The effects of annulation on cyclobutadiene bond alternation: A comparative study of semiempirical and ab initio methods

A great deal of interest has been directed toward cyclobutadiene (CBD) in the past because of its low activation barrier for automerization. The high reactivity of CBD has limited experimental investigations largely to tert-butyl substituted species. Little is known about the effect of specific molecular design perturbation, such as annulation, on the automerization energies. Herein, we describe this annulation effect and present a comparative study between semiempirical and ab initio methods. The calculations indicate that method selection is essential in these strained cyclic species. The semiempirical methods (AM1, MNDO and PM3) exhibit a reversal in the trend of isomerization energies from that obtained with ab initio methods (RHF/6-31G* and MP2(fc)/6-31G*//RHF/6-31G*) with respect to annulation.