Conformational Effects in Photoelectron Spectra of Tetrasilanes

Four cyclic carbosilanes [(SiMe 2 ) 4 (CH 2 ) 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 compound the SiSiSiSi dihedral angle ω is constrained within relatively narrow limits: 1, 0°; 2, 35−55°; 3, 45−65° (30° for a higher-energy conformer); and 4, 60−80°. It is argued that other structural differences between 1 − 4 and the linear chain analogue, decamethyltetrasilane Si 4 Me 10 ( 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 σ 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 σ conjugation in saturated systems.