In Pursuit of the Elusive Bond-Stretch Isomers by ab Initio MethodsBenzocyclobutene, Benzo[1,2:4,5]dicyclobutadiene, and Some Related Substituted Systems

Abstract

The problem of bond-stretch isomerism in benzocyclobutene (1) and benzo[1,2:4,5]- dicyclobutadiene is addressed 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 not exhibit 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 wave function, 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 potential energy barrier (0.4-1.0 kcal/mol), it is concluded that 3 prevails and that bond-stretch isomerism probably does not occur in the case of benzo[1,2:4,5]dicyclobutadiene either. However, the final asnwer will be given only by high level multireference (MR) ab initio calculations. It is important to stress that 3 exhibits a pattern of 10π electrons fully delocalized over the CC bonds of the molecular perimeter and thus represents a planar pseudo-[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 judiciously selected derivatives in the laboratory. In particular, it is shown that a number of substituted pseudo-[10]annulene systems are possible.