Kinetics and Mechanism of the Tropospheric Oxidation of Vinyl Acetate Initiated by OH Radical: A Theoretical Study

New mechanistic understanding for atmospheric oxidation of isoprene initiated by atomic chlorine

Isoprene is the most abundant non-methane VOC and a significant SOA contributor. The atmospheric oxidation initiated by atomic chlorine is an important sink for isoprene, especially in certain regions with high Cl concentration, while its detailed oxidation mechanism remains unclear. In this work, we comprehensively investigated the reaction mechanism of isoprene with Cl using quantum chemistry calculation, and first elaborated the specific reaction mechanisms of chloroalkenyl peroxy radicals with HO₂/NO and the formation of 2-methylbut-3-enal, highlighting their important roles in the SOA formation. For the initial reactions, Cl additions to terminal carbons and H abstraction from CH₃ moiety of isoprene are the predominant reactions, which is consistent with previous research. Following the initial reactions, their subsequent reactions with O₂ and HO₂ (or NO) under different atmospheric conditions could lead to the formation of 17 highly oxidized molecules (HOMs), of which P10, P12, P16, P17, P19 and P33 generated by the subsequent reactions of the major first-generation products (MVK, CMBO, CMBA and MBO) have been detected in the reaction process of isoprene with Cl in the chamber experiment. In addition to auto-oxidation process, the reaction of chloroalkenyl peroxy radicals with HO₂/NO and their subsequent reactions are all easy to occur under atmospheric conditions, which could be crucial contributors to the formation of HOMs and SOA arising from the Cl initiated oxidation of isoprene. This study would be conducive to clarifying the atmospheric oxidation process of isoprene initiated by Cl and providing a new understanding of its SOA formation.

Tropospheric degradation of 2-fluoropropene (CH3CFCH2) initiated by hydroxyl radical: Reaction mechanisms, kinetics and atmospheric implications from DFT study

Degradation of hydrofluoro-olefins (HFOs) with oxidants plays a significant role in the troposphere. Thus, we have investigated detail theoretical calculations of hydroxyl radical (^•OH) initiated oxidation of 2-fluoropropene (CH₃CFCH₂) using M06-2X/6-311++G(d,p) level of theory. Here, we have considered different possible H-abstraction and OH addition for the degradation of CH₃CFCH₂ molecule. The potential energy analysis shows that OH-addition channels are more dominant than H-abstraction channels. The calculated reaction enthalpies (Δ_rH°) and Gibbs free energies (Δ_rG°) also suggest that OH-addition reaction channels are more favourable than H-abstraction channels. The overall rate coefficients for CH₃CFCH₂ + ^•OH reaction is calculated within the temperature range of 250-450 K. The observed overall rate coefficient (2.01 × 10^-11 cm³ molecule^-1 s^-1) at 298 K for the titled reaction is found to be in good agreement with the earlier reported experimental rate coefficient. The calculated percentage branching ratio shows that the contribution of OH-addition to α-carbon and β-carbon of CH₃CFCH₂ molecule are 85.10% and 14.20% to the overall rate coefficient while H-abstractions have a negligible contribution. Based on the kinetics calculations, the atmospheric lifetime of the titled molecule is found to be 0.6 days. Further, we have also explored the degradation pathways of OH-addition product radicals and found acetyl fluoride (CH₃CFO) and formaldehyde (HCHO) are the end degradation products.

Oxidation pathways, kinetics and branching ratios of chloromethyl ethyl ether (CMEE) initiated by OH radicals and the fate of its product radical: an insight from a computational study

