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

Abstract

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.

Computed Pre-reactive Complex Association Lifetimes Explain Trends in Experimental Reaction Rates for Peroxy Radical Recombinations

“…Recent experiments have measured a surprisingly high reactivity for this cross-reaction ( k = 200 × 10 –13 cm –3 molecule –1 s –1 ), even higher than the reactivity of the AcOO self-reaction ( k = 130 × 10 –13 cm –3 molecule –1 s –1 ) Our modeling would instead be consistent with a value of k for the cross-reaction in between the values for MeOO and AcOO self-reactions. This intuitive prediction is also consistent with our recent calculations of the energetics in these systems . We also note that a semi-empirical calculation of k for the MeOO + AcOO cross-reaction arrived at a value of 70 × 10 –13 cm –3 molecule –1 s –1 , which would fall in line with our model.…”

“…On the other hand, acetonyl peroxy collisions had a significantly higher chance to remain associated for a longer time. We noted that this was consistent with the fact that the self-reaction rate of acetonyl peroxy radicals has been experimentally found to be ∼10–20× higher than that of methyl peroxy radicals. ,, On the basis of both computations and experimentally determined effective activation energies, acetonyl peroxy, like methyl peroxy, is found to have a negligible energy barrier, while t -butyl peroxy has a barrier of several kcal/mol.…”

“…In recent work, we and others have used computational methods to study the energetics − of peroxy radical self- and cross-reactions. Very recently, we also extended our work to include dynamics .…”

Theoretical Investigations of the OH-Initialized Oxidation of 4-Methyl-3-Penten-2-One in the Atmosphere

“…Similar to other ROO • radicals, the self-reaction of CH 3 OO • also has three main channels (see Scheme ) and several studies have been done using different quantum chemical methods. ,− For the reaction pathway of producing 2CH 3 O • + O 2 , previous studies found that it can occur through a nonconcerted mechanism involving two tetraoxide intermediates, the dissociation of a weakly bound complex 3 (CH 3 O • ) 2 ··· 3 O 2 with an overall singlet multiplicity , (i.e., PC in Scheme ), or a barrierless concerted dissociation from tetraoxide intermediate . Then, the production of 2CH 3 O • + O 2 via either a concerted or nonconcerted process can occur easily.…”

“…an overall singlet multiplicity 58,59 (i.e., PC in Scheme 2), or a barrierless concerted dissociation from tetraoxide intermediate. 52 Then, the production of 2CH 3 O • + O 2 via either a concerted or nonconcerted process can occur easily.…”