Computational studies of gas-phase accretion product formation involving RO2
Authors: Kurtén Theo , Iyer Siddharth , Salo Vili-Taneli , Hasan Galib , Rissanen Matti , Valiev Rashid 
Published: March 2020

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