scholarly journals Rate Constants and Branching Ratios in the Oxidation of Aliphatic Aldehydes by OH Radicals under Atmospheric Conditions

2017 ◽  
Vol 56 (3) ◽  
Author(s):  
Romina Castañeda ◽  
Cristina Iuga ◽  
J. Raúl Álvarez-Idaboy ◽  
Annik Vivier-Bunge
Keyword(s):  
Theoretical Study ◽  
Hydrogen Abstraction ◽  
Branching Ratio ◽  
Branching Ratios ◽  
Rate Coefficients ◽  
Oh Radicals ◽  
Atmospheric Conditions ◽  
Total Rate ◽  
Aliphatic Aldehydes ◽  
Oh Reactions

In this work, a theoretical study is presented on the mechanism of OH reactions with C1-C5 aliphatic aldehydes. We have shown that, starting from butanal, the Cβ H-abstraction channel becomes relatively important and it contributes moderately to the total rate constant. Calculated overall rate coefficients at the CCSD(T)/6-311++G**//BHandHLYP/6-311++G** level are in excellent agreement with experimental data, supporting the proposed mechanisms. Negative activation energies are found to be in agreement with the temperature dependence observed for aldehydes. The branching ratio between the aldehydic and Cβ hydrogen abstraction is not significantly modified as temperature increases from 230 to 330 K.

New Theoretical Insight into the Reaction Kinetics of Toluene and Hydroxyl Radicals

2020 ◽  
Author(s):  
Xiaoqing Wu ◽  
Can Huang ◽  
Shiyao Niu ◽  
Feng Zhang
Keyword(s):  
Reaction Kinetics ◽  
Quantum Chemical ◽  
Branching Ratios ◽  
Rate Coefficients ◽  
Total Rate ◽  
Barrier Heights ◽  
Reaction Channels ◽  
High Level ◽  
Kinetics Of ◽  
Insight Into

<p><a></a><a>Toluene’s removal mechanism in the atmosphere is mainly attributed to OH radical, which includes major OH-addition and minor H-abstraction reactions. The cresols and RO2 derived from OHadducts reacting to O2 have significant impacts on the generation of secondary organic aerosols (SOA) and O3. However, computed branching ratios of various OH-adducts at various theoretical levels are largely inconsistent, mainly because previously reported barrier heights of OH-addition reaction showed a strong method dependence. In the present study, we demonstrate that this reaction involves a nonnegligible anharmonic effect (during the process of OH moving to the benzene ring), which has been overlooked by previous studies. The reaction kinetics of toluene + OH was systematically studied by a high-level quantum chemical method (CCSD(T)-F12/cc-pVQZ-F12//B2PLYP-D3/6-311++G(d,p)) combined with RRKM/master equation simulations. The particle-in-a-box approximation was used to treat the anharmonicity in this system. The final total rate coefficient is calculated to be 2.60 × 10−12 cm3 molecule−1 s−1 at 300 K and 1 atm. The main products for toluene + OH are computed as ortho-adducts (50.8%), benzyl radical + H2O (21.1%), ipso-adduct (16.3%), para-adduct (6.1%), and meta-adduct (4.6%). Our results indicate that both high level quantum chemical calculations for the crucial barrier heights and appropriate treatments for the anharmonicity determine the accuracy of the final computed total rate coefficients and branching ratios. Further analysis on the branching ratios of various reaction channels provides insight into the atmosphere-initiated oxidation of toluene. </a></p>

New Theoretical Insight into the Reaction Kinetics of Toluene and Hydroxyl Radicals

2020 ◽  
Author(s):  
Xiaoqing Wu ◽  
Can Huang ◽  
Shiyao Niu ◽  
Feng Zhang
Keyword(s):  
Reaction Kinetics ◽  
Quantum Chemical ◽  
Branching Ratios ◽  
Rate Coefficients ◽  
Total Rate ◽  
Barrier Heights ◽  
Reaction Channels ◽  
High Level ◽  
Kinetics Of ◽  
Insight Into

