CYCLISATION OF BUTADIENE BY CO-ROTATION AND BY DIS-ROTATION

We discuss the use of Longuet-Higgins and Abrahamson's method for predicting valence isomerization of butadiene into cyclobutene or vice versa in the ground and the excited state. With the use of an approximation to the extended Hückel method, we are able to demonstrate that the type of correlation diagramm for the M.O. in the conrotatory mode but not in the disrotatory mode depends on the interatomic distances and correlatively on the electronic state. Employing the method itself we observe the same phenomena. We find that, while the ground state reaction does proceed in a conrotatory fashion and the excited state reaction disrotatory, this cannot be attributed to potential barriers but should be explained by more or less easy de-excitation. © 1968 Springer-Verlag.