Tropospheric oxidation of methyl hydrotrioxide (CH3OOOH) by hydroxyl radical

Methanetriol─Formation of an Impossible Molecule

Orthocarboxylic acids─organic molecules carrying three hydroxyl groups at the same carbon atom─have been distinguished as vital reactive intermediates by the atmospheric science and physical (organic) chemistry communities as transients in the atmospheric aerosol cycle. Predicted short lifetimes and their tendency to dehydrate to a carboxylic acid, free orthocarboxylic acids, signify one of the most elusive classes of organic reactive intermediates, with even the simplest representative methanetriol (CH(OH)3)─historically known as orthoformic acid─not previously been detected experimentally. Here, we report the first synthesis of the previously elusive methanetriol molecule in low-temperature mixed methanol (CH3OH) and molecular oxygen (O2) ices subjected to energetic irradiation. Supported by electronic structure calculations, methanetriol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies and the detection of photoionization fragments. The first synthesis and detection of methanetriol (CH(OH)3) reveals its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition. These findings progress our fundamental understanding of the chemistry and chemical bonding of methanetriol, hydroxyperoxymethane (CH3OOOH), and hydroxyperoxymethanol (CH2(OH)OOH), which are all prototype molecules in the oxidation chemistry of the atmosphere.

Hydrotrioxide (ROOOH) formation in the atmosphere

Organic hydrotrioxides (ROOOH) are known to be strong oxidants used in organic synthesis. Previously, it has been speculated that they are formed in the atmosphere through the gas-phase reaction of organic peroxy radicals (RO₂) with hydroxyl radicals (OH). Here, we report direct observation of ROOOH formation from several atmospherically relevant RO₂ radicals. Kinetic analysis confirmed rapid RO₂ + OH reactions forming ROOOH, with rate coefficients close to the collision limit. For the OH-initiated degradation of isoprene, global modeling predicts molar hydrotrioxide formation yields of up to 1%, which represents an annual ROOOH formation of about 10 million metric tons. The atmospheric lifetime of ROOOH is estimated to be minutes to hours. Hydrotrioxides represent a previously omitted substance class in the atmosphere, the impact of which needs to be examined.

Radical chemistry in oxidation flow reactors for atmospheric chemistry research

We summarize the studies on the chemistry in oxidation flow reactor and discuss its atmospheric relevance.

Impact of water on the BrO + HO2 gas-phase reaction: mechanism, kinetics and products

The BrO + HO2 reaction, which participates in the cycle of ozone removal via BrOH formation, was explored both in the absence and in the presence of water using ab initio calculations. Two main sets of products, (i) HBr + O3 and (ii) BrOH + O2, are formed regardless of the presence of water, following a hydrogen abstraction mechanism. The HBr + O3 products are formed from the intermediate BrOOOH adduct, whereas BrOH + O2 are formed either from the intermediate OBrOOH adduct or via a barrierless hydrogen transfer from HO2 to BrO. Owing to the formation of molecular oxygen that can bear different spin configurations, the formation of BrOH + O2 products was examined both on the singlet and the triplet surfaces. Under relevant atmospheric temperatures and pressure, the formation of products (i) is energetically and kinetically less favorable than that of products (ii). The rate coefficient at 298 K for the HBr + O3 formation was determined to be 2.00 × 10-20 cm3 molecule-1 s-1, and found to decrease by 1-2 orders of magnitude when one or both reactants are clustered with water. For the formation of BrOH + O2, a rate coefficient of 2.21 × 10-11 cm3 molecule-1 s-1 is determined on both singlet and triplet surfaces in the absence of water. Though this rate coefficient slightly decreases for the hydrated reactions, the fractions of the reactants that are effectively complexed with water are not high enough to shift the overall BrOH + O2 formation rate. The current study further indicates that humidity plays a negligible role in ozone removal via the BrO + HO2 reaction.

Pressure-Dependent Kinetics of the Reaction between CH3OO and OH Focusing on the Product Yield of Methyltrioxide (CH3OOOH)

The reaction kinetics of methyl peroxy radical (CH₃OO) and hydroxyl radical (OH) has attracted an increasing level of interest in the past decade, while the branching yields of various product channels are still under debate. In this work, a comprehensive theoretical effort was made to investigate the branching yield of the stabilized methyltrioxide (CH₃OOOH, TRIOX) adduct, which has recently been a research focus. Our computed branching ratio of TRIOX at 298 K and 760 Torr is ∼0.04, in agreement with the result of multiplexed photoionization mass spectrometry. We show that the large branching yield obtained in an early theoretical study mainly originated from the collision-induced strong stabilization presented in their simulation. Our findings clarify the controversial product yield results for this important species in recent studies. The computed rate constants over wide temperature and pressure ranges allow better integration of this reaction into global atmospheric models and low-temperature combustion kinetic models.