The gas‐phase ozonolysis reaction of methylbutenol: A mechanistic study

Gas-Phase Reaction Kinetic Study of a Series of Methyl-Butenols with Ozone under Atmospherically Relevant Conditions

Methyl-butenols are a category of oxygenated biogenic volatile organic compounds emitted by plants as part of their natural metabolic processes. This study examines the gas-phase reactions of ozone (O3) with five methyl-butenols (2-methyl-3-buten-2-ol, 3-methyl-2-buten-1-ol, 3-methyl-3-buten-1-ol, 2-methyl-3-buten-1-ol, and 3-methyl-3-buten-2-ol) under atmospheric conditions at a temperature of (298 ± 2) K and pressure of (1000 ± 10) mbar. The experimental values for the gas-phase reaction rate coefficients obtained in this study, by using the relative rate method, are as follows (in cm3 molecule-1 s-1): k(3-methyl-2-buten-1-ol + O3) = (311 ± 20) × 10-18, k(2-methyl-3-buten-2-ol + O3) = (9.55 ± 1.04) × 10-18, k(3-methyl-3-buten-1-ol + O3) = (7.29 ± 0.46) × 10-18, k(2-methyl-3-buten-1-ol + O3) = (4.25 ± 0.29) × 10-18, and k(3-methyl-3-buten-2-ol + O3) = (62.9 ± 6.8) × 10-18. The results are discussed in detail, with particular emphasis on the degree and type of substitutions of the double bond. The determined rate coefficient values are also compared to the available literature data and with estimates of the structure-activity relationship. Additionally, the atmospheric implications toward the tropospheric lifetime and photochemical ozone generation potential for the investigated compounds are provided, which highlight the atmospheric impact of methyl-butenol decomposition into the lower atmosphere.

Gas-Phase Kinetic Investigation of the OH-Initiated Oxidation of a Series of Methyl-Butenols under Simulated Atmospheric Conditions

Five biogenic unsaturated alcohols have been investigated under simulated atmospheric conditions regarding their gas-phase OH reactivity. The gas-phase rate coefficients of OH radicals with 2-methyl-3-buten-2-ol (k1), 3-methyl-2-buten-1-ol (k2), 3-methyl-3-buten-1-ol (k3), 2-methyl-3-buten-1-ol (k4), and 3-methyl-3-buten-2-ol (k5) at 298 ± 2 K and 1000 ± 10 mbar total pressure of synthetic air were determined under low- and high-NOx conditions using the relative kinetic technique. The present work provides for the first time the rate coefficients of gas-phase reactions of hydroxyl radicals with 2-methyl-3-buten-1-ol and 3-methyl-3-buten-2-ol. The following rate constants were measured (in 10-11 cm3 molecule-1 s-1): k1 = 6.32 ± 0.49, k2 = 14.55 ± 0.93, k3 = 10.04 ± 0.78, k4 = 5.31 ± 0.37, and k5 = 11.71 ± 1.29. No significant differences in the measured rate coefficients were obtained when either 365 nm photolysis of CH3ONO in the presence of NO or 254 nm photolysis of H2O2 was used as a source of OH radicals. Reactivity toward other classes of related compounds such as alkenes and saturated alcohols is discussed. A comparison of the structure-activity relationship (SAR) estimates derived from the available accepted methodologies with experimental data available for unsaturated alcohols is provided. Atmospheric lifetimes for the investigated series of alkenols with respect to the main atmospheric oxidants are given and discussed.

Unimolecular Reactions of E-Glycolaldehyde Oxide and Its Reactions with One and Two Water Molecules

