Whether tetra-tert-butyl-s-indacene is a symmetric D-2h structure or a bond-alternating
C-2h structure remains a standing puzzle. Close agreement between experimental and
computed proton chemical shifts based on minima structures optimized at the M06-2X,
?B97X-D, and M11 levels confirm a bond-localized C-2h symmetry, which is consistent
with the expected strong antiaromaticity of TtB-s-indacene.