The OH-initiated oxidation reactions of chloromethyl ethyl ether (CH2ClOCH2CH3) have been presented by using quantum calculation methods. The Minnesota functional (M06-2X) of the density functional theory method along with a polarization and diffuse 6-311++G(d,p) basis set is chosen for optimization and frequency calculations for H-abstractions from CH2ClOCH2CH3 molecules by OH radicals. Furthermore, the CCSD(T) method along with the same basis set is used for energy refinement of all optimized structures to obtain more accurate energies of the species. Our thermo-chemical calculation results show that the C˙HClOCH2CH3 product radical is more stable, corresponding to hydrogen atom abstraction from the -CH2Cl site, than others while the energy profile results indicate that the H-atom abstracted from the -OCH2 site follows the minimum energy path compared to other channels. The rate constants are computed using canonical transition state theory (CTST) within the temperature range of 250-450 K at 1 atm. The overall rate constant (at 298 K) for the abstraction reactions is found to be consistent with the earlier reported rate constant. The percentage branching ratios of different abstraction channels and the lifetime of chloromethyl ethyl ether are also given herein. We further investigated the unimolecular decomposition pathways of the CH2ClOCH(O˙)CH3 radical and found that unimolecular C-C bond scission is the kinetically and thermodynamically more feasible pathway compared to other unimolecular decomposition reactions.

Detailed Investigation of the Outstanding Peroxyl Radical Scavenging Activity of Two Novel Amino-Pyridinol-Based Compounds

The ability of two novel amino-pyridinol based compounds (NPyr6 and NPyr7) as peroxyl radical scavengers was investigated in silico. The gathered data indicate that they are exceptionally efficient in that role. However, solvent polarity influences their relative efficiency for that purpose. NPyr6 was identified as the best peroxyl radical scavenger in lipid solution, while NPyr7 takes that place in aqueous solution. Both compounds present two acid-base equilibria, which influence their reactivity in aqueous solution. The associated pKa values were estimated. Several reaction mechanisms were explored. Hydrogen transfer from the phenolic group was identified as the chemical route with the highest contribution to the antioxidant behavior of the investigated compounds in both, nonpolar medium and aqueous solution (at 2 ≤ pH ≤ 10). At higher pH other reaction pathways become the most relevant ones. In addition, their bioavailability, cell permeability, safety, and manufacturability were evaluated. According to these, particularly toxicity, NPyr7 seems to be a better candidate for use as an oral drug to fight oxidative stress than NPyr6.

Quantum calculation on night-time degradation of 2-chloroethyl ethyl ether (CH3CH2OCH2CH2Cl) initiated by NO3 radical

A dual-level quantum chemical calculations have been carried out on the initiation of night-time degradation of 2-chloroethyl ethyl ether (CH3CH2OCH2CH2Cl) via H-abstraction by NO3 radical. Within the scope of density functional theory, the electronic structure of all the species involved in the titled reaction has been optimized at M06-2X functional along with 6-31[Formula: see text]G(d,p) basis set. A higher level of couple cluster CCSD(T) method in conjunction with 6-311[Formula: see text]G(d,p) basis set has been used for the refined energy of the species. All minima and saddle states involved in the reaction channel have been characterized on the potential energy surface (PES). From PES, it is confirmed that H-abstraction from methylene (–CH2–) of ethyl (CH3CH2–) part of CH3CH2OCH2CH2Cl follows the minimum energy path. The rate constants (individual and overall) of the titled reaction are obtained using Canonical Transition State Theory (CTST) over the temperature range of 250–350[Formula: see text]K. The atmospheric lifetime and radiative efficiency of the titled molecule have also been estimated, amounting to 0.23 years and 0.024 years, respectively. The Global Warming Potentials of the 2-chloroethyl ethyl ether in 20 years, 100 years and 500 years time horizon were found to be 0.13, 0.04 and 0.01, respectively.

Dual antioxidant/pro-oxidant behavior of the tryptophan metabolite 3-hydroxyanthranilic acid: a theoretical investigation of reaction mechanisms and kinetics

Potent antioxidant in the absence of metal ions, responsible for the activity usually attributed to tryptophan. Pro-oxidant in the presence of metal ions; this effect increases with the pH.