The kinetics of Criegee intermediates are important for atmospheric modeling. However, the quantitative kinetics of Criegee intermediates are still very limited, especially for those with hydroxy groups. Here, we calculate rate constants for the unimolecular reaction of E-glycolaldehyde oxide [E-hydroxyethanal oxide, E-(CH₂OH)CHOO], for its reactions with H₂O and (H₂O)₂, and for the reaction of the E-(CH₂OH)CHOO…H₂O complex with H₂O. For the highest level of electronic structure, we use W3X-L//CCSD(T)-F12a/cc-pVDZ-F12 for the unimolecular reaction and the reaction with water and W3X-L//DF-CCSD(T)-F12b/jun-cc-pVDZ for the reaction with 2 water molecules. For the dynamics, we use a dual-level strategy that combines conventional transition state theory with the highest level of electronic structure and multistructural canonical variational transition state theory with small-curvature tunneling with a validated density functional for the electronic structure. This dynamical treatment includes high-frequency anharmonicity, torsional anharmonicity, recrossing effects, and tunneling. We find that the unimolecular reaction of E-(CH₂OH)CHOO depends on both temperature and pressure. The calculated results show that E-(CH₂OH)CHOO…H₂O + H₂O is the dominant entrance channel, while previous investigations only considered Criegee intermediates + (H₂O)₂. In addition, we find that the atmospheric lifetime of E-(CH₂OH)CHOO with respect to 2 water molecules is particularly short with a value of 1.71 × 10^-6 s at 0 km, which is about 2 orders of magnitude shorter than those usually assumed for Criegee intermediate reactions with water dimer. We also find that the OH group in E-(CH₂OH)CHOO enhances its reactivity.

Experimental and theoretical study of the reactivity of a series of epoxides with chlorine atoms at 298 K

Evaluating the reactivity of epoxides in the gas phase is very important due to their wide distribution in the atmosphere, potential health implications and atmospheric impact. The kinetic rate constants for the oxidation of epoxides have been very little studied until now. From the experimental data obtained in this work has been observed that there is an increase in reactivity towards chlorine atoms as a CH2 group is added to the hydrocarbon chain. The Structure Activity Relationship (SAR) method usually provides a good approximation of the rate constant for a wide series of compounds especially for those without complex structure and multiple organic functions. However, a good determination of the factors included in SAR estimations depends largely on the database of these compounds, which in the case of epoxides is very limited. The SAR estimation method also does not take into account other possible factors that could affect reactivity, such as the geometry of the molecule. The aim of this work is to further evaluate the reactivity of epoxides with chlorine atoms using experimental determinations, theoretical calculations and SAR estimations. For this, rate coefficients have been measured at 298 ± 2 K and 1000 ± 4 mbar pressure of synthetic air in a 1080 l Quartz Reactor (QUAREC) and a 480 l Duran glass reactor for the reaction of chlorine atoms with cyclohexene oxide (CHO), 1,2-epoxyhexane (12EHX), 1,2-epoxybutane (12EB), trans-2,3-epoxybutane (tEB) and cis-2,3-epoxybutane (cEB). Theoretical calculations for the reactions studied are in good agreement with our experimental findings and provide insights about the position of the H atom abstraction and reactivity trends for a series of epoxides. The importance of taking into consideration the geometrical distribution and the ring influence to improve SAR calculations is discussed.

Computational Study of the Dissociation Reactions of Secondary Ozonide

This contribution presents a comprehensive computational study on the reactions of secondary ozonide (SOZ) with ammonia and water molecules. The mechanisms were studied in both a vacuum and the aqueous medium. All the molecular geometries were optimized using the B3LYP functional in conjunction with several basis sets. M06-2X, APFD, and ωB97XD functionals with the full basis set were also used. In addition, single-point energy calculations were performed with the G4MP2 and G3MP2 methods. Five different mechanistic pathways were studied for the reaction of SOZ with ammonia and water molecules. The most plausible mechanism for the reaction of SOZ with ammonia yields HC(O)OH, NH3, and HCHO as products, with ammonia herein acting as a mediator. This pathway is exothermic and exergonic, with an overall barrier height of only 157 kJ mol−1 using the G3MP2 method. All the reaction pathways between SOZ and water molecules are endothermic and endergonic reactions. The most likely reaction pathway for the reaction of SOZ with water involves a water dimer, in which the second water molecule acts as a mediator, with an overall barrier height of only 135 kJ mol−1 using the G3MP2 method. Solvent effects were found to incur a significant reduction in activation energies. When the second H2O molecule acts as a mediator in the reaction of SOZ with water, the barrier height of the rate-determining step state decreases significantly.