Tetra-tert-butyl-s-indacene is a Bond Localized C2h Structure: Even More Bad News for B3LYP

Whether tetra-tert-butyl-s-indacene (TtB-s-indacene) is a symmetric D2h structure or a bond alternating C2h structure remains a standing puzzle. Agreement between 1H NMR data and computed proton chemical shifts based on minima structures optimized at the M06-2X, B97X-D, and M11 levels confirm a bond localized C2h symmetryconsistent with its antiaromaticity. X-ray structures and computed B3LYP geometries of D2h TtB-s-indacene poorly reproduce experimental NMR data. The limitations and complications of using B3LYP geometries for interpreting the structures and paratropicities of -expanded antiaromatic systems are discussed.