Hückel's rule
Hückel's rule of aromaticity treats molecules in their singlet ground states (S0). The stability trends of the compounds described here are found to be reversed in the lowest lying triplet and singlet excited states (T1 and S1), according to Baird's rule. This means that compounds like benzene, with 4"n" + 2 π-electrons and aromatic properties in the ground state, become antiaromatic and often adopt less symmetric structures in the excited state.