ELECTRON-ATTACHMENT TO THE LOWEST UNOCCUPIED ORBITALS IN LINEAR AND BRANCHED PERMETHYLATED POLYSILANES AND HEXAMETHYLDIGERMANE AND HEXAMETHYLDISTANNANE

The peculiar properties of polysilanes have stimulated many studies of their electronic structure. While the nature and ordering of the fill ed MOs have been studied in some detail, the theoretical predictions f or the empty counterparts have never been tested experimentally. Here the electron transmission spectra of Me(SiMe(2))(n)Me (with Me = CH3 a nd 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 ass ignment of the lowest unoccupied levels. With the change from monomers to dimers, the presence of a heteroatom-heteroatom bond causes a larg e electron affinity increase; the LUMO is ascribed to a sigma MO esse ntially heteroatom in character, whereas the lowest sigma(c-heteroato m) MOs lie at about 1.5 eV higher energy. The spectra of the linear tr imer and tetramer of silicon show that the energy separation between t he lowest-lying empty orbitals is even larger than that observed in th eir filled counterparts. The lowest-lying empty MOs are thus well delo calized on the silicon skeleton but not on the substituents. The reson ance energies are also compared with the unoccupied orbital energies c alculated by means of ab initio 3-21G and MNDO methods. The results p resented here do not support the previously proposed pi model based u pon ab initio calculations.