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 la rgely to tert-butyl substituted species. Little is known about the eff ect of specific molecular design perturbation, such as annulation, on the automerization energies. Herein, we describe this annulation effec t 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, MND O and PM3) exhibit a reversal in the trend of isomerization energies f rom that obtained with ab initio methods (RHF/6-31G and MP2(fc)/6-31G //RHF/6-31G*) with respect to annulation.