Ab Initio Molecular Orbital Study of the First Four Si/C Alternately Substituted Annulenes

The ground and low-lying excited states of four alternating Si/C annulenes, H _nSi _n/2C _n/2 with n = 4, 6, 8, and 10, have been investigated by ab initio molecular orbital methods and compared to those of their all-carbon and all-silicon analogues. In the ground state, all of the Si/C-mixed annulenes, except for the largest 10-membered annulene (H₁₀Si₅C₅), assume equal-bond-length structures by adopting a closed-shell electronic structure in the possible highest symmetry. For the largest H₁₀Si₅C₅, the trend of the bond delocalization still remains but the circular structure is considerably distorted and nonplanar due to severe angle strain. In the low-lying singlet (S₁) and triplet (T₁) states, the geometry of the compounds tends to be nonplanar as the excitations produce silyl radical character. Relative energies of the T₁ and S₁ states of the 6-membered ring, compared to those of the respective ground states (S₀), are higher than those of the 4- and 8-membered rings, suggesting a special stability for H₆Si₃C₃. The planar rhombus shape of the formally antiaromatic H₄Si₂C₂ suggests that a synthetic effort is merited. Bonding analyses are given to support the conclusions reached on the basis of geometric structures and excited-state energetics.