rk and that the more diffuse set of 3d orbitals do not make a major co ntribution.