Direct ab initio dynamics calculations of thermal rate constants for the CH4 + O2 = CH3 + HO2 reaction

Ab initio kinetics of the CH3NH + NO2 reaction: formation of nitramines and N-alkyl nitroxides

This study provides a detailed understanding of how the reaction between CH3NH, one of the primary products of the CH3NH2 + OH/Cl reactions, and NOx occurs in the atmosphere since the reaction is expected to be a dominant sink for the tropospheric CH3NH radical. First, we focus on the reaction of the aminyl radical CH3NH with NO2, complementing the known reaction between CH3NH and NO, to provide the overall picture of the CH3NH + NOx system. The reaction was meticulously examined across the extended range of temperature (298-2000 K) and pressure (0.76-76 000 torr) using quantum chemistry calculations and kinetic modeling based on the framework of the Rice-Ramsperger-Kassel-Marcus (RRKM)-based master equation. Highly correlated electronic structure calculations unveil that the intricate reaction mechanism of the CH3NH + NO2 reaction, which can proceed through O-addition or N-addition to form NO2, encompasses numerous steps, channels, and various intermediates and products. The temperature-/pressure-dependent kinetic behaviors and product distribution of the CH3NH + NO2 reaction are revealed under atmospheric and combustion conditions. The main products under atmospheric conditions are found to be CH3NHO and NO, as well as CH3NHNO2, while under combustion conditions, the primary products are only CH3NHO and NO. Given its stability under ambient conditions, CH3NHNO2, a nitramine, is believed to have the potential to induce DNA damage, which can ultimately result in severe cancers. Secondly, by building upon prior research on the CH3NH + NO system, this study shows that the reaction of CH3NH with NOx holds greater importance in urban areas with elevated NOx emissions than other oxidants like O2. Furthermore, this reaction occurs swiftly and results in the creation of various compounds, such as the carcinogenic nitrosamine (CH3NHNO), carcinogenic nitramine (CH3NHNO2), CH3NNOH, (CH3NN + H2O) and (CH3NHO + NO).

Mechanistic and Kinetic Insights into OH-Initiated Atmospheric Oxidation of Hymexazol: A Computational Study

Hymexazol is a volatile fungicide widely used in agriculture, causing its abundance in the atmosphere; thus, its atmospheric fate and conversion are of great importance when assessing its environmental impacts. Herein, we report a theoretical kinetic mechanism for the oxidation of hymexazol by OH radicals, as well as the subsequent reactions of its main products with O2 and then with NO by using the Rice-Ramsperger-Kassel-Marcus-based Master equation kinetic model on the potential energy surface explored at the ROCBS-QB3//M06-2X/aug-cc-pVTZ level. The predicted total rate constants ktotal(T, P) for the reaction between hymexazol and OH radicals show excellent agreement with scarcely available experimental values (e.g., 3.6 × 10-12 vs (4.4 ± 0.8) × 10-12 cm3/molecule/s at T = 300 K and P = 760 Torr); thus, the calculated kinetic parameters can be confidently used for modeling/simulation of N-heterocycle-related applications under atmospheric and even combustion conditions. The model shows that 3,4-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-5-yl (IM2), 3,5-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-4-yl (IM3), and (3-hydroxy-1,2-oxazol-5-yl)methyl (P8) are the main primary intermediates, which form the main secondary species of (3,4-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-5-yl)dioxidanyl (IM4), (3,5-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-4-yl)dioxidanyl (IM7), and ([(3-hydroxy-1,2-oxazol-5-yl)methyl]dioxidanyl (IM11), respectively, through the reactions with O2. The main secondary species then can react with NO to form the main tertiary species, namely, (3,4-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-5-yl)oxidanyl (P19), (3,5-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-4-yl)oxidanyl (P21), and [(3-hydroxy-1,2-oxazol-5-yl)methyl]oxidanyl (P23), respectively, together with NO2. Besides, hymexazol could be a persistent organic pollutant in the troposphere due to its calculated half-life τ1/2 of 13.7-68.1 h, depending on the altitude.

