| Abstract
| - The problem of bond-stretch isomerism in benzocyclobutene (1) and benzo[1,2:4,5]- dicyclobutadiene isaddressed by single-state (SS-CASSCF and SS-CASPT2) as well by two-state (TWS-CASSCF and TWS-CASPT2) theoretical formalisms. It is shown beyond reasonable doubt that benzocyclobutene 1 does notexhibit bond-stretch isomerism. The situation is more complex and interesting in the case of benzo[1,2:4,5]-dicyclobutadiene. The TWS-CASPT2(π)+σ approach, based on the TWS-CASSCF(10,10)π zeroth-order wavefunction, yields two bond-stretch isomers 2 and 3. Their total energies, including the ZPVE contributions,indicate that the structure 3 is slightly favored. Because they are separated by an almost vanishing potentialenergy barrier (0.4−1.0 kcal/mol), it is concluded that 3 prevails and that bond-stretch isomerism probablydoes not occur in the case of benzo[1,2:4,5]dicyclobutadiene either. However, the final asnwer will be givenonly by high level multireference (MR) ab initio calculations. It is important to stress that 3 exhibits a patternof 10π electrons fully delocalized over the CC bonds of the molecular perimeter and thus represents a planarpseudo-[10]annulene system par excellence. Finally, it is argued that strategic substitution of benzo[1,2:4,5]dicyclobutadiene might lead to a discrimination of the isomers 2 and 3 and to the isolation of some judiciouslyselected derivatives in the laboratory. In particular, it is shown that a number of substituted pseudo-[10]annulenesystems are possible.
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