Search

In this paper we study collisions between polyatomic radicals - an important process in fields ranging from biology to combustion. Energy transfer, formation of intermediate complexes and recombination reactions are treated, with applications to peroxy radicals in atmospheric chemistry. Multi-reference perturbation theory, supplemented by coupled-cluster calculations, describes the potential energy surfaces with high accuracy, including the interaction of singlet and triplet spin states during radical recombination. Our multi-reference molecular dynamics (MD) trajectories on methyl peroxy radicals confirm the reaction mechanism postulated in earlier studies. Specifically, they show that if suitable pre-reactive complexes are formed, they will rapidly lead to the formation and subsequent decomposition of tetroxide intermediates. However, generating multi-reference MD trajectories is exceedingly computationally demanding, and we cannot adequately sample the whole conformational space. To answer this challenge, we promote the use of a novel simplified semi-empirical MD methodology. It assumes the collision is governed by two states, a singlet (S-0) and a triplet (T-1) state. The method predicts differences between collisions on S-0 and T-1 surfaces, and qualitatively includes not only pre-reactive complex formation, but also recombination processes such as tetroxide formation. Finally, classical MD simulations using force-fields for non-reactive collisions are employed to generate thousands of collision trajectories, to verify that the semi-empirical method is sampling collisions adequately, and to carry out preliminary investigations of larger systems. For systems with low activation energies, the experimental rate coefficient is surprisingly well reproduced by simply multiplying the gas-kinetic collision rate by the simulated probability for long-lived complex formation.

The lifetimes of pre-reactive complexes, although implicitly part of the equations used to model many gas-phase bimolecular reactions, have seldom been included in quantitative calculations of rate coefficients. Here, we demonstrate the application of empirical molecular dynamics simulations of collisions between peroxy radicals to model association lifetimes. With the exception of the methyl peroxy−acetyl peroxy system, measurements of the lifetimes based on a phenomenological model are shown to correlate well with available experimental data for recombination reactions of peroxy radicals in cases where the rate-limiting transition state lies below the reactants in energy. Further, we predict reaction rates for larger α-pinene-derived peroxy radicals, and we interpret our results in tandem with available experimental data on these systems, which are of great relevance to improve our understanding of atmospheric aerosol formation.

The recombination ("dimerization") of peroxyl radicals (RO2 center dot) 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 RO2 center dot + R'O-2 center dot reactions are still unknown. In this work, we have studied the formation of tetroxide intermediates (RO4R'): their formation from two peroxyl radicals and their decomposition to triplet molecular oxygen (O-3(2)) and a triplet pair of alkoxyl radicals (RO center dot). 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 (RO2) is a key 5 3 step in the gas-phase generation of low-volatility vapors, leading to atmospheric aerosol particles. We have recently demonstrated that S 1 this recombination reaction very likely proceeds via an intermediate complex of two alkoxy radicals (RO center dot center dot center dot 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 center dot center dot center dot OR' systems actually relevant to atmospheric aerosol formation. Here, we carry out systematic conformational sampling and ISC rate calculations on (3)(RO center dot center dot center dot OR') clusters formed in the recombination reactions of different diastereomers of the first-generation peroxyl radicals originating in both OH- and NO3 -initiated reactions of alpha-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 alpha-pinene-derived RO2 and likely explains the experimentally observed gas-phase formation of C-20 compounds in alpha-pinene oxidation.

The formation of accretion products (“dimers”) from recombination reactions of peroxyl radicals (RO2) 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 NO3-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 106 s–1, and many have rates exceeding 1010 s–1. Especially the latter value demonstrates that accretion product formation via the suggested pathway is a competitive process also for α-pinene-derived RO2 and likely explains the experimentally observed gas-phase formation of C20 compounds in α-pinene oxidation.

