OH-initiated mechanistic pathways and kinetics of camphene and fate of product radical: a DFT approach

Atmospheric insight into the reaction mechanism and kinetics of isopropenyl methyl ether (i-PME) initiated by OH radicals and subsequent oxidation of product radicals

Studies on primary gas-phase reactions of emitted saturated and unsaturated ethers with oxidants and subsequent secondary reactions of product radicals with O₂ in the presence of NO are important in their atmospheric chemical processes. To accomplish these findings, we have examined the chemistry of OH-initiated oxidation of isopropenyl methyl ether (i-PME) CH₃C(CH₂)OCH₃ by electronic structure ca using density functional theory. Our energetic calculations show that OH additions to carbon-carbon double bonds of i-PME are more favorable reaction pathways than H-abstraction reactions from the various CH sites of the titled molecule. The rate constant values which are obtained from the transition state theory also signify that OH-addition reactions have faster reaction rates than H-abstraction reactions. Our calculated total rate constant of the reaction is found 9.90 × 10^-11 cm³ molecule^-1 s^-1. The percentage branching ratio calculations imply that OH-addition reactions have 98.09% contribution in the total rate constant. The atmospheric lifetime of i-PME is found to be 2.8 h. Further, we have identified 2-hydroxy-2-methoxypropanol, methyl acetate, methy-1,2-hydroxyacetate and 1-hydroxypropane-2-one, 1,2-dihydroxypropan-2-yl format, 2-hydroxyacetic acid, acetic acid, and formaldehyde from the secondary oxidation of product radicals.

Kinetics and degradation of camphene with OH radicals and its subsequent fate under the atmospheric O2 and NO radicals - A theoretical study

Camphene (C₁₀H₁₆) is an abundant bicyclic monoterpene in the atmosphere which can be easily oxidized by the atmospheric OH radicals. In this study, the oxidation of camphene with OH radicals and its subsequent reactions are studied using quantum chemical method. Thermochemical parameters show that the addition of OH radicals to the terminal C10 atom of camphene is thermodynamically more stable than the addition of OH radicals to the internal C7 atom of camphene. The reaction force profile demonstrates that the formation of two hydroxyalkoxy radical intermediates (I1a and I2a) are mainly dominated by the structural rearrangement with 94.28% and 99.43% of the total energy, respectively. The overall reaction rate coefficient for camphene + OH radical is 2.1⨯10^-12 cm³ molecule^-1 sec^-1 at 298 K and 1 atm which agree well with the experimental reaction rate coefficient (5.58⨯10^-11 cm³ molecule^-1 sec^-1) for the reaction of camphene with OH radical. The branching ratio for the addition of OH radical to the C10 position of camphene is 68.32%, and the C7 position of camphene is 31.68% at 298 K. The calculated lifetime reveals that camphene degrades quickly in the atmosphere owing to its short lifetime of 5.3 h. The obtained mechanistic and kinetic results reveal that the addition of OH radical to the C10 position is more dominant than the C7 position, and it is more stable and spontaneous in the atmosphere.

Mechanism and kinetics of diuron oxidation by hydroxyl radical addition reaction

Diuron is a phenyl urea herbicide used to control weeds in agricultural lands. The degradation of diuron in the atmosphere takes place dominantly via reaction with OH radicals. In this work, the OH addition reaction of diuron has been studied by using density functional theory methods M06-2X, ωB97X-D and MPWB1K with 6-31G(d,p) basis set. The calculated thermochemical parameters show that OH addition reaction occurs favourably at C₂ position of diuron. The rate constant is calculated for the favourable initial reaction pathway by using canonical variational transition state theory with small curvature tunnelling (SCT) correction over the temperature range of 200-1000 K. The reaction of initially formed diuron-OH adduct intermediate with O₂ leads to the formation of peroxy radical intermediate. The reaction of peroxy radical intermediate with HO₂ and NO_x (x = 1, 2) radicals is studied in detail. The results obtained from time-dependent density functional theory (TDDFT) calculations show that the intermediates and products formed from oxidation of diuron can be easily photolyzed in the sunlight. This study provides thermodynamical and kinetic data for the atmospheric oxidation of diuron by OH radical addition reaction and demonstrates the atmospheric chemistry of diuron and its derivatives.

Quantum mechanical study on the oxidation of ethyl vinyl ketone initiated by an OH radical

Oxidation of ethyl vinyl ketone (CH2CHCOCH2CH3) by an OH radical was carried out using the M06-2X/6-311++G(d,p) level of theory. For the OH-initiated oxidation of ethyl vinyl ketone (EVK), we have considered six H-atom abstractions and three addition reactions. From the energetic calculation of the species involved therein, the potential energy surface (PES) of all the reaction channels was constructed. From the energy profile, we found that the H-atom abstraction from the methylene group (-CH2-) of CH2CHCOCH2CH3 is energetically more favourable than the other H-abstraction channels. Moreover, we also observed that OH-addition to the α-carbon of the carbon-carbon double bond of the title molecule is energetically and thermodynamically more dominant than β-carbon and carbonyl carbon. The rate coefficients for all the reaction channels were calculated using the canonical transition state theory at the temperature range of 250-450 K and it reveals that among all the reaction channels, OH-addition to α-carbon is kinetically more dominant to the total rate constant. The total rate coefficient for the reaction at 298 K is found to be in good agreement with the reported experimental rate constant. Finally, we have determined the atmospheric lifetime of the title molecule.

A quantum theory investigation on atmospheric oxidation mechanisms of acrylic acid by OH radical and its implication for atmospheric chemistry

The hydroxyl radical, as the most important oxidant, controls the removal of some volatile organic compounds (VOCs) in the atmosphere. In this work, the atmospheric oxidation processes of acrylic acid by OH radical have been investigated by density functional theory (DFT). The energetic routes of the reaction of CH₂CHCOOH with OH radical have been calculated accurately at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G(d,p) level. It is implicated that the oxidation has five elementary reaction pathways mostly hinging on how hydroxyl radical approaches to the carbon skeleton of acrylic acid. The atmospheric degradation mechanisms of the CH₂CHCOOH by OH radical are the formation of reactive intermediates IM1 and IM2. Meanwhile, the further oxidation mechanisms of IM1 and IM2 by O₃ and NO are also investigated. The rate coefficients have been computed using tight transition state theory of the variflex code. The calculated rate coefficient is 2.3 × 10^-11 cm³ molecule^-1 s^-1 at standard pressure and 298 K, which is very close to the laboratory data (1.75 ± 0.47 × 10^-11 cm³ molecule^-1 s^-1). Moreover, the atmospheric lifetime of acrylic acid is about 6 h at 298 K and 1 atm, implying that the fast sinks of acrylic acid by hydroxyl radical.