Application of the VBSSA Rule

We now consider the isomerization of Dewar benzene into benzene  

In the above reaction, the product benzene is of D6h symmetry, but only C2v symmetry elements are common for both Dewar benzene and benzene. We can perform therefore the symmetry analysis of the VB structures within the C2v point group framework. There are 6 essential valence electrons involved in the reaction, numbered from 1 to 6. These six valence electrons form three localized bonds (two ji bonds and one o bonds) in Dewar benzene, and a n delocalized bond in benzene, which can be described by five independent BTs within the OEO formalism (see Eq.(65)). Their corresponding BTs are 

Applying the projection operators associated with C2v point group on these BTs results in Dewar benzene 

Obviously, the conversion from Dewar benzene to benzene is forbidden by the VBSSA rule because no A2 symmetrized VB structure segment in the reactant Dewar benzene can match the A2 symmetrized VB structure segment of benzene. More examples, including generally allowed and forbidden chemical process, can be found in our previous study[69], 

The VBSSA rule can be used to select relevant VB structures and construct the semiquantitative and quantitative curve-crossing VB diagram. Combining the MO method and VB calculations, we investigated atom exchange reactions H + HLi — H2 + Li and H + LiH— HLi + H, and discussed the effect of energy differences between the VB structures on the activation energy and properties of the transition state [70], 

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