The Electronic Structure of the Criegee Intermediate. Ramifications for the Mechanism of Ozonolysis

Generalized valence bond (GVB) and configuration interaction (Cl) calculations using a double ζ basis set have been carried out on methylene peroxide (H₂COO), the reactive intermediate in the Criegee mechanism for ozonolysis of olefins. The ground state of methylene peroxide (using an open geometry) is shown to be a singlet biradical rather than a zwitterion. A strong analogy between methylene peroxide and its isoelectronic counterpart, ozone, is developed. The calculations also show that the ring state of methylene peroxide is 1 eV lower than the open form. Moreover, the ring state may reopen to give the dioxymethane biradical. The ab initio results are combined with thermochemical data in order to analyze the stability of the Criegee intermediate as well as the possible modes of reaction in ozonolysis. With regard to ozonolysis in solution, the mechanism for epoxide formation is elucidated and the possible role of methylene peroxide rearrangement to dioxymethane is considered in interpreting the ¹⁸O isotope experiments. With regard to ozonolysis in the gas phase, the production of many of the chemiluminescent species observed by Pitts and coworkers is explained. The production of reactive radicals such as OH and HO₂ in the course of ozonolysis, which may have important consequences for understanding the generation of photochemical air pollution, is also delineated.