Huckel model annulenes

Following the Longuet-Higgins—Salem approach, Buck studied the tendency of aromatic monocyclic compounds to bond alternation with increasing ring size. He used the Huckel model with variable P for the 7T electrons and a Morse potential for the o electrons. His distortion energies show that the o system resists distortion and the tt system favors distortions from D h to D i2)h symmetry in C H annulenes with 77 = 6, 8, 10, 12, 14, 16. The symmetry of the HOMO and LUMO is used to predict whether the C30H30 bond alternation due to the tt electrons should occur. It was subsequently shownthat this method can also be applied to the characterization of antiaromatic polycycles. 

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