The benzvalene-benzene interconversion allowed or forbidden

Empirically, there is no doubt that the energetic demands of simultaneous rupture of both a bonds is too demanding. Computations with extensive configuration interaction [27] confirm the results of an earlier SCF calculation [28] according to which the two bonds are lengthened unequally in the transition 

In contrast to the thermal isomerization of benzvalene to benzene, those of the other two well known valence isomers, Dewarbenzene and prismane are genuinely forbidden Not only do the occupied orbitals fail to correlate along 

Concerted Two or more primitive changes are said to be concerted (or to constitute a concerted process) if they occur within the same elementary reaction. Such changes will normally (though perhaps not inevitably) be energetically coupled . 

Synchronous A concerted process in which the primitive changes concerned (generally bond rupture and bond formation) have progressed to the same (or comparable) extent at the transition state is said to be synchronous [29] 

The relatively facile spin-non-conservative isomerization of Dewarbenzene [30] will be taken up in a subsequent chapter. 

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Figure 5.10. The benzvalene-benzene interconversion allowed or forbidden" (For clarity the arrows are drawn to depict the reverse reaction [ 2 + 2] cycloaddition, to which the same Rules apply)...

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