Protonated para-diheteroaromatics: For the diheteraromatics with E = E´ atoms at para-positions and with aromatic S0 states (9, 17, and 19) we also explored the changes in (anti)aromatic character upon protonation of their 3 n,* states. This leads to 9H + , 3, and 19H + , respectively. These three species remain aromatic in S0, but less than when unprotonated. Importantly, though, the residuals of their 3 n,* states remain aromatic in character in each of these species, indicating that protonation reduces the aromaticity of both the S0 and 3 n,* states to similar relative amounts. In contrast, when 14 is protonated at the negative C atom, leading to 1, there is no aromatic but rather an antiaromatic residual. From this we conclude that the two features that lead to an aromatic residual are high symmetry (D2h or nearly so) and negative charge (excess of electrons).