Elettra | CiPo Understanding the mechanisms behind ion-driven reactions is essential for unraveling atmospheric chemical processes, especially those influencing cloud formation and climate. Among the ionic pathways with significant atmospheric implications is the reaction between methanol radical cation (CH₃OH⁺) and molecular oxygen (O₂), which leads to the formation of hydroperoxyl radicals (HO₂) and protonated formaldehyde (CH₂OH⁺). This study presents experimental and theoretical investigations into this exothermic reaction, offering insight into its dynamics and relevance in the upper atmosphere. Gas-phase ion chemistry is pivotal in atmospheric reactions due to the high reactivity of ions compared to neutral species—often by orders of magnitude. Indeed, in regions of the upper troposphere and stratosphere, cosmic rays generate ions that participate in atmospheric reactions, contributing to the formation of cloud condensation nuclei (CCN). The significance of these processes prompted the CLOUD experiment at CERN to examine the effects of galactic cosmic rays on aerosol and cloud formation. In this context, understanding the interactions of trace gases such as methanol with oxygen is critical, as methanol is a major volatile organic compound (VOC) contributing up to 20% of total VOC emissions. The focus of this study is on the reaction: CH₃OH⁺ + O₂ → CH₂OH⁺ + HO₂ which is exothermic by –118.4 kJ/mol and produces hydroperoxyl radical and protonated formaldehyde (Fig. 1 a). This channel is experimentally observed to be the dominant, if not the exclusive, pathway under the conditions used. Less exothermic alternatives involving the formation of H₂CO and H₂O₂ were predicted thermochemically but not observed. Using synchrotron radiation in the CIPO Beamline, CH₃OH⁺ ions were generated and made to react with O₂ under controlled pressures and photon energies. Mass spectrometry showed a clear product ion at m/z = 31, attributed to CH₂OH⁺. Deuterium labeling experiments with CD₃OH confirmed that the hydrogen (or deuterium) transferred to O₂ comes from the methyl group rather than the hydroxyl group (Fig. 1 b,c). The reaction thus proceeds via hydrogen abstraction from the C–H bond rather than the O–H bond, the latter being energetically less favorable by approximately 212.3 kJ/mol. Further experiments monitored the reactivity over a photon energy range of 10.8–11.6 eV. A declining trend in product-to-reactant ion ratios with increasing photon energy suggests that excess internal energy in CH₃OH⁺ reduces the efficiency of the reaction (Fig. 1 d). This aligns with a mechanistic interpretation where the reaction is capture-limited and primarily governed by long-range electrostatic interactions, consistent with capture theory. Theoretical calculations supported the experimental data. The initial interaction leads to the formation of a weakly bound molecular adduct (M1), characterized by a hydrogen bond between O₂ and a hydrogen of the CH₃ group (Fig. 1 e). The minimum energy path (MEP) shows a shallow barrier (6.4 kJ/mol) before the hydrogen transfer to O₂, forming HO₂ and CH₂OH⁺. A second adduct (M2), stabilized by a hydrogen bond between the two products, precedes dissociation into the final species. Spin and charge analyses along the MEP reveal gradual charge rearrangements and confirm the reaction progresses without substantial barriers. The calculated reaction rate at ionization threshold is 1.2 ± 0.6 × 10⁻⁹ cm³molecule⁻¹s⁻¹, in close agreement with the Langevin capture rate, indicating a fast, barrierless process. As photon energy increases, the rate declines modestly due to reduced stabilization of the intermediate complex. Atmospherically, this reaction is noteworthy for several reasons. First, HO₂ is a key species in photochemical cycles, including ozone formation and degradation. It also participates in the oxidation of SO₂ and the transformation of Criegee intermediates, as well as in smog formation through reactions with NO and NO₂. HO₂ formation during lightning events highlights its connection to ionization phenomena. Second, CH₂OH⁺, a protonated form of formaldehyde, can rapidly transfer its proton to bases like methanol or ammonia to produce neutral H₂CO, which affects ozone budgets, contributes to particulate formation, and influences the acidity of precipitation. Methanol itself plays a versatile role in atmospheric chemistry, acting as both a source and sink for various species and participating in reactions affecting OH, CO, and O₃ cycles. Moreover, its presence in interstellar environments broadens the significance of these findings beyond Earth’s atmosphere. In conclusion, the reaction between CH₃OH⁺ and O₂ represents a highly efficient, ionic pathway to the formation of atmospherically relevant species. The results demonstrate that long-range interactions facilitate the reaction, which proceeds rapidly without significant energetic barriers. This underscores the importance of ion–molecule reactions in atmospheric models and the need for their integration into climate and photochemical simulations. In this context, several numerical models have been developed, typically using average approaches. However, these models are not designed to predict the ionization states of ions or their spatial distributions. To address this, we have recently begun implementing interaction models in a Monte Carlo simulation of atmospheric ionization, focusing on O₂ and N₂ using the Geant4-DNA toolkit. The goal is to extend this work to other key atmospheric molecules, such as H₂O, SO₂, CO₂, CH₄, CH₃OH, NOx, and O₃. The Ion Chemistry of Methanol and Oxygen: A Path to HO₂ and protonated Formaldehyde Elettra Highlights 2024–2025 42
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