Theoretical study of Si/C alternately substituted annulenes with a belt structure

Alternating Si/C belt-shaped annulenes, H_2<i>n</i>Si_<i>n</i>C_<i>n</i> (<i>n</i> = 3, 4, 5, 6, and 10), representing a third class of annulenes beyond the planar and Möbius types, were investigated through quantum chemical calculations. Notably, the Si-C bond length alternation is not observed regardless of the number of π electrons (<i>e.g.</i>, 4<i>n</i> or 4<i>n</i> + 2). For the smaller molecules (<i>n</i> = 3 and 4), the belt-shaped isomers were found to be less thermodynamically stable than their planar counterparts, benzene (<i>n</i> = 3) and cyclooctatetraene (<i>n</i> = 4), due to distorted π orbitals and strained ring structures. The quasi-atomic orbital (QUAO) analysis reveals that the planar <i>n</i> = 3 Si/C annulene exhibits delocalized π bonding with weak aromatic stabilization, while its belt-shaped counterpart shows hybridization-induced π localization and antiaromatic character. Both <i>n</i> = 4 systems (planar and belt-shaped) are intrinsically antiaromatic, although geometric distortion in the belt isomer partially alleviates this destabilization. As the ring size increases (<i>n</i> ≥ 5), the Si-C π orbitals become increasingly localized due to geometric constraints, in contrast with the uniform delocalization observed in the all-carbon analog H₂₀C₂₀. Notably, in the larger annulenes (<i>n</i> = 5, 6, and 10), the curvature of the belt structure imposes a ceiling on π conjugation. These results underscore the key role of geometry and QUAO asymmetry in modulating antiaromaticity in Si/C belt systems.