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Reactions of organic peroxy radicals, RO2, with substituted and biogenic alkenes at room temperature: unsuspected sinks for some RO2 in the atmosphere?† † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc02263f

ABSTRACT: Until now the reactions of organic peroxy radicals (RO2) with alkenes in the gas phase have been essentially studied at high temperature (T ≥ 360 K) and in the context of combustion processes, while considered negligible in the Earth's atmosphere. In this work, the reactions of methyl-, 1-pentyl- and acetylperoxy radicals (CH3O2, C5H11O2, and CH3C(O)O2, respectively) with 2-methyl-2-butene, 2,3-dimethyl-2-butene and for the first time the atmospherically relevant isoprene, α-pinene, and limonene were studied at room temperature (298 ± 5 K). Monitoring directly the radicals with chemical ionization mass spectrometry led to rate coefficients larger than expected from previous combustion studies but following similar trends in terms of alkenes, with (in molecule−1 cm3 s−1) = 10−18 to 10−17 × 2/2 and = 10−14 to 10−13 × 5/5. While these reactions would be negligible for CH3O2 and aliphatic RO2 at room temperature, this might not be the case for acyl-, and perhaps hydroxy-, allyl- and other substituted RO2. Combining our results with the Structure–Activity Relationship (SAR) predicts kII(298 K) ∼10−14 molecule−1 cm3 s−1 for hydroxy- and allyl-RO2 from isoprene oxidation, potentially accounting for up to 14% of their sinks in biogenic-rich regions of the atmosphere and much more in laboratory studies. The reactions of organic peroxy radicals with alkenes, overlooked until now, could be more significant than expected for some RO2 in the atmosphere.

SUBMITTER: Noziere B

PROVIDER: S-EPMC8442694 | biostudies-literature | 2021 Jul

REPOSITORIES: biostudies-literature

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Project description:A computational protocol is employed to glean new insight into the kinetics of several 1,5-hydrogen atom (H) shift reactions subsequent to first- and second-generation OH/O2 additions to isoprene. The M06-2X density functional was initially used with the Nudged Elastic Band (NEB) method to determine the potential energy surface of OH/O2 addition reactions, the 1,5-H shift reactions, and the fragmentation exit channels. The Master Equation Solver for Multi-Energy Well Reactions (MESMER) was applied to determine the rate constants for OH addition and the 1,5-H shifts. M06-2X was capable of quantifying the rate constants of OH addition to the first and second double bonds of isoprene with deviations less than 17% from the experimentally determined values. However, M06-2X underestimated the 1,5-H shift rate constants of second-generation isoprene peroxy radicals. Consequently, MN15, ωB97X-D, and CBS-QB3 methods were employed to compute average barrier heights to first- and second-generation 1,5-H shifts. In the first generation, the rate constants of H abstraction by β-(1,2) and (4,3) isoprene hydroxy-peroxy radicals from the neighboring hydroxyl group are 1.1 × 10-3 and 2.4 × 10-3 s-1, respectively. These values are determined primarily by the barrier of the H shift reaction and, to a smaller albeit nonnegligible extent, by the stability of the resulting alkoxy radical and the exit barrier leading to C-C bond dissociation. In contrast, the average second-generation rate constant of 1,5-H shifts from H-R-OH sites to the peroxy radical is 1.8 × 10-1 s-1, with tunneling playing the significant role of increasing this value relative to first-generation 1,5-H shifts. Under low NO x conditions, first-generation isoprene oxidation reactions may recycle HO x at levels ranging from 10 to 30% due in large part to 1,5-H shifts, with the recycling efficiency being sensitive to HO2 concentrations and temperature. HO x recycling is expected to increase to levels beyond 80% in second-generation reactions of oxidized isoprene species because of isoprene epoxydiol (IEPOX) formation and further 1,5-H shifts that are kinetically favorable.

Dataset Information

Reactions of organic peroxy radicals, RO2, with substituted and biogenic alkenes at room temperature: unsuspected sinks for some RO2 in the atmosphere?† † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc02263f

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