<p><a></a><a>Toluene’s removal mechanism in the atmosphere is mainly attributed to OH radical, which includes major OH-addition and minor H-abstraction reactions. The cresols and RO2 derived from OHadducts reacting to O2 have significant impacts on the generation of secondary organic aerosols (SOA) and O3. However, computed branching ratios of various OH-adducts at various theoretical levels are largely inconsistent, mainly because previously reported barrier heights of OH-addition reaction showed a strong method dependence. In the present study, we demonstrate that this reaction involves a nonnegligible anharmonic effect (during the process of OH moving to the benzene ring), which has been overlooked by previous studies. The reaction kinetics of toluene + OH was systematically studied by a high-level quantum chemical method (CCSD(T)-F12/cc-pVQZ-F12//B2PLYP-D3/6-311++G(d,p)) combined with RRKM/master equation simulations. The particle-in-a-box approximation was used to treat the anharmonicity in this system. The final total rate coefficient is calculated to be 2.60 × 10−12 cm3 molecule−1 s−1 at 300 K and 1 atm. The main products for toluene + OH are computed as ortho-adducts (50.8%), benzyl radical + H2O (21.1%), ipso-adduct (16.3%), para-adduct (6.1%), and meta-adduct (4.6%). Our results indicate that both high level quantum chemical calculations for the crucial barrier heights and appropriate treatments for the anharmonicity determine the accuracy of the final computed total rate coefficients and branching ratios. Further analysis on the branching ratios of various reaction channels provides insight into the atmosphere-initiated oxidation of toluene. </a></p>

scholarly journals Ferrous-salt-promoted damage to deoxyribose and benzoate. The increased effectiveness of hydroxyl-radical scavengers in the presence of EDTA

1987 ◽  
Vol 243 (3) ◽  
pp. 709-714 ◽  
Author(s):  
J M C Gutteridge
Keyword(s):  
Rate Constants ◽  
Metal Ion ◽  
Hydrogen Abstraction ◽  
Oh Radicals ◽  
Free Solution ◽  
Abstraction Reaction ◽  
Cheap Method ◽  
Oh Reactions ◽  
A Site ◽  
Deoxyribose Assay

Hydroxyl radicals (OH.) in free solution react with scavengers at rates predictable from their known second-order rate constants. However, when OH. radicals are produced in biological systems by metal-ion-dependent Fenton-type reactions scavengers do not always appear to conform to these established rate constants. The detector molecules deoxyribose and benzoate were used to study damage by OH. involving a hydrogen-abstraction reaction and an aromatic hydroxylation. In the presence of EDTA the rate constant for the reaction of scavengers with OH. was generally higher than in the absence of EDTA. This radiomimetic effect of EDTA can be explained by the removal of iron from the detector molecule, where it brings about a site-specific reaction, by EDTA allowing more OH. radicals to escape into free solution to react with added scavengers. The deoxyribose assay, although chemically complex, in the presence of EDTA appears to give a simple and cheap method of obtaining rate constants for OH. reactions that compare well with those obtained by using pulse radiolysis.

scholarly journals Theoretical studies on the kinetics of the hydrogen-abstraction reactions from 1,3,5-trioxane and 1,4-dioxane by OH radicals

2020 ◽  
Vol 45 ◽  
pp. 146867831989925 ◽  
Author(s):  
Vahid Saheb ◽  
Aidin Bahadori
Keyword(s):  
Transition State ◽  
Transition State Theory ◽  
Hydrogen Abstraction ◽  
Transition States ◽  
State Theory ◽  
Rate Coefficients ◽  
Cyclic Ethers ◽  
Oh Radicals ◽  
Kinetics Of ◽  
Bonded Complexes

Theoretical investigations have been performed on the kinetics of bimolecular hydrogen-abstraction reactions of 1,3,5-trioxane and 1,4-dioxane cyclic ethers with OH radicals. Hydrogen abstraction from both axial and equatorial positions of 1,3,5-trioxane and 1,4-dioxane was considered. Optimization of the structures, and the calculation of energies, vibrational frequencies and moments of inertia for all the stationary points including reactants, hydrogen-bonded complexes, transition states and products were carried out using density functional theory at the M06-2X level together with the MG3S basis set. Single-point energy calculations on the optimized points were obtained at the CBS-QB3 level. The calculations show that the title reactions proceed through relatively strong hydrogen-bonded complexes due to the hydrogen bonding between the OH radicals and the oxygen atoms of the cyclic ethers. A two-transition state model (an inner tight transition state and an outer loose transition state) was employed to compute the hydrogen-abstraction rate coefficients. The rate coefficients were also computed using conventional transition state theory considering a tight transition state for the purpose of comparison. It was found that when the reactions proceed via inner transition states with relative energies higher than the reactants, the computed rate coefficients are underestimated by conventional transition state theory.

