<p>Field and laboratory studies have indirectly but conclusively established that reactions involving peroxy radicals (RO<sub>2</sub>) play a key role in the gas-phase formation of accretion products, also commonly referred to as &#8220;dimers&#8221;, 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.</p><p>First, direct and rapid recombination of peroxy and alkoxy (RO) radicals, analogous to the recently characterized RO<sub>2</sub> + OH reaction, leads to the formation of metastable RO<sub>3</sub>R&#8217; 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 RO<sub>2</sub> + R&#8217;O pathway is likely to be a minor, though not necessarily negligible, pathway for atmospheric dimer formation.</p><p>Second, we have shown that recombination of two peroxy radicals &#8211; phenomenologically known to be responsible for the formation of ROOR&#8217; &#8211; type dimers &#8211; very likely occurs through a multi-step mechanism involving an intersystem crossing (ISC). [2]&#160; In contrast to earlier predictions, we find that the rate-limiting step for the overall RO<sub>2</sub>&#160; + R&#8217;O<sub>2</sub> reaction is the initial formation of a short-lived RO<sub>4</sub>R&#8217; tetroxide intermediate. For tertiary RO<sub>2</sub>, the barrier for the tetroxide formation can be substantial. However, for all studied species the tetroxide decomposition is rapid, forming ground-state triplet O<sub>2</sub>, and a weakly bound triplet complex of two alkoxy radicals. The branching ratios of the different RO<sub>2</sub> + R&#8217;O<sub>2</sub> 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&#8217;O), H-abstraction on the triplet surface (leading to RC=O + R&#8217;OH), and ISC and subsequent recombination on the singlet surface (leading to ROOR&#8217;). All of these can potentially be competive with each other, with rates very roughly on the order of 10<sup>9</sup> s<sup>-1</sup>. For more complex RO<sub>2</sub> parents, rapid unimolecular reactions of the daughter RO (such as alkoxy scissions) open up even more potential reaction channels, for example direct alkoxy &#8211; alkyl recombination to form (either singlet or triplet) ether-type (ROR&#8217;) dimers.</p><p>[1] Iyer, S., Rissanen, M. P. and Kurt&#233;n, T. Reaction Between Peroxy and Alkoxy Radicals can Form Stable Adducts. Journal of Physical Chemistry Letters, Vol. 10, 2051-2057, 2019.</p><p>[2] Valiev, R., Hasan, G., Salo, V.-T., Kube&#269;ka, J. and Kurt&#233;n, T. Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. Journal of Physical Chemistry A, Vol. 123, 6596-6604, 2019.</p><p>&#160;</p>
The physical chemistry of Criegee intermediates in the gas phase