Ab Initio Kinetics of Initial Thermal Pyrolysis of Isopropyl Propionate: A Revisited Study

This work reports a detailed mechanism of the initial thermal pyrolysis of isopropyl propionate, (C₂H₅C(=O)OCH(CH₃)₂), an important biodiesel additive/surrogate, for a wide range of T = 500-2000 K and P = 7.6-76 000 Torr. The detailed kinetic behaviors of the title reaction on the potential energy surface constructed at the CBS-QB3 level were investigated using the RRKM-based master equation (RRKM-ME) rate model, including hindered internal rotation (HIR) and tunneling corrections. It is revealed that the C₃H₆ elimination occurring via a six-centered retro-ene transition state is dominant at low temperatures, while the homolytic fission of the C-C bonds becomes more competitive at higher temperatures. The tunneling treatment is found to slightly increase the rate constant at low temperatures (e.g., ∼1.59 times at 563 K), while the HIR treatment, being important at high temperatures, decreases the rate (e.g., by 5.9 times at 2000 K). Showing a good agreement with experiments in low-temperature kinetics, the kinetic model reveals that the pressure effect should be taken into account at high temperatures. Finally, the temperature- and pressure-dependent kinetic mechanism, consisting of the calculated thermodynamic and kinetic data, is provided for further modeling and simulation of any related systems.

Mai T, Assali M, Nguyen TTD, Nguyen H, Szőri M, Huynh LK, Fittschen C, Farooq A. Reaction Kinetics of 1,4-Cyclohexadiene with OH radicals : An Experimental and Theoretical Study

This work presents OH-initiated oxidation kinetics of 1,4-cyclochexadiene (1,4-CHD). Temperature dependence of the reaction was investigated by utilizing laser flash photolysis flow reactor and laser-induced fluorescence (LPFR/LIF) technique over the...

New Mechanistic Insights into Atmospheric Oxidation of Aniline Initiated by OH Radicals

This study theoretically reports the comprehensive kinetic mechanism of the aniline + OH reaction in the range of 200-2000 K and 0.76-7600 Torr. The temperature- and pressure-dependent behaviors, including time-resolved species profiles and rate coefficients, were studied within the stochastic RRKM-based master equation framework with the reaction energy profile, together with molecular properties of the species involved, characterized at the M06-2X/aug-cc-pVTZ level. Hindered internal rotation and Eckart tunneling treatments were included. The H-abstraction from the -NH₂ moiety (to form C₆H₅NH (P1)) is found to prevail over the OH-addition on the C atom at the ortho site of aniline (to form 6-hydroxy-1-methylcyclohexa-2,4-dien-1-yl (I2)) with the atmospheric rate expressions (in cm³/molecule/s) as k^abstraction(P1) = 3.41 × 10¹ × T^-4.56 × exp (-255.2 K/T) for 200-2000 K and k^addition(I2) = 3.68 × 10⁹ × T^-7.39 × exp (-1163.9 K/T) for 200-800 K. The U-shaped temperature-dependent characteristics and weakly positive pressure dependence at low temperatures (e.g., T ≤ 800 K and P = 760 Torr) of k_total(T) are also observed. The disagreement in k_total(T) between the previous calculations and experimental studies is also resolved, and atmospheric aniline is found to be primarily removed by OH radicals (τ_OH ∼ 1.1 h) in the daytime. Also, via TD-DFT simulations, it is recommended to include P1 and I2 in any atmospheric photolysis-related model.

Comment on “Atmospheric chemistry of oxazole: the mechanism and kinetic studies on oxidation reaction initiated by OH radicals” by A. Shiroudi, M. A. Abdel-Rahman, A. M. El-Nahas and M. Altarawneh, New J. Chem., 2021, , 2237

The large kinetic discrepancy between computational and experimental studies is resolved using the rigorous stochastic RRKM-based master-equation rate model. Detailed mechanistic insights are also revealed to advance its related applications.

Ab initio kinetic mechanism of OH-initiated atmospheric oxidation of pyrrole

The comprehensive kinetic mechanism of the OH-initiated gas-phase oxidation of pyrrole is first theoretically reported in a broad range of conditions (T = 200-2000 K &P = 1-7600 Torr). On the potential energy surface constructed at the M06-2X/aug-cc-pVTZ level, the temperature- and pressure-dependent behaviors of the title reaction were characterized using the stochastic Rice-Ramsperger-Kassel-Marcus based Master Equation (RRKM-ME) rate model. The corrections of the hindered internal rotation and quantum tunneling treatments were included. The calculated results reveal the competition between the two distinct pathways: OH-addition and direct H-abstraction. The former channels are found favorable at low-temperature and high-pressure range (e.g., T < 900 K and P = 760 Torr) where non-Arrhenius and positive pressure-dependent behaviors of the rate constants are noticeably observed, while the latter predominate at temperatures higher than 900 K at atmospheric pressure and no pressure dependence on the rate constant is found. The predicted global rate constants are in excellent agreement with laboratory values; thus, the derived kinetic parameters are recommended for modeling/simulation of N-heterocycle-related applications in atmospheric and even in combustion conditions. Besides, pyrrole should not be considered as a persistent organic pollutant owing to its short atmospheric lifetime (∼1 h) towards OH radicals. The secondary mechanisms of the subsequent reactions of two OH-pyrrole adducts (namely, I1 and I2) with two abundant species, O₂/NO, which are relevant to the atmospheric degradation process, were also investigated. It is also revealed by TD-DFT calculations that two OH-pyrrole adducts (I1 &I2), nine intermediates, Ii (i = 3-11) and four products (P1, P2, P3 and P6) can undergo photodissociation under the sunlight.

