Hiickel and Mobius Aromaticity of Transition Structures

The view that the transition structures of allowed pericyclic reactions are aromatic is supported by a more detailed analysis. For example, both the suprafacial [1,5] hydrogen shift of 1,3-pentadiene and the disrotatory 

Curved arrow notation for benzene resonance and for aromatic transition structures in pericyclic reactions invoiving six eiectrons. (Adapted from reference 19.) 

Antiaromatic transition structures having four electrons. 

A hypothetical array of twisted p orbitals leading to a Mobius n system. 

In HMO theory, the cyclopropenyl cation is aromatic (i.e., it is a closed shell system with large delocalization energy), since both electrons are in the orbital with E = a -I- 26. The Huckel cyclopropenyl anion is antiaromatic because it is an open shell system (having one electron in each of the = a - 6 orbitals) with zero delocalization energy. In contrast, the Mobius cyclopropenyl anion is aromatic, since it is a closed shell system with all four electrons in bonding orbitals, and the Mobius cyclopropenyl cation is antiaromatic. 

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