Conformational effects in UV absorption spectra of tetrasilanes

UV absorption of cyclic carbosilanes (SiMe₂)₄(CH₂)_n, N = 1–4 (1–4), and Si₄Me₁₀ (5) provides an experimental counterpart to the singlet transition energy and intensity correlation diagrams for the syn-anti conformational transformation in tetrasilane. A new third transition is found between the two previously known singlet transitions. Transition energies are nearly independent of the dihedral angle, while intensities vary widely. All trends agree with CIS/3–21G^*//HF/3–21G^* calculations. The Sandorfy C and ladder C models of σ conjugation fail to describe electronically excited states of tetrasilane, since they do not consider the lateral bonds to substituents.