OH-initiated tropospheric photooxidation of allyl acetate: a theoretical study

The mechanisms of OH-initiated oxidation of allyl acetate in the presence of O2/NO have been investigated by performing density functional theory calculations. Two patterns (OH-addition and H-abstraction) of the initial reaction and the subsequent reactions of the primarily produced intermediates (IM1, IM2, and IM4) have been proposed. The OH-addition reactions are more favorable than the H-abstraction reactions, but H-abstraction from the CH2 group cannot be ignored. The major degradation products have been identified. The rate coefficients and the branching ratios of the primary reactions are obtained over the temperature of 200–500 K and the pressure range of 0.001–1000 atm. The total rate coefficient is 3.17 × 10−11 cm3 molecule−1 s−1 at 298 K and 1 atm. With respect to the typical concentration of OH radical (2.0 × 106 molecule cm−3), the atmospheric lifetime of AAC is estimated to be 4.40 h.

Quantum Chemical Calculations Resolved Identification of Methylnitrocatechols in Atmospheric Aerosols

Methylnitrocatechols (MNCs) are secondary organic aerosol (SOA) tracers and major contributors to atmospheric brown carbon; however, their formation and aging processes in atmospheric waters are unknown. To investigate the importance of aqueous-phase electrophilic substitution of 3-methylcatechol with nitronium ion (NO2(+)), we performed quantum calculations of their favorable pathways. The calculations predicted the formation of 3-methyl-5-nitrocatechol (3M5NC), 3-methyl-4-nitrocatechol (3M4NC), and a negligible amount of 3-methyl-6-nitrocatechol (3M6NC). MNCs in atmospheric PM2 samples were further inspected by LC/(-)ESI-MS/MS using commercial as well as de novo synthesized authentic standards. We detected 3M5NC and, for the first time, 3M4NC. In contrast to previous reports, 3M6NC was not observed. Agreement between calculated and observed 3M5NC/3M4NC ratios cannot unambiguously confirm the electrophilic mechanism as the exclusive formation pathway of MNCs in aerosol water. However, the examined nitration by NO2(+) is supported by (1) the absence of 3M6NC in the ambient aerosols analyzed and (2) the constant 3M5NC/3M4NC ratio in field aerosol samples, which indicates their common formation pathway. The magnitude of error one could make by incorrectly identifying 3M4NC as 3M6NC in ambient aerosols was also assessed, suggesting the importance of evaluating the literature regarding MNCs with special care.

A theoretical insight into atmospheric chemistry of HFE-7100 and perfluoro-butyl formate: reactions with OH radicals and Cl atoms and the fate of alkoxy radicals

The calculated rate constants for C₄F₉OCH₃ + OH/Cl reactions are found to be 1.94 × 10⁻¹⁴ and 1.74 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively, at 298 K. The atmospheric lifetime and global warming potential for HFE-7100 are computed to be 2.12 years and 155.3, respectively.

Assessing the Protective Activity of a Recently Discovered Phenolic Compound against Oxidative Stress Using Computational Chemistry

The protection exerted by 3,5-dihydroxy-4-methoxybenzyl alcohol (DHMBA), a phenolic compound recently isolated from the Pacific oyster, against oxidative stress (OS) is investigated using the density functional theory. Our results indicate that DHMBA is an outstanding peroxyl radical scavenger, being about 15 times and 4 orders of magnitude better than Trolox for that purpose in lipid and aqueous media, respectively. It was also found to react faster with HOO(•) than other known antioxidants such as resveratrol and ascorbic acid. DHMBA is also predicted to be able to sequester Cu(II) ions, consequently inhibiting the OS induced by Cu(II)-ascorbate mixtures and downgrading the (•)OH production via the Haber-Weiss reaction. However, it is proposed that DHMBA is more efficient as a primary antioxidant (free radical scavenger), than as a secondary antioxidant (metal ion chelator). In addition, it was found that DHMBA can be efficiently regenerated in aqueous solution, at physiological pH. Such regeneration is expected to contribute to increase the antioxidant protection exerted by DHMBA. These results suggest that probably synthetic routes for this compound should be pursued, because albeit its abundance in nature is rather low, its antioxidant activity is exceptional.

