Peralkylated Tetrasilanes: Conformational Dependence of the Photoelectron Spectrum

Photoelectron spectra of conformationally constrained methylated tetrasilanes 1 − 5 were measured, extending to the full 0−180 ° range the previously available 0−80° region of SiSiSiSi dihedral angles ω provided by the monocyclic analogues 6 − 9 . The resulting complete picture of the conformational dependence of photoionization of the n -tetrasilane moiety is unfortunately somewhat distorted by differential polarizability effects of the alkyl chains used for conformational constraint. The lowest three ionization potentials are attributed to σ SiSi bonding orbitals. The first and the third of these decrease a little as ω goes from 0 to 180°, whereas the second one increases distinctly. Within the framework of the Koopmans' approximation, these trends in backbone MO energies agree with those computed in the HF/TZ//MP2/TZ approximation and those obtained from the simple Hückel ladder C model.