CONFORMATIONAL EFFECTS IN PHOTOELECTRON-SPECTRA OF TETRASILANES

Four cyclic carbosilanes [(SiMe2)(4)(CH2)(n)], n = 1-4 (1-4), have bee n synthesized and used as models of conformationally constrained tetra silane. Results of matrix isolation IR spectroscopy, annealing, and ph otodestruction, combined with HF/3-21G calculations on low-energy con formers, suggest that in each each compound the SiSiSiSi dihedral angl e omega is constrained within relatively narrow Limits: 1, 0 degrees; 2, 35-55 degrees; 3, 45-65 degrees (30 degrees for a higher-energy con former); and 4, 60-80 degrees. It is argued that other structural diff erences between 1-4 and the linear chain analogue, decamethyltetrasila ne Si4Me10 (5), are of secondary importance, that Koopmans' theorem ca n be used within a family of closely related structures, and that meas urements of photoelectron spectra of 1-5 permit the construction of an experimental counterpart to the orbital energy correlation diagram fo r the syn-anti conformational transformation in tetrasilane. The trend s found for the first three ionization potentials, assigned to electro n removal from the three sigma(SiSi) molecular orbitals, are readily u nderstood qualitatively by reference to the ladder C model of saturate d chain structure. They show clearly that the even simpler Sandorfy C model is not appropriate for the description of sigma conjugation in s aturated systems.