Theoretical study of the chemiluminescent decomposition of dioxetanone.

The unimolecular chemiluminescent decomposition of unsubstituted dioxetanone was studied at the complete active space self-consistent field level of theory combined with the multistate second-order multiconfigurational perturbation theory energy correction. The calculations revealed interesting features. Two transition states, two conical intersections, and one intermediate stable biradical structure along the lowest energy reaction path were identified. It was noted that the conical intersections are found at or in very close proximity to the transition states. The first and second transition states correspond to O-O and C-C cleavages, respectively. In particular, a planar structure is supported by the (1)(sigma,sigma*) state during the O-O dissociation up to the first transition state and conical intersection. At this point the (1)(sigma,sigma*) state dissociation path bifurcates, corresponding to a torsion of the O-C-C-O angle. Simultaneously, the (1)(n,sigma*) state becomes lower in energy while still favoring a planar structure. As the lowest-energy reaction path proceeds toward the second transition state and conical intersection, the (1)(n,sigma*), (3)(n,sigma*), and (1)(sigma,sigma*) states are close in energy. This work suggests that the vibrational distribution at the first conical intersection and the interactions among the states as the reaction proceeds between the two transition states are the origin of the population of the chemiluminescent (n,sigma*) states.