Comparing Reaction Routes for ³(RO···OR′) Intermediates Formed in Peroxy Radical Self- and Cross-Reactions

Abstract

Organic peroxy radicals (RO 2 ) are key intermediates in the chemistry of the atmosphere. One of the main sink reactions of RO 2 is the recombination reaction RO 2 + R'O 2 , which has three main channels (all with O 2 as a co-product): 1) R -H =O + R'OH, 2) RO + R'O, and 3) ROOR'. The RO + R'O "alkoxy" channel promotes radical and oxidant recycling, while the ROOR' "dimer" channel leads to low-volatility products relevant to aerosol processes. The ROOR' channel has only recently been discovered to play a role in the gas phase. Recent computational studies indicate that all these channels first go through an intermediate complex 1 (RO… 3 O 2 …OR'). Here, 3 O 2 is very weakly bound, and will likely evaporate from the system, giving a triplet cluster of two alkoxy radicals: 3 (RO… OR'). In this study, we systematically investigate the three reaction channels for an atmospherically representative set of RO + R'O radicals formed in the corresponding RO 2 + R'O 2 reaction. First, we systematically sample the possible conformations of the RO…OR' clusters on the triplet potential energy surface. Next, we compute energetic parameters, and attempt to estimate reaction rate coefficients for the three channels: evaporation/dissociation to RO + R'O, a hydrogen shift leading to the formation of R' -H =O + ROH, and "spin-flip" (intersystem crossing) leading to, or at least allowing, the formation of ROOR' dimers. While large uncertainties in the computed energetics prevents a quantitative comparison of reaction rates, all three channels were found to be very fast (with typical rates greater than 10 6 s -1 ). This qualitatively demonstrates that the computationally proposed novel RO 2 + R'O 2 reaction mechanism is compatible with experimental data showing non-negligible branching ratios for all three channels, at least for sufficiently complex RO 2 .

Computational Investigation of the Formation of Peroxide (ROOR) Accretion Products in the OH- and NO3-Initiated Oxidation of α-Pinene

“…(Data for local minima are given in Section S2 of the Supporting Information.) In our previous work comparing a series of relatively simple functionalized RO, 9 , 10 we observed that the ISC rates strongly depended on the conformation of the 3 (RO···OR′) clusters and displayed extreme stereoselectivity for the two systems included in the study that possessed stereocenters: R,S -BuOH-O···O-BuOH and R,S -PrNO 3 -O···O-PrNO 3 . The present systems also display substantial stereoselectivity: the ISC rates for the global minimum conformers of different stereoisomer pairs of hydroxy-alkoxy and nitroxy-alkoxy triplet clusters vary by almost three and over four orders of magnitude, respectively.…”

Anthropogenic Volatile Organic Compound (AVOC) Autoxidation as a Source of Highly Oxygenated Organic Molecules (HOM)

“…RO undergo much more rapid H-shift reactions than RO 2 , and their more energetic nature makes for more varied branching pathways . Alkoxy radicals are also prone to decompose, and their special role in the breaking of carbon rings that hinder HOM formation has been noted previously. , Especially in the naked, straight-chain, unbranched alkanes the role of RO is heightened, as the parent compounds do not have feasible RO 2 hydrogen-shift reactions available. , Alkoxy radicals are even indirectly related to the molecular mechanism of peroxide “dimer” formation by RO 2 + RO 2 reactions, in which two ROs will stay entangled before the O 2 breaks apart and the peroxide −C-O-O-C– bridge is formed . Very recently alkoxy radicals were also shown to undergo bimolecular reactions even with another (stabile) radical species such as RO 2 , which highlights the poor understanding we have of their importance for the atmospheric particulate matter formation and on atmospheric chemistry in general.…”

“… 33 , 36 Alkoxy radicals are even indirectly related to the molecular mechanism of peroxide “dimer” formation by RO 2 + RO 2 reactions, in which two ROs will stay entangled before the O 2 breaks apart and the peroxide −C-O-O-C– bridge is formed. 72 Very recently alkoxy radicals were also shown to undergo bimolecular reactions even with another (stabile) radical species such as RO 2 , 84 which highlights the poor understanding we have of their importance for the atmospheric particulate matter formation and on atmospheric chemistry in general.…”

“…A coupled technical hurdle comes from the fact that several of the key reactions require state-of-the-art computational tools, which consume a vast amount of computational resources. For example, recently the decades old mystery of gas-phase “dimerization” of peroxy radicals into oxygen-bridged peroxides, − which have special importance in growing atmospheric SOA, ,, was resolved only by an application of far more sophisticated computational methodology (i.e., multireference computations with intersystem crossing rates having their origin in relativistic quantum mechanics). Thus, parts of the potential energy surfaces (PES) must be described very accurately, which consumes a lot of effort and, consequently, time.…”

Gas-to-Particle Partitioning of Cyclohexene- and α-Pinene-Derived Highly Oxygenated Dimers Evaluated Using COSMOtherm

“…This mechanism has previously been thought to occur solely in the condensed phase 25 but has recently been demonstrated to be feasible also in the gas phase. 26 − 28 Another possible formation pathway of dimers is the reaction between two closed-shell reactants, where either a hydroperoxide (ROOH) or an alcohol (ROH) reacts with an aldehyde (R=O) to form a peroxyhemiacetal or hemiacetal, respectively 29 …”