Pericyclic Reactions: Aromaticity of Transition States
Aromaticity of Transition States: Visualization of Delocalized Electrons
The principle of aromaticity becomes especially clarified by making delocalized π electrons visual. The ACID method (anisotropy of the induced current density) can be used for this purpose. The density of delocalized electrons is quantum-mechanically calculated and visualized as a yellow surface.
Cycohexene, cyclohexadiene and benzene are compared as examples for the distribution of delocalized electrons in molecules . In the case of cyclohexene, delocalized electrons are found only in the area of the double bond. Both electrons within the π system of the double bond are delocalized over both p orbitals of -hybridized centers; the electrons are allowed to move "freely" within the π system. As indicated by small yellow spots, there is little or no probability to find delocalized electrons at C-C single and C-H bonds, because the bonds contain localized electrons. Delocalization of electrons over both double bonds is shown in cyclohexadiene with delocalization within the double bonds to be stronger than in the area of conjugation between them. This is consistent with the fact the π bond in a double bond is stronger than the π interaction between two conjugated double bonds. Benzene represents a closed conjugated system. Depending on the number of delocalized electrons, fully delocalized cyclic systems are either aromatic or antiaromatic.
The ACID method is being used as evidence for aromatic transition states in pericyclic reactions. Many examples of 3D animations of reactions with ACID surfaces are available. The transition states of allowed pericyclic reactions show a cyclic conjugated system, i.e., an aromatic system of delocalized electrons.
Therefore, a comparison between the ACID visualizations of transition states of the Diels-Alder reaction (pericyclic) and elimination (not pericyclic) shows distinct differences: the transition state of the Diels-Alder reaction is displayed as a cyclic delocalized aromatic system of π electrons while the transition state of the elimination represents a linear non-aromatic system.