Multireference and Coupled-Cluster Study of Dimethyltetroxide (MeO4Me) Formation and Decomposition.

Peroxyl radicals (RO2) are important intermediates in the atmospheric oxidation processes. The RO2 can react with other RO2 to form reactive intermediates known as tetroxides, RO4R. The reaction mechanisms of RO4R formation and its various decomposition channels have been investigated in multiple computational studies, but previous approaches have not been able to provide a unified methodology that is able to connect all relevant reactions together. An apparent difficulty in modeling the RO4R formation and its decomposition is the involvement of open-shell singlet electronic states along the reaction pathway. Modeling such electronic states requires multireference (MR) methods, which we use in the present study to investigate in detail a model reaction of MeO2 + MeO2 → MeO4Me, and its decomposition, MeO4Me → MeO + O2 + MeO, as well as the two-body product complexes MeO···O2 + MeO and MeO···MeO + O2. The used MR methods are benchmarked against relative energies of MeO2 + MeO2, MeO4Me, and MeO + MeO + O2, calculated with CCSD(T)/CBS, W2X, and W3X-L methods. We found that the calculated relative energies of the overall MeO2 + MeO2 → MeO4Me → MeO + O2 + MeO reaction are very sensitive to the chosen MR method and that the CASPT2(22e,14o)-IPEA method is able to reproduce the relative energies obtained by the various coupled-cluster methods. Furthermore, CASPT2(22e,14o)-IPEA and W3X-L results show that the MeO···O2 product complex + MeO is lower in energy than the MeO···MeO complex + O2. The formation of MeO4Me is exothermic, and its decomposition is endothermic, relative to the tetroxide. Both the formation and decomposition reactions appear to follow pathways with no saddle points. According to potential energy surface scans of the decomposition pathway, the concerted cleavage of both MeO···O bonds in MeO4Me is energetically preferred over the corresponding sequential decomposition.

Computational Investigation of Substituent Effects on the Alcohol + Carbonyl Channel of Peroxy Radical Self- and Cross-Reactions.

Organic peroxy radicals (RO2) are key intermediates in atmospheric chemistry and can undergo a large variety of both uni- and bimolecular reactions. One of the least understood reaction classes of RO2 are their self- and cross-reactions: RO2 + R'O2. In our previous work, we have investigated how RO2 + R'O2 reactions can lead to the formation of ROOR' accretion products through intersystem crossings and subsequent recombination of a triplet intermediate complex 3(RO···OR'). Accretion products can potentially have very low saturation vapor pressures, and may therefore participate in the formation of aerosol particles. In this work, we investigate the competing H-shift channel, which leads to the formation of more volatile carbonyl and alcohol products. This is one of the main, and sometimes the dominant, RO2 + R'O2 reaction channels for small RO2. We investigate how substituents (R and R' groups) affect the H-shift barriers and rates for a set of 3(RO···OR') complexes. The variation in barrier heights and rates is found to be surprisingly small, and most computed H-shift rates are fast: around 108-109 s-1. We find that the barrier height is affected by three competing factors: (1) the weakening of the breaking C-H bond due to interactions with adjacent functional groups; (2) the overall binding energy of the 3(RO···OR'), which tends to increase the barrier height; and (3) the thermodynamic stability of the reaction products. We also calculated intersystem crossing rate coefficients (ISC) for the same systems and found that most of them were of the same order of magnitude as the H-shift rates. This suggests that both studied channels are competitive for small and medium-sized RO2. However, for complex enough R or R' groups, the binding energy effect may render the H-shift channel uncompetitive with intersystem crossings (and thus ROOR' formation), as the rate of the latter, while variable, seems to be largely independent of system size. This may help explain the experimental observation that accretion product formation becomes highly effective for large and multifunctional RO2.

Gas-Phase Peroxyl Radical Recombination Reactions: A Computational Study of Formation and Decomposition of Tetroxides.

The recombination ("dimerization") of peroxyl radicals (RO2•) 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• + R'O2• 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 (3O2) 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.

Energy transfer, pre-reactive complex formation and recombination reactions during the collision of peroxy radicals.

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 (S0) and a triplet (T1) state. The method predicts differences between collisions on S0 and T1 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.

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

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.

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

Organic peroxy radicals (RO2) are key intermediates in the chemistry of the atmosphere. One of the main sink reactions of RO2 is the recombination reaction RO2 + R'O2, which has three main channels (all with O2 as a coproduct): (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 of these channels first go through an intermediate complex 1(RO···3O2···OR'). Here, 3O2 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 RO2 + R'O2 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 prevent a quantitative comparison of reaction rates, all three channels were found to be very fast (with typical rates greater than 106 s-1). This qualitatively demonstrates that the computationally proposed novel RO2 + R'O2 reaction mechanism is compatible with experimental data showing non-negligible branching ratios for all three channels, at least for sufficiently complex RO2.

Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation.

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···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···R'O) system, at least one low-energy conformer with an ISC rate above 108 s-1 can be found. This demonstrates that gas-phase dimer formation in the atmosphere very likely involves ISCs originating in relativistic quantum mechanics.

A One-Pot Synthesis of N-Aryl-2-Oxazolidinones and Cyclic Urethanes by the Lewis Base Catalyzed Fixation of Carbon Dioxide into Anilines and Bromoalkanes.

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.