Dioxetane decomposition revisited: A semi‐empirical study of the potential energy surface

Abstract

AbstractThe potential energy surface describing the unimolecular thermolysis of 1,2‐dioxetane into two formaldehydes is explored via semi‐empirical calculations using the PM3 Hamiltonian with multi‐electron configuration interaction. An active space of the four highest occupied orbitals and the two lowest virtual orbitals is used. Several reaction coordinates were examined, and the in‐plane OO distance was used most extensively. The results indicate that the activation barrier to fission is on the ground state surface and is about 18 kcal mol−1, in good agreement with experimental results. Furthermore, calculations of the vertical triplet‐state energies show that the ground and triplet states are nearly degenerate for OO distances beyond that corresponding to the 18 kcal mol−1 barrier, until the molecule abruptly dissociates into two formaldehydes (at ca 2·55 A). This picture is indicative of a diradical pathway, where the activation energy is associated with motion of the system on the ground‐state surface, not the triplet surface, but where generation of a triplet product can be expected. Similar results were obtained for dimethyl‐ and tetramethyldioxetanes, which require higher Ea for dissociation, and for dimethyldioxetanone. Thermochemical calculations pertaining to these reactions and to the formation of triplet products are also presented.