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

The oxidation of most organic matter emitted to the atmosphere proceeds by radical reaction steps, where peroxy radicals, ROO^•, are critical intermediates formed by addition of O₂ molecules to carbon-based radicals. The chemistry of these RO₂ radicals in high-NOx conditions is well-known, forming alkoxy radicals and NO₂. In low-NOx and pristine conditions, the RO₂ radicals react with HO₂ and other R'O₂ 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 O₂ 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. RO₂ 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.

Despite the interest in RO₂ 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 RO₂ unimolecular reactions, covering aliphatic, branched, and unsaturated RO₂ with oxo, hydroxy, hydroperoxy, nitrate, carboxylic acid, and ether substitutions.

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 RO₂), 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 RO₂ classes. The fastest H-migrations are found to be for unsaturated RO₂, with the double bond outside the H-migration TS ring. Ring closure of unsaturated RO₂ are likewise fast if the product radical carbon is exocyclic to the newly formed peroxide ring.