Computational studies of gas-phase accretion product formation involving RO2

Field and laboratory studies have indirectly but conclusively established that reactions involving peroxy radicals (RO₂) play a key role in the gas-phase formation of accretion products, also commonly referred to as “dimers”, as they typically contain roughly twice the number of carbon atoms compared to their hydrocarbon precursors. Using computational tools, we have recently presented two different potential mechanisms for this process.

First, direct and rapid recombination of peroxy and alkoxy (RO) radicals, analogous to the recently characterized RO₂ + OH reaction, leads to the formation of metastable RO₃R’ trioxides, which may have lifetimes on the order of a hundred seconds. [1] However, due to both the limited lifetime of the trioxides, and the low concentration of alkoxy radicals, the RO₂ + R’O pathway is likely to be a minor, though not necessarily negligible, pathway for atmospheric dimer formation.

Second, we have shown that recombination of two peroxy radicals – phenomenologically known to be responsible for the formation of ROOR’ – type dimers – very likely occurs through a multi-step mechanism involving an intersystem crossing (ISC). [2]  In contrast to earlier predictions, we find that the rate-limiting step for the overall RO₂  + R’O₂ reaction is the initial formation of a short-lived RO₄R’ tetroxide intermediate. For tertiary RO₂, the barrier for the tetroxide formation can be substantial. However, for all studied species the tetroxide decomposition is rapid, forming ground-state triplet O₂, and a weakly bound triplet complex of two alkoxy radicals. The branching ratios of the different RO₂ + R’O₂ reaction channels are then determined by a three-way competition of this complex. For simple systems, the possible channels are dissociation (leading to RO + R’O), H-abstraction on the triplet surface (leading to RC=O + R’OH), and ISC and subsequent recombination on the singlet surface (leading to ROOR’). All of these can potentially be competive with each other, with rates very roughly on the order of 10⁹ s^-1. For more complex RO₂ parents, rapid unimolecular reactions of the daughter RO (such as alkoxy scissions) open up even more potential reaction channels, for example direct alkoxy – alkyl recombination to form (either singlet or triplet) ether-type (ROR’) dimers.

[1] Iyer, S., Rissanen, M. P. and Kurtén, T. Reaction Between Peroxy and Alkoxy Radicals can Form Stable Adducts. Journal of Physical Chemistry Letters, Vol. 10, 2051-2057, 2019.

[2] Valiev, R., Hasan, G., Salo, V.-T., Kubečka, J. and Kurtén, T. Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. Journal of Physical Chemistry A, Vol. 123, 6596-6604, 2019.