Uncovering the Fundamental Chemistry of Alkyl + O₂ Reactions via Measurements of Product Formation

Abstract

The reactions of alkyl radicals (R) with molecular oxygen (O(2)) are critical components in chemical models of tropospheric chemistry, hydrocarbon flames, and autoignition phenomena. The fundamental kinetics of the R + O(2) reactions is governed by a rich interplay of elementary physical chemistry processes. At low temperatures and moderate pressures, the reactions form stabilized alkylperoxy radicals (RO(2)), which are key chain carriers in the atmospheric oxidation of hydrocarbons. At higher temperatures, thermal dissociation of the alkylperoxy radicals becomes more rapid and the formation of hydroperoxyl radicals (HO(2)) and the conjugate alkenes begins to dominate the reaction. Internal isomerization of the RO(2) radicals to produce hydroperoxyalkyl radicals, often denoted by QOOH, leads to the production of OH and cyclic ether products. More crucially for combustion chemistry, reactions of the ephemeral QOOH species are also thought to be the key to chain branching in autoignition chemistry. Over the past decade, the understanding of these important reactions has changed greatly. A recognition, arising from classical kinetics experiments but firmly established by recent high-level theoretical studies, that HO(2) elimination occurs directly from an alkylperoxy radical without intervening isomerization has helped resolve tenacious controversies regarding HO(2) formation in these reactions. Second, the importance of including formally direct chemical activation pathways, especially for the formation of products but also for the formation of the QOOH species, in kinetic modeling of R + O(2) chemistry has been demonstrated. In addition, it appears that the crucial rate coefficient for the isomerization of RO(2) radicals to QOOH may be significantly larger than previously thought. These reinterpretations of this class of reactions have been supported by comparison of detailed theoretical calculations to new experimental results that monitor the formation of products of hydrocarbon radical oxidation following a pulsed-photolytic initiation. In this article, these recent experiments are discussed and their contributions to improving general models of alkyl + O(2) reactions are highlighted. Finally, several prospects are discussed for extending the experimental investigations to the pivotal questions of QOOH radical chemistry.

Atmospheric autoxidation is increasingly important in urban and suburban North America

“…We use measurements of the organonitrates (RONO 2 ), produced as minor products in the RO 2 + NO channel, to probe the bimolecular chemistry. Following oxidation, the RONO 2 isomers were separated by gas chromatography and detected with chemical ionization mass spectrometry (GC-CIMS) using CF 3 O − (m/z = 85) as a reagent ion (Fig. 1).…”

“…It is well established that reactions are initiated by oxidants including OH, NO 3 , and O 3 , and that, in the presence of oxygen, organic peroxy radicals (RO 2 ) are usually formed (1). The subsequent chemistry of RO 2 is diverse and depends on the chemical state of the atmosphere.…”

“…The critical reaction in autoxidation, which generally governs the overall kinetics, is an intramolecular hydrogen shift to the RO 2 . This reaction produces hydroperoxyalkyl radicals (often denoted QOOH), which are known intermediates in autoignition (2,3). QOOH have only recently been observed (4).…”

Detailed chemical kinetic reaction mechanisms for primary reference fuels for diesel cetane number and spark-ignition octane number

“…Examples of improvements that will be included in the future include recent work of Taatjes et al [23] on alternative reaction pathways for reactions of alkyl radicals and O 2 to directly produce olefins via a concerted molecular elimination pathway will be implemented, and site-specific reactions of olefins can be improved following recent studies of Mehl et al [24] and VanHove et al [25].…”

Communication: Theoretical prediction of the structure and spectroscopic properties of the $\tilde{\mathrm{X}}$X̃ and $\tilde{\mathrm{A}}$Ã states of hydroxymethyl peroxy (HOCH2OO) radical

“…[1][2][3][4][5][6] The photochemistry and oxidation of these species are resulting in the production of α-hydroxyalkyl peroxy radicals (see Fig. 1).…”