Among the other compounds with E = E', 15, 17, and 19 have residuals in their n,* states with considerable aromatic character. Hence, it is obvious that the placement of two equal heteroatoms with in-plane lone-pairs in para-positions, leading to D2h symmetric molecules, provides for n,* states with strong aromatic character of the residuals. For 17 and 18, it is also notable that the MCI-components are significant, presumably a result of the fact that the excitation involves excitation of an -electron into an orbital with diffuse character as a result of the low electronegativity of carbon. Although MICD was not computed for these species, NICS supports the MCI results as the para-isomers are the least antiaromatic isomers (Figure ).