Detailed kinetics of hydrogen abstraction from trans-decalin by OH radicals: the role of hindered internal rotation treatment

In this study, the detailed kinetic mechanism of the trans-decalin + OH reaction is firstly investigated for a wide range of conditions (i.e., T = 200-2000 K & P = 0.76-76 000 Torr) using the M06-2X/aug-cc-pVTZ level and stochastic Rice-Ramsperger-Kassel-Marcus based master equation (RRKM-ME) rate model, which includes corrections of the hindered internal rotor (HIR) and tunneling effects. Our predicted global rate constant excellently matches with the scarce experimental measurement (R. Atkinson, et al. Int. J. Chem. Kinet., 1983, 15, 37-50). The H-abstraction channel from Cα of trans-decalin is found to be dominant at low temperatures. A U-shaped temperature-dependent behavior and slightly positive pressure-dependence at low temperatures (e.g., T ≤ 400 K & P = 760 Torr) of the total rate constants are also observed. Detailed analysis reveals that the HIR treatment is essential to capture the kinetic behavior while the tunneling correction only plays a minor role.

In Silico Evaluation of the Radical Scavenging Mechanism of Mactanamide

Mactanamide (MA) is a diketopiperazine isolated from marine fungi of the genus Aspergillus. This compound is known as a natural antioxidant from experimental studies, yet this activity has not been successfully modeled thus far. In this work, the hydroperoxyl radical scavenging activity of MA was evaluated in the gas phase and physiological environments by thermodynamic and kinetic calculations. The results revealed that the HOO• radical scavenging of MA in the lipid media follows the formal hydrogen transfer mechanism via hydrogen abstraction at the O11-H bond. In the aqueous solution, however, the antioxidant activity follows the sequential proton loss electron transfer mechanism. The rate constant of the HOO• scavenging of MA in the polar environment is about 1045 times (k overall = 2.23 × 106 M-1 s-1) higher than that in the lipid medium (k overall = 2.20 × 103 M-1 s-1). In polar media, the HOO• radical scavenging activity of MA is therefore 18 times higher than that of Trolox, the reference compound when assessing antioxidant activity. The results presented here align well with the experimental data, validating the mechanistic pathways and thus providing useful insights into the antioxidant activity of MA.

Is Indolinonic Hydroxylamine a Promising Artificial Antioxidant?

Indolinonic hydroxylamine (IH) is a new-generation artificial antioxidant that, due to its ability to fractionate into apolar environments, is considered for prevention against lipid peroxidation. For this reason, it is important to understand, and compare, its activity in polar and nonpolar environments. In this study, the antioxidant activity of IH has been evaluated against HO^• and HOO^• radicals in water and, for a lipid-mimetic environment, pentyl ethanoate solvent, using kinetic calculations. It was found that the overall reaction rate constant of the HO^• radical scavenging is more than 7 times higher in aqueous (8.98 × 10⁹ M^-1 s^-1) than in apolar (1.22 × 10⁹ M^-1 s^-1) media. However, HOO^• scavenging was 35 times faster in apolar media (1.00 × 10⁵ M^-1 s^-1 vs 2.80 × 10³ M^-1 s^-1). In a lipid environment, the HAT mechanism was favored for the antioxidant activity for both radical species, whereas in aqueous solution the SET mechanism defined the HO^• scavenging, while HAT described the HOO^• scavenging. IH was shown to be one of the most active antioxidants in lipid environment, an essential characteristic for the protection of biological systems.

