On the bond-stretch isomerism in the benzo[1,2:4,5]dicyclobutadiene system - An ab initio MR-AQCC study

Bond-stretch isomerism in benzo[1,2:4,5]dicyclobutadiene (BDCB) has been investigated using the MR-AQCC/6-31G(d) method, a high-level multireference ab initio approach including size-extensivity corrections. The applied theoretical approach includes both nondynamical and dynamical electron correlation effects. Full MR-AQCC geometry optimizations of localized (1) and delocalized (3) isomers as well as the transition structure (TS) have been determined using D _2h symmetry restriction. The calculations show that both isomers are approximately of equal stability separated by a barrier with a height of about 5 kcalmol^-1. Thus, the present results strongly indicate that benzo[1,3:4,5]dicyclobutadiene is a very good candidate for an organic compound exhibiting bond-stretch isomerism, since isomers 1 and 3 correspond to true minima on the double-well potential energy surface, which are separated by a sufficiently high barrier. It is particularly important to emphasize that isomer 3 represents a realization of the highly elusive quasi-[10]annulene.