High-molecular weight "ROOR'" dimers, likely formed in the gas phase through self-and cross-reactions of complex peroxy radicals (RO2), have been suggested to play a key role in forming ultrafine aerosol particles in the atmosphere. However, the molecular-level reaction mechanism producing these dimers remains unknown. Using multireference quantum chemical methods, we explore one potentially competitive pathway for ROOR' production, involving the initial formation of triplet alkoxy radical (RO) pairs, followed by extremely rapid intersystem crossings (ISC) to the singlet surface, permitting subsequent recombination to ROO R'. Using CH3OO + CH3OO as a model system, we show that the initial steps of this reaction mechanism are likely to be very fast, as the transition states for both the formation and the

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 .

The recombination (“dimerization”) of peroxyl radicals (RO 2 •) is one of the pathways suggested in the literature for the formation of peroxides (ROOR′, often referred to as dimers or accretion products in the literature) in the atmosphere. It is generally accepted that these dimers play a major role in the first steps of the formation of submicron aerosol particles. However, the precise reaction pathways and energetics of RO 2 • + R′O 2 • reactions are still unknown. In this work, we have studied the formation of tetroxide intermediates (RO 4 R′): their formation from two peroxyl radicals and their decomposition to triplet molecular oxygen ( 3 O 2 ) and a triplet pair of alkoxyl radicals (RO•). We demonstrate this mechanism for several atmospherically relevant primary and secondary peroxyl radicals. The potential energy surface corresponds to an overall singlet state. The subsequent reaction channels of the alkoxyl radicals include, but are not limited to, their dimerization into ROOR′. Our work considers the multiconfigurational character of the tetroxides and the intermediate phases of the reaction, leading to reliable mechanistic insights for the formation and decomposition of the tetroxides. Despite substantial uncertainties in the computed energetics, our results demonstrate that the barrier heights along the reaction path are invariably small for these systems. This suggests that the reaction mechanism, previously validated at a multireference level only for methyl peroxyl radicals, is a plausible pathway for the formation of aerosol-relevant larger peroxides in the atmosphere.

The formation of accretion products (“dimers”) from recombination reactions of peroxyl radicals (RO 2 ) is a key step in the gas-phase generation of low-volatility vapors, leading to atmospheric aerosol particles. We have recently demonstrated that this recombination reaction very likely proceeds via an intermediate complex of two alkoxy radicals (RO···OR′) and that the accretion product pathway involves an intersystem crossing (ISC) of this complex from the triplet to the singlet surface. However, ISC rates have hitherto not been computed for large and chemically complex RO···OR′ systems actually relevant to atmospheric aerosol formation. Here, we carry out systematic conformational sampling and ISC rate calculations on 3 (RO···OR′) clusters formed in the recombination reactions of different diastereomers of the first-generation peroxyl radicals originating in both OH- and NO 3 -initiated reactions of α-pinene, a key biogenic hydrocarbon for atmospheric aerosol formation. While we find large differences between the ISC rates of different diastereomer pairs, all systems have ISC rates of at least 10 6 s –1 , and many have rates exceeding 10 10 s –1 . Especially the latter value demonstrates that accretion product formation via the suggested pathway is a competitive process also for α-pinene-derived RO 2 and likely explains the experimentally observed gas-phase formation of C 20 compounds in α-pinene oxidation.

Collisions between methyl peroxy radicals do not react in the triplet excited state, and lead to tetroxide formation in the singlet ground state.