Ab initio kinetics of the C2H2 + NH2 reaction: a revisited study

This work provides a rigorous detailed kinetic study on the C₂H₂ + NH₂ reaction in a wide range of conditions (T = 250-2000 K & P = 1-76000 Torr). In particular, the composite method W1U was used to construct the potential energy surface on which the kinetic behaviors were characterized within the state-of-the-art master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) framework. Corrections of the hindered internal rotation (HIR) treatment and quantum tunneling effect were included. A clear reaction mechanism shift with respect to both temperature and pressure was revealed via detailed kinetic and species analyses. In particular, bimolecular products (i.e., CH₂[double bond, length as m-dash]C[double bond, length as m-dash]NH + H, CH[triple bond, length as m-dash]CNH₂ + H, CH₃CN + H, CH[triple bond, length as m-dash]C· + NH₃ in the decreasing mole fraction order) can be formed directly from the reactants at high temperature and/or low pressure while they can be produced indirectly via intermediates (e.g., ·CH[double bond, length as m-dash]CHNH₂(cis), ·CH[double bond, length as m-dash]CHNH₂(trans), CH₂[double bond, length as m-dash]C·NH₂,…) at low temperature and/or high pressure. The calculated rate constants are in good agreement with the literature data from ab initio calculations without any adjustment; thus, the proposed temperature- and pressure-dependent rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling NH₂-related systems under atmospheric and combustion conditions.

mHDFS-HoF: A generalized multilevel homodesmotic fragment-separation reaction based program for heat-of-formation calculation for acyclic hydrocarbons

Based on our modified classification of elemental species, a framework for automatic generation of multilevel Homodesmotic fragment-separation (mHDFS) reactions for chemical species was proposed. Combined the mHDFS framework with a database of heat of formation (HoF) and the calculated electronic structure data for the elemental mHD species, the mHDFS-HoF program was constructed in C/C++ language to calculate heat of formation for a species of interest on-the-fly. Using the electronic structure data calculated at CBS-QB3 level of theory for the elemental mHD species, applications and robustness of the code were discussed with several acyclic hydrocarbon systems including neutral and radical species. On-going work and extension to other systems were also discussed. The program and the supporting files can be freely downloaded at https://sites.google.com/view/mhdfs/. © 2019 Wiley Periodicals, Inc.

Comment on “Atmospheric oxidation reactions of imidazole initiated by hydroxyl radicals: kinetics and mechanism of reactions and atmospheric implications” by Safaei et al., Phys. Chem. Chem. Phys., 2019, 21, 8445

This communication resolves the disagreement in the chemical kinetics of the imidazole + OH reaction using a more rigorous master equation/Rice–Ramsperger–Kassel–Marcus rate model on a more comprehensive potential energy surface.

Ab initio dynamics of hydrogen abstraction from N2H4 by OH radicals: an RRKM-based master equation study

The detailed kinetic mechanism of the N₂H₄ + OH reaction is comprehensively reported for a wide condition range of conditions (i.e., 200–3000 K & 1–7600 Torr) using the CCSD(T)/CBS//M06-2X/6-311++G(3df,2p) level and the RRKM-based master equation rate model.

Theoretical rate constant of methane oxidation from the conventional transition-state theory

The potential energy surface for the first step of the methane oxidation CH₄ + O₂➔CH₃ + HO₂ was studied using the London-Eyring-Polanyi-Sato equation (LEPS) and the conventional transition-state theory (CTST). The calculated activation energy and rate constant values were in good agreement with the experimental and theoretical values reported in the literature using the shock tube technique and coupled cluster method respectively. The rate equation from CTST, although simple, provides good results to study the H-shift between methane and the oxygen molecules.

On-the-fly kinetics of hydrogen abstraction from polycyclic aromatic hydrocarbons by methyl/ethyl radicals

This work provides a rigorous procedure, within the framework of the Reaction Class Transition State Theory (RC-TST) and the Structure-Activity Relationship (SAR), for predicting reliable thermal rate constants on-the-fly for hydrogen abstraction reactions by methyl/ethyl radicals from Polycyclic Aromatic Hydrocarbons (PAHs) in a temperature range of 300-3000 K. All necessary RC-TST parameters were derived from ab initio calculations for a representative set of 36 reactions on which different error analyses and comparisons with available literature data were carried out. In addition to the good agreement between the RC-TST rate constants and the literature data, the detailed error analyses show that RC-TST/SAR, utilizing either the Linear Energy Relationship (LER) where only the reaction energy is needed or Barrier Height Grouping (BHG) where no additional data is needed, can predict the thermal rate constants for any reaction in the title reaction class with an average systematic error of less than 50% when compared to the explicit rate calculations. Therefore, the constructed RC-TST procedure can be confidently used to obtain reliable rate constants on the fly in an attempt to effectively construct detailed kinetic mechanisms for PAH-related fuels.

