Direct dynamics simulation of dioxetane formation and decomposition via the singlet ·O–O–CH₂–CH₂· biradical: Non-RRKM dynamics

Electronic structure calculations and direct chemical dynamics simulations are used to study the formation and decomposition of dioxetane on its ground state singlet potential energy surface. The stationary points for ¹O₂ + C₂H₄, the singlet ·O–O–CH₂–CH₂· biradical, the transition state (TS) connecting this biradical with dioxetane, and the two transition states and gauche ·O–CH₂–CH₂–O· biradical connecting dioxetane with the formaldehyde product molecules are investigated at different levels of electronic structure theory including UB3LYP, UMP2, MRMP2, and CASSCF and a range of basis sets. The UB3LYP/6-31G* method was found to give representative energies for the reactive system and was used as a model for the simulations. UB3LYP/6-31G* direct dynamics trajectories were initiated at the TS connecting the ·O–O–CH₂–CH₂· biradical and dioxetane by sampling the TS's vibrational energy levels, and rotational and reaction coordinate energies, with Boltzmann distributions at 300, 1000, and 1500 K. This corresponds to the transition state theory model for trajectories that pass the TS. The trajectories were directed randomly towards both the biradical and dioxetane. A small fraction of the trajectories directed towards the biradical recrossed the TS and formed dioxetane. The remainder formed ¹O₂ + C₂H₄ and of these ∼ 40% went directly from the TS to ¹O₂ + C₂H₄ without getting trapped and forming an intermediate in the ·O–O–CH₂–CH₂· biradical potential energy minimum, a non-statistical result. The dioxetane molecules which are formed dissociate to two formaldehyde molecules with a rate constant two orders of magnitude smaller than that predicted by Rice–Ramsperger–Kassel–Marcus theory. The reaction dynamics from dioxetane to the formaldehyde molecules do not follow the intrinsic reaction coordinate or involve trapping in the gauche ·O–CH₂–CH₂–O· biradical potential energy minimum. Important non-statistical dynamics are exhibited for this reactive system.