Electron Attachment to the Lowest Unoccupied Orbitals in Linear and Branched Permethylated Polysilanes and Hexamethyldigermane and -distannane

The peculiar properties of polysilanes have stimulated many studies of their electronic structure. While the nature and ordering of the filled MOs have been studied in some detail, the theoretical predictions for the empty counterparts have never been tested experimentally. Here the electron transmission spectra of Me(SiMe 2 ) n Me (with Me = CH 3 and n = 1−4), MeSi(SiMe 3 ) 3, Si(SiMe 3 ) 4, Ge 2 Me 6, and Sn 2 Me 6 are presented and the energies of the observed resonances allow assignment of the lowest unoccupied levels. With the change from monomers to dimers, the presence of a heteroatom−heteroatom bond causes a large electron affinity increase; the LUMO is ascribed to a σ* MO essentially heteroatom in character, whereas the lowest σ* C-heteroatom MOs lie at about 1.5 eV higher energy. The spectra of the linear trimer and tetramer of silicon show that the energy separation between the lowest-lying empty orbitals is even larger than that observed in their filled counterparts. The lowest-lying empty MOs are thus well delocalized on the silicon skeleton but not on the substituents. The resonance energies are also compared with the unoccupied orbital energies calculated by means of ab initio 3-21G* and MNDO methods. The results presented here do not support the previously proposed π* model based upon ab initio calculations.