Conformational effects in photoelectron spectra of tetrasilanes

Four cyclic carbosilanes [(SiMe2)(4)(CH2)(n)], n = 1-4 (1-4), have been synthesized and used as models of conformationally constrained tetrasilane. Results of matrix isolation IR spectroscopy, annealing, and photodestruction, combined with HF/3-21G* calculations on low-energy conformers, suggest that in each each compound the SiSiSiSi dihedral angle omega is constrained within relatively narrow Limits: 1, 0 degrees; 2, 35-55 degrees; 3, 45-65 degrees (30 degrees for a higher-energy conformer); and 4, 60-80 degrees. It is argued that other structural differences between 1-4 and the linear chain analogue, decamethyltetrasilane Si4Me10 (5), are of secondary importance, that Koopmans' theorem can be used within a family of closely related structures, and that measurements of photoelectron spectra of 1-5 permit the construction of an experimental counterpart to the orbital energy correlation diagram for the syn-anti conformational transformation in tetrasilane. The trends found for the first three ionization potentials, assigned to electron removal from the three sigma(SiSi) molecular orbitals, are readily understood qualitatively by reference to the ladder C model of saturated chain structure. They show clearly that the even simpler Sandorfy C model is not appropriate for the description of sigma conjugation in saturated systems.