Rate coefficients and product branching ratios for (E)-2-butenal + H reactions

2020 ◽  
Vol 22 (25) ◽  
pp. 14246-14254
Author(s):  
Maiara Oliveira Passos ◽  
Igor Araujo Lins ◽  
Tiago Vinicius Alves
Keyword(s):  
Rate Constants ◽  
Hydrogen Abstraction ◽  
Branching Ratios ◽  
Rate Coefficients ◽  
Variational Theory ◽  
Hydrogen Atoms ◽  
Small Curvature ◽  
Thermal Rate Constants ◽  
First Time ◽  
Product Branching

Thermal rate constants for the hydrogen abstraction reactions of (E)-2-butenal by hydrogen atoms were calculated, for the first time, using the multipath canonical variational theory with small-curvature tunneling (MP-CVT/SCT).

scholarly journals Atmospheric fate of a series of saturated alcohols: kinetic and mechanistic study

2020 ◽  
Vol 20 (2) ◽  
pp. 699-720
Author(s):  
Inmaculada Colmenar ◽  
Pilar Martin ◽  
Beatriz Cabañas ◽  
Sagrario Salgado ◽  
Araceli Tapia ◽  
...  
Keyword(s):  
Carbon Chain ◽  
Rate Coefficients ◽  
Mechanistic Study ◽  
Oh Radicals ◽  
Reaction Products ◽  
Atmospheric Fate ◽  
No3 Radical ◽  
Oh Reactions ◽  
Polyfunctional Compounds ◽  
Cl Atoms

Abstract. The atmospheric fate of a series of saturated alcohols (SAs) was evaluated through kinetic and reaction product studies with the main atmospheric oxidants. These SAs are alcohols that could be used as fuel additives. Rate coefficients (in cm3 molecule−1 s−1) measured at ∼298 K and atmospheric pressure (720±20 Torr) were as follows: k1 ((E)-4-methylcyclohexanol + Cl) = (3.70±0.16) ×10-10, k2 ((E)-4-methylcyclohexanol + OH) = (1.87±0.14) ×10-11, k3 ((E)-4-methylcyclohexanol + NO3) = (2.69±0.37) ×10-15, k4 (3,3-dimethyl-1-butanol + Cl) = (2.69±0.16) ×10-10, k5 (3,3-dimethyl-1-butanol + OH) = (5.33±0.16) ×10-12, k6 (3,3-dimethyl-2-butanol + Cl) = (1.21±0.07) ×10-10, and k7 (3,3-dimethyl-2-butanol + OH) = (10.50±0.25) ×10-12. The main products detected in the reaction of SAs with Cl atoms (in the absence/presence of NOx), OH radicals, and NO3 radicals were (E)-4-methylcyclohexanone for the reactions of (E)-4-methylcyclohexanol, 3,3-dimethylbutanal for the reactions of 3,3-dimethyl-1-butanol, and 3,3-dimethyl-2-butanone for the reactions of 3,3-dimethyl-2-butanol. Other products such as formaldehyde, 2,2-dimethylpropanal, and acetone have also been identified in the reactions of Cl atoms and OH radicals with 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol. In addition, the molar yields of the reaction products were estimated. The products detected indicate a hydrogen atom abstraction mechanism at different sites on the carbon chain of alcohol in the case of Cl reactions and a predominant site in the case of OH and NO3 reactions, confirming the predictions of structure–activity relationship (SAR) methods. Tropospheric lifetimes (τ) of these SAs have been calculated using the experimental rate coefficients. Lifetimes are in the range of 0.6–2 d for OH reactions, 7–13 d for NO3 radical reactions, and 1–3 months for Cl atoms. In coastal areas, the lifetime due to the reaction with Cl decreases to hours. The calculated global tropospheric lifetimes, and the polyfunctional compounds detected as reaction products in this work, imply that SAs could contribute to the formation of ozone and nitrated compounds at local, regional, and even global scales. Therefore, the use of saturated alcohols as additives in diesel blends should be considered with caution.

scholarly journals Atmospheric fate of a series of Methyl Saturated Alcohols (MSA): Kinetic and Mechanistic study

2019 ◽  
Author(s):  
Inmaculada Colmenar ◽  
Pilar Martin ◽  
Beatriz Cabañas ◽  
Sagrario Salgado ◽  
Araceli Tapia ◽  
...  
Keyword(s):  
Hydrogen Abstraction ◽  
Global Scale ◽  
Rate Coefficients ◽  
Mechanistic Study ◽  
Oh Radical ◽  
Reaction Products ◽  
Atmospheric Fate ◽  
No3 Radical ◽  
Oh Reactions ◽  
Polyfunctional Compounds

