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

Peroxyl radicals (RO) are important intermediates in the atmospheric oxidation processes. The RO can react with other RO to form reactive intermediates known as tetroxides, ROR. The reaction mechanisms of ROR 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 ROR formation and its decomposition is the involvement of open-shell 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 + MeO → MeOMe, and its decomposition, MeOMe → MeO + O + MeO, as well as the two-body product complexes MeO···O + MeO and MeO···MeO + O. The used MR methods are benchmarked against relative energies of MeO + MeO, MeOMe, and MeO + MeO + O, calculated with CCSD(T)/CBS, W2X, and W3X-L methods. We found that the calculated relative energies of the overall MeO + MeO → MeOMe → MeO + O + 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 product complex + MeO is lower in energy than the MeO···MeO complex + O. The formation of MeOMe 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 MeOMe is energetically preferred over the corresponding sequential decomposition.