PM3 and ab initio studies of the C8H8 potential energy surface.: Thermal isomerism of syn- and anti-tricyclo[4.2.0.02,5]octa-3,7-dienes

We have identified for the first time symmetry allowed pathways for rearrangement of both the syn (1) and anti (2) isomers of the cyclobutadiene dimer in the closed shell potential energy surface for the isomerisation to cyclooctatetraene. At the B3LYP/6-31G(d) level of theory, the calculated activation energy for the reaction of 1 is 3.2 kcal mol(-1) lower than that for 2, in good agreement with the experimental difference of 3.6 kcal mol(-1). The possible influence of the a strain on the synchronicity of the pericyclic reactions of the systems is discussed.