Genuinely Forbidden Valence Isomerizations

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. 

Unlike Figs. 5.1 and 5.2, the correspondence diagrams in Fig. 5.11 are not rudimentary , because they incorporate all of the information necessary to deduce the allowedness of the thermal isomerizations. The six combinations of the (Tec-bonding orbitals of benzene retain their C2V labels in prismane and Dewarbenzene as do its six CH-bonding combinations. The analysis of the reaction is thus reduced to a six-electron problem the occupied orbitals involved are the three Hiickel-MOs of benzene - labelled as in Fig. 1.2, the long a-bonding orbital ((725) in its two valence isomers, and the symmetric (-I-) and antisymmetric (—) combinations of (Ti3 and 74 in prismane or of TTie and 34 in Dewarbenzene. 

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