Structure-activity relationships for unimolecular reactions of peroxy radicals, RO2, at atmospheric temperatures

Abstract

<p>The oxidation of most organic matter emitted to the atmosphere proceeds by radical reaction steps, where peroxy radicals, ROO<sup>•</sup>, are critical intermediates formed by addition of O<sub>2</sub> molecules to carbon-based radicals. The chemistry of these RO<sub>2</sub> radicals in high-NOx conditions is well-known, forming alkoxy radicals and NO<sub>2</sub>. In low-NOx and pristine conditions, the RO<sub>2</sub> radicals react with HO<sub>2</sub> and other R'O<sub>2</sub> radicals, but can have a sufficiently long lifetime to also undergo unimolecular reactions. Hydrogen atom migration, forming a hydroperoxide (-OOH) and a new peroxy radical site after addition of an additional O<sub>2</sub> on the newly formed radical site, has been studied extensively in some compounds, such as isoprene where it was shown to be the a critical step in OH radical regeneration. RO<sub>2</sub> ring closure reactions have likewise been studied, where for β-pinene it has been shown to be a critical step governing the yield of the decomposition products such as acetone and nopinone.</p><p>Despite the interest in RO<sub>2</sub> unimolecular reactions, and the potential impact on atmospheric chemistry, no widely applicable structure-activity relationships (SARs) have been proposed to allow systematic incorporation of such unimolecular reactions in gas phase atmospheric kinetic models. In this work, we present a series of systematic theoretical predictions on the site-specific rate coefficients for such reactions for a wide range of molecular substitutions. Combined with extensive literature data this allows for the formulation of a SAR for RO<sub>2</sub> unimolecular reactions, covering aliphatic, branched, and unsaturated RO<sub>2</sub> with oxo, hydroxy, hydroperoxy, nitrate, carboxylic acid, and ether substitutions.</p><p>The predictions are compared to experimental and theoretical data, including multi-functionalized species. Though some molecular classes are well represented in the training set (e.g. aliphatic RO<sub>2</sub>), other classes have little data available and additional work is needed to enhance and validate the reliability of the SAR. Direct experimental data is scarce for all RO<sub>2</sub> classes. The fastest H-migrations are found to be for unsaturated RO<sub>2</sub>, with the double bond outside the H-migration TS ring. Ring closure of unsaturated RO<sub>2</sub> are likewise fast if the product radical carbon is exocyclic to the newly formed peroxide ring.</p>