Bond length alternation in s-indacene

Tetra-tert-butyl-s-indacene is a Bond-Localized C2h Structure and a Challenge for Computational Chemistry

Whether tetra-tert-butyl-s-indacene is a symmetric D2h structure or a bond-alternating C2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical shifts based on minima structures optimized at the M06-2X, ωB97X-D, and M11 levels confirm a bond-localized C2h symmetry, which is consistent with the expected strong antiaromaticity of TtB-s-indacene.

Enhancing the Antiaromaticity of s-Indacene through Naphthothiophene Fusion

Addressing the instability of antiaromatic compounds often involves protection with bulky groups and/or fusion of aromatic rings, thus decreasing paratropicity. We report four naphthothiophene-fused s-indacene isomers, one of which is more antiaromatic than parent s-indacene. This surprising result is examined computationally through nucleus-independent chemical shift XY calculations and experimentally via nuclear magnetic resonance spectroscopy, X-ray crystallography, ultraviolet-visible spectrophotometry, and cyclic voltammetry, with the latter two indicating that this molecule possesses the lowest highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap observed for heterocycle-fused s-indacene.