Organic peroxy radicals (RO

Field and laboratory studies have indirectly but conclusively established that reactions involving peroxy radicals (RO2) play a key role in the gas-phase formation of accretion products, also commonly referred to as “dimers”, as they typically contain roughly twice the number of carbon atoms compared to their hydrocarbon precursors. Using computational tools, we have recently presented two different potential mechanisms for this process.First, direct and rapid recombination of peroxy and alkoxy (RO) radicals, analogous to the recently characterized RO2 + OH reaction, leads to the formation of metastable RO3R’ trioxides, which may have lifetimes on the order of a hundred seconds. [1] However, due to both the limited lifetime of the trioxides, and the low concentration of alkoxy radicals, the RO2 + R’O pathway is likely to be a minor, though not necessarily negligible, pathway for atmospheric dimer formation.Second, we have shown that recombination of two peroxy radicals – phenomenologically known to be responsible for the formation of ROOR’ – type dimers – very likely occurs through a multi-step mechanism involving an intersystem crossing (ISC). [2] In contrast to earlier predictions, we find that the rate-limiting step for the overall RO2 + R’O2 reaction is the initial formation of a short-lived RO4R’ tetroxide intermediate. For tertiary RO2, the barrier for the tetroxide formation can be substantial. However, for all studied species the tetroxide decomposition is rapid, forming ground-state triplet O2, and a weakly bound triplet complex of two alkoxy radicals. The branching ratios of the different RO2 + R’O2 reaction channels are then determined by a three-way competition of this complex. For simple systems, the possible channels are dissociation (leading to RO + R’O), H-abstraction on the triplet surface (leading to RC=O + R’OH), and ISC and subsequent recombination on the singlet surface (leading to ROOR’). All of these can potentially be competive with each other, with rates very roughly on the order of 109 s-1. For more complex RO2 parents, rapid unimolecular reactions of the daughter RO (such as alkoxy scissions) open up even more potential reaction channels, for example direct alkoxy – alkyl recombination to form (either singlet or triplet) ether-type (ROR’) dimers.[1] Iyer, S., Rissanen, M. P. and Kurtén, T. Reaction Between Peroxy and Alkoxy Radicals can Form Stable Adducts. Journal of Physical Chemistry Letters, Vol. 10, 2051-2057, 2019.[2] Valiev, R., Hasan, G., Salo, V.-T., Kubečka, J. and Kurtén, T. Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. Journal of Physical Chemistry A, Vol. 123, 6596-6604, 2019.

Ions enhance the formation rate of atmospheric aerosol particles, which play an important role in Earth’s radiative balance. Ion-induced nucleation involves the stepwise accretion of neutral monomers onto a molecular cluster containing an ion, which helps to stabilize the cluster against evaporation. Although theoretical frameworks exist to calculate the collision rate coefficients between neutral molecules and ions, they need to be experimentally confirmed, ideally under atmospherically relevant conditions of around 1000 ion pairs cm−3. Here, in experiments performed under atmospheric conditions in the CERN CLOUD chamber, we have measured the collision rate coefficients between neutral iodic acid (HIO3) monomers and charged iodic acid molecular clusters containing up to 11 iodine atoms. Three methods were analytically derived to calculate ion-polar molecule collision rate coefficients. After evaluation with a kinetic model, the 50% appearance time method is found to be the most robust. The measured collision rate coefficient, averaged over all iodine clusters, is (2.4 ± 0.8)×10−9 cm3 s−1, which is close to the expectation from the surface charge capture theory.

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 ROOR'. 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 decomposition of the CH3O4CH3 tetroxide intermediate are far below the reactants in energy. Next, we compute ISC rates for seven different atmospherically relevant (3)(RO center dot center dot center dot R'O) complexes. The ISC rates vary significantly depending on the conformation of the complex and also exhibit strong stereoselectivity. Furthermore, the fastest ISC process is usually not between the lowest-energy triplet and singlet states but between the triplet ground state and an exited singlet state. For each studied (RO center dot center dot center dot R'O) system, at least one low-energy conformer with an ISC rate above 10(8) s(-1) can be found. This demonstrates that gas-phase dimer formation in the atmosphere very likely involves ISCs originating in relativistic quantum mechanics.

We present measurements utilizing the Filter Inlet for Gases and Aerosols (FIGAERO) applied to chamber measurements of isoprene-derived epoxydiol (IEPOX) reactive uptake to aqueous acidic particles and associated SOA formation. Similar to recent field observations with the same instrument, we detect two molecular components desorbing from the IEPOX SOA in high abundance: C5H12O4 and C5H10O3. The thermal desorption signal of the former, presumably 2-methyltetrols, exhibits two distinct maxima, suggesting it arises from at least two different SOA components with significantly different effective volatilities. Isothermal evaporation experiments illustrate that the most abundant component giving rise to C5H12O4 is semi-volatile, undergoing nearly complete evaporation within 1 hour, while the second, less volatile, component remains unperturbed and even increases in abundance. We thus confirm, using controlled laboratory studies, recent analyses of ambient SOA measurements showing that IEPOX SOA is of very low volatility and commonly measured IEPOX SOA tracers, such as 2-methyltetrols and C5-alkene triols, result predominantly from artifacts of measurement techniques associated with thermal decomposition and/or hydrolysis. We further show that IEPOX SOA volatility continues to evolve via acidity enhanced accretion chemistry on the timescale of hours, potentially involving both 2-methyltetrols and organosulfates.

The multicomponent assembly of pharmaceutically relevant N-aryl-oxazolidinones through the direct insertion of carbon dioxide into readily available anilines and dibromoalkanes is described. The addition of catalytic amounts of an organosuperbase such as Barton's base enables this transformation to proceed with high yields and exquisite substrate functional-group tolerance under ambient CO2 pressure and mild temperature. This report also provides the first proof-of-principle for the single-operation synthesis of elusive seven-membered ring cyclic urethanes.