Heilbronner and co-workers described how this double-bond delocalization is promoted by using donors at positions with pronounced LUMO coefficients or acceptors at HOMO positions, which alleviates the antiaromaticity of the parent molecules. In this way, we designed several double-bond delocalized pentalenes and s-indacenes which exhibit inverted gaps. However, there are many other non-alternant hydrocarbons that are highly symmetric but nonetheless exhibit small positive singlet-triplet gaps, despite having minimal HOMO-LUMO overlap.