Abstract. The atmospheric fate of a series of Methyl Saturated Alcohols (MSA) has been evaluated through the kinetic and reaction product studies with the main atmospheric oxidants. Rate coefficients (in cm3 molecule−1 s−1 unit) measured at ~ 298 K and atmospheric pressure (~ 740 Torr) were as follows: (3.71 ± 0.53) × 10−10, (1.91 ± 0.65) × 10−11 and (2.92 ± 1.38) × 10−15 for reaction of E-4-methyl-cyclohexanol with Cl, OH and NO3, respectively. (2.70 ± 0.55) × 10−10 and (5.57 ± 0.66) × 10−12 for reaction of 3,3-dimethyl-1-butanol with Cl and OH radical respectively and (1.21 ± 0.37) × 10−10 and (10.51 ± 0.81) × 10−12 for reaction of 3,3-dimethyl-2-butanol with Cl and OH radical respectively. The main detected products were 4-methylcyclohexanone, 3,3-dimethylbutanal and 3,3-dimethyl-2-butanone for the reactions of E-4-methyl-cyclohexanol, 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol respectively with the three oxidants. A tentative estimation of yields have been done obtaining the following ranges (25–60) % for 4-methylcyclohexanone, (40–60) % for 3,3-dimethylbutanal and (40–80) % for 3,3-dimethyl-2-butanone. Other products as HCOH, 2,2-dimethylpropanal and acetone have been identified in the reaction of 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol. The yields of these products indicate a hydrogen abstraction mechanism at different sites of the alkyl chain in the case of Cl reaction and a predominant site in the case of OH and NO3 reactions, supported by SAR methods prediction. Tropospheric lifetimes (τ) of these MSA have been calculated using the experimental rate coefficients. Lifetimes are in the range of 0.6–2 days for OH reactions, 8–13 days for NO3 radical reactions and 1–3 months for Cl atoms. In coastal areas the lifetime due to the reaction with Cl decreases to hours. The global tropospheric lifetimes calculated, and the polyfunctional compounds detected as reaction products in this work, imply that the Methyl Saturated Alcohols could contribute to ozone and nitrated compound formation at local, but also regional and even to global scale. Therefore, the use of large saturated alcohols as additives in biofuels must be taken with caution.

Rate coefficients for the reaction of OH radicals with cis-3-hexene: an experimental and theoretical study

2015 ◽  
Vol 17 (14) ◽  
pp. 8714-8722 ◽  
Author(s):  
Thaís da Silva Barbosa ◽  
Silvina Peirone ◽  
Javier A. Barrera ◽  
Juan P. A. Abrate ◽  
Silvia I. Lane ◽  
...  
Keyword(s):  
Experimental Data ◽  
Theoretical Study ◽  
Rate Coefficients ◽  
Oh Radicals ◽  
Experimental Rate ◽  
Theoretical Results ◽  
Good Agreement

Microcanonical variational rate coefficients and experimental rate coefficients for the OH addition to cis-3-hexene have been determined. Theoretical results showed a non-Arrhenius profile and good agreement with the experimental data.

scholarly journals Investigations on the gas-phase photolysis and OH radical kinetics of nitrocatechols: Implications of intramolecular interactions on their atmospheric behavior

2021 ◽  
Author(s):  
Claudiu Roman ◽  
Cecilia Arsene ◽  
Iustinian Gabriel Bejan ◽  
Romeo-Iulian Olariu
Keyword(s):  
Gas Phase ◽  
Rate Coefficient ◽  
Relative Rate ◽  
Structural Features ◽  
Intramolecular Interactions ◽  
Rate Coefficients ◽  
Oh Radicals ◽  
Phase Reaction ◽  
Oh Radical ◽  
Atmospheric Conditions

Abstract. The Environmental Simulation Chamber made of Quartz from the University “Alexandru Ioan Cuza” from Iasi (ESC-Q-UAIC), Romania, was used to investigate for the first time the gas-phase reaction rate coefficients for four nitrocatechols towards OH radicals under simulated atmospheric conditions. Employing relative rate technique at a temperature of 298 ± 2 K and total air pressure of 1 atm, the obtained rate coefficients (in 10−12 cm3×s−1) were as followed: k3NCAT = (3.41 ± 0.37) for 3-nitrocatechol, k4NCAT = (1.27 ± 0.19) for 4-nitrocatechol, k5M3NCAT = (5.55 ± 0.45) for 5-methyl-3-nitrocatechol and k4M5NCAT = (0.92 ± 0.14) for 4-methyl-5-nitrocatechol. For the investigated compounds the photolysis rates in the actinic region, scaled to atmospheric relevant conditions, were evaluated as well. In this case the photolysis rate coefficient values were obtained only for 3-nitrocatechol and 5-methyl-3-nitrocatechol: J3NCAT = (3.06 ± 0.16) × 10−4 s−1 and J5M3NCAT = (2.14 ± 0.18) × 10−4 s−1, respectively. Considering the obtained results our study suggests that photolysis may be the main degradation process for 3-nitrocatechol and 5-methyl-3-nitrocatechol in the atmosphere. Results are discussed in terms of the reactivity of the investigated four nitrocatechols towards OH-radical initiated oxidation and their structural features. The rate coefficient values are also compared with those estimated from the structure-activity relationship for monocyclic aromatic hydrocarbons. Additional comparison with similar compounds is also presented underlining the implications towards possible degradation pathways and atmospheric behavior.

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