Ab initio kinetics of the HOSO2 + 3O2 → SO3 + HO2 reaction

The detailed kinetic mechanism of the HOSO₂ + ³O₂ reaction, which plays a pivotal role in the atmospheric oxidation of SO₂, was investigated using accurate electronic structure calculations and novel statistical thermodynamic/kinetic models. Explored using the accurate composite method W1U, the detailed potential energy surface (PES) revealed that the addition of O₂ to a HOSO₂ radical to form the adduct (HOSO₄) proceeds via a transition state with a slightly positive barrier (i.e., 0.7 kcal mol^-1 at 0 K). Such a finding compromises a long-term hypothesis about this channel of being a barrierless process. Moreover, the overall reaction was found to be slightly exothermic by 1.7 kcal mol^-1 at 0 K, which is in good agreement with recent studies. On the newly-constructed PES, the temperature- and pressure-dependent behaviors of the title reaction were characterized in a wide range of conditions (T = 200-1000 K & P = 10-760 Torr) using the integrated deterministic and stochastic master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) rate model in which corrections for hindered internal rotation (HIR) and tunneling treatments were included. The calculated numbers were found to be in excellent agreement with literature data. The sensitivity analyses on the derived rate coefficients with respect to the ab initio input parameters (i.e., barrier height and energy transfer) were also performed to further understand the kinetic behaviors of the title reaction. The detailed kinetic mechanism, consisting of thermodynamic and kinetic data (in NASA polynomial and modified Arrhenius formats, respectively), was also provided at different T & P for further use in the modeling/simulation of any related systems.

Kinetics of Thermal Unimolecular Decomposition of Acetic Anhydride: An Integrated Deterministic and Stochastic Model

An integrated deterministic and stochastic model within the master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) framework was first used to characterize temperature- and pressure-dependent behaviors of thermal decomposition of acetic anhydride in a wide range of conditions (i.e., 300-1500 K and 0.001-100 atm). Particularly, using potential energy surface and molecular properties obtained from high-level electronic structure calculations at CCSD(T)/CBS, macroscopic thermodynamic properties and rate coefficients of the title reaction were derived with corrections for hindered internal rotation and tunneling treatments. Being in excellent agreement with the scattered experimental data, the results from deterministic and stochastic frameworks confirmed and complemented each other to reveal that the main decomposition pathway proceeds via a 6-membered-ring transition state with the 0 K barrier of 35.2 kcal·mol^-1. This observation was further understood and confirmed by the sensitivity analysis on the time-resolved species profiles and the derived rate coefficients with respect to the ab initio barriers. Such an agreement suggests the integrated model can be confidently used for a wide range of conditions as a powerful postfacto and predictive tool in detailed chemical kinetic modeling and simulation for the title reaction and thus can be extended to complex chemical reactions.

A combined high-temperature experimental and theoretical kinetic study of the reaction of dimethyl carbonate with OH radicals

The reaction kinetics of dimethyl carbonate (DMC) and OH radicals were investigated behind reflected shock waves over the temperature range of 872–1295 K and at pressures near 1.5 atm.

Theoretical Study of the Reactions of Methane and Ethane with Electronically Excited N2(A(3)Σu(+))

Comprehensive quantum chemical analysis with the usage of density functional theory and post-Hartree-Fock approaches were carried out to study the processes in the N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 systems. The energetically favorable reaction pathways have been revealed on the basis of the examination of potential energy surfaces. It has been shown that the reactions N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 occur with very small or even zero activation barriers and, primarily, lead to the formation of N2H + CH3 and N2H + C2H5 products, respectively. Further, the interaction of these species can give rise the ground state N2(X(1)Σg(+)) and CH4 (or C2H6) products, i.e., quenching of N2(A(3)Σu(+)) by CH4 and C2H6 molecules is the complex two-step process. The possibility of dissociative quenching in the course of the interaction of N2(A(3)Σu(+)) with CH4 and C2H6 molecules has been analyzed on the basis of RRKM theory. It has been revealed that, for the reaction of N2(A(3)Σu(+)) with CH4, the dissociative quenching channel could occur with rather high probability, whereas in the N2(A(3)Σu(+)) + C2H6 reacting system, an analogous process was little probable. Appropriate rate constants for revealed reaction channels have been estimated by using a canonical variational theory and capture approximation. The estimations showed that the rate constant of the N2(A(3)Σu(+)) + C2H6 reaction path is considerably greater than that for the N2(A(3)Σu(+)) + CH4 one.