Multireference and Coupled-Cluster Study of Dimethyltetroxide (MeO₄Me) Formation and Decomposition

Abstract

Peroxyl radicals (RO 2 ) are important intermediates in the atmospheric oxidation processes. The RO 2 can react with other RO 2 to form reactive intermediates known as tetroxides, RO 4 R. The reaction mechanisms of RO 4 R formation and its various decomposition channels have been investigated in multiple computational studies, but previous approaches have not been able to provide a unified methodology that is able to connect all relevant reactions together. An apparent difficulty in modeling the RO 4 R formation and its decomposition is the involvement of openshell singlet electronic states along the reaction pathway. Modeling such electronic states requires multireference (MR) methods, which we use in the present study to investigate in detail a model reaction of MeO 2 + MeO 2 → MeO 4 Me, and its decomposition, MeO 4 Me → MeO + O 2 + MeO, as well as the two-body product complexes MeO•••O 2 + MeO and MeO•••MeO + O 2 . The used MR methods are benchmarked against relative energies of MeO 2 + MeO 2 , MeO 4 Me, and MeO + MeO + O 2 , calculated with CCSD(T)/ CBS, W2X, and W3X-L methods. We found that the calculated relative energies of the overall MeO 2 + MeO 2 → MeO 4 Me → MeO + O 2 + MeO reaction are very sensitive to the chosen MR method and that the CASPT2(22e,14o)-IPEA method is able to reproduce the relative energies obtained by the various coupled-cluster methods. Furthermore, CASPT2(22e,14o)-IPEA and W3X-L results show that the MeO•••O 2 product complex + MeO is lower in energy than the MeO•••MeO complex + O 2 . The formation of MeO 4 Me is exothermic, and its decomposition is endothermic, relative to the tetroxide. Both the formation and decomposition reactions appear to follow pathways with no saddle points. According to potential energy surface scans of the decomposition pathway, the concerted cleavage of both MeO•••O bonds in MeO 4 Me is energetically preferred over the corresponding sequential decomposition.

Lifetimes of pre-reactive complexes of peroxy radicals revisited: thermostat effects, temperature dependence and highly oxygenated molecules

“…Our research group, among others, has long been involved in using theoretical chemistry methods to understand bi-molecular reactions between peroxy radicals. 8–16 In particular, we have been attempting to extend our studies to include the dynamics of the reactions. 13,14 In many cases, experimental results 5,17–21 show a decrease in overall reactivity as temperature is increased, consistent with a negative activation energy E a in the framework of an Arrhenius relation, k = A e − E a / RT .…”