Adrenaline and noradrenaline: protectors against oxidative stress or molecular targets?

Density functional theory was used to investigate the potential role of neurotransmitters adrenaline and noradrenaline regarding oxidative stress. It is predicted that they can be efficient as free radical scavengers both in lipid and aqueous media, with the main reaction mechanism being the hydrogen transfer and the sequential proton loss electron transfer, respectively. Despite the polarity of the environment, adrenaline and noradrenaline react with (•)OOH faster than Trolox, which suggests that they are better peroxyl radical scavengers than the reference compound. Both catecholamines are also proposed to be capable of efficiently inhibiting the oxidative stress induced by copper(II)-ascorbate mixtures, and the (•)OH production via Haber-Weiss reaction, albeit the effects on the later are only partial. They exert such beneficial effects by sequestering Cu(II) ions. In summary, these catecholamines can be capable of reducing oxidative stress, by scavenging free radicals and by sequestering metal ions. However, at the same time they might lose their functions in the process due to the associated structural modifications. Consequently, adrenaline and noradrenaline can be considered as both protectors and molecular targets of oxidative stress. Fortunately, under the proper conditions, both catecholamines can be regenerated to their original form so their functions are restored.

Reaction mechanism and kinetic study of the •OH initiated tropospheric oxidation of 3-methyl-2-buten-1-ol: A quantum chemical investigation

In this article a theoretical study of mechanism and kinetics of the OH -initiated oxidation reaction of 3-methyl-2-buten-1-ol (MBO-321) is presented. This degradation mechanism of MBO-321 consists of H -abstraction reaction and OH -addition to olefinic bond. In addition, probable product analysis alongwith the mechanism for secondary reactions in presence of O 2 and NO has been investigated. Energetics have been studied at the CCSD(T)/6-311++g(d,p)//MP2 = full/6-31+G(d) level of theory. Thermochemical analysis has been done using CBS-QB3 method starting from the geometry obtained in the MP2 method. All the H -abstraction reactions pass through positive energy barrier except H4 -abstraction channel. OH -addition to the double bond takes place via formation of the pre-reactive complex as these reaction channel pass through negative activation barrier. OH -addition is the predominant reaction pathway for the overall oxidation process. Using TST theory at 1 atm pressure and in the temperature range of 200 K–400 K, the calculated rate constant and lifetime of OH -addition is [Formula: see text] molecule-1 s-1 and 1.88 h, which is consistent with the previous experimental data, [Formula: see text] molecule-1 s-1 and 1.9 h respectively (Imamura et al., Rate coefficients for the gas phase reactions of OH radical with methylbutenols at 298 K, Int J Chem Kinet36:379–385, 2004).

Theoretical studies on kinetics, mechanism and thermochemistry of gas-phase reactions of HFE-449mec-f with OH radicals and Cl atom

A theoretical study on the mechanism and kinetics of the gas phase reactions of CF3CHFCF2OCH2CF3 (HFE-449mec-f) with the OH radicals and Cl atom have been performed using meta-hybrid modern density functional M06-2X using 6-31+G(d,p) basis set. Two conformers have been identified for CF3CHFCF2OCH2CF3 and the most stable one is considered for detailed study. Reaction profiles for OH-initiated hydrogen abstraction are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels. Our calculations reveal that hydrogen abstraction from the CH2 group is thermodynamically and kinetically more facile than that from the CHF group. Using group-balanced isodesmic reactions, the standard enthalpies of formation for HFE-449mecf and radicals generated by hydrogen abstraction, are also reported. The calculated bond dissociation energies for CH bonds are in good agreement with experimental results. The rate constants of the two reactions are determined for the first time in a wide temperature range of 250-450K. The calculated rate constant values are found to be 9.10×10(-15) and 4.77×10(-17)cm(3)molecule(-1)s(-1) for reactions with OH radicals and Cl atom, respectively. At 298K, the total calculated rate coefficient for reactions with OH radical is in good agreement with the experimental results. The atmospheric life time of HFE-449mec-f is estimated to be 0.287 years.