Kinetics of Hydrogen Abstraction Reactions of Butene Isomers by OH Radical

Prediction of Hydrogen Abstraction Rate Constants at the Allylic Site between Alkenes and OH with Multiple Machine Learning Models

Hydrogen abstraction reactions between hydrocarbons and hydroxyl radicals are important propagation steps in radical chain reactions, playing a crucial role in atmospheric and combustion chemistry. This study focuses on predicting the rate constants of the prototype of the reaction class of hydrogen abstractions, i.e., the primary allylic hydrogen abstraction from alkenes by the OH radical, via utilizing machine learning (ML) methods. Specifically, three distinct models, namely, feedforward neural network (FNN), support vector regression (SVR), and Gaussian process regression (GPR), have been employed to construct robust ML models for prediction. We proposed a novel strategy that seamlessly integrates descriptor preprocessing, a pairwise linear correlation analysis, and a model-specific Wrapper method to enhance the effectiveness of the feature selection procedure. The selected feature subset was then evaluated using two cross-validation techniques, i.e., leave-one-group-out (LOGO) and K-fold cross-validations, for each of the three ML models (FNN, SVR, and GPR) to assess their predictive and stability performance. The results demonstrate that the FNN model, trained with seven representative descriptors, achieves superior performance compared to the other two methods. For the FNN model, the average percentage deviation is 39.06% on the test set by performing LOGO cross-validation, while the repeated 10-fold cross-validation achieves a percentage prediction deviation of 19.1%. Two larger alkenes with 10 carbons were selected to test the prediction performance of the trained FNN model on primary allylic hydrogen abstraction. Results show that the kinetic predictions follow well the modified three-parameter Arrhenius equation, indicating the reliable performance of FNN in predicting hydrogen abstraction rate constants, especially for the primary allylic site. Hopefully, this work can shed useful light on the application of ML in generating chemical kinetic parameters of hydrocarbon combustion chemistry.

A Combined Experimental and Theoretical Study on the Gas Phase Reaction of OH Radicals with Ethyl Propyl Ether

The reaction of ethyl propyl ether (EnPE) with OH radicals was studied using proton-transfer-reaction mass spectrum (PTR-MS), and the rate constant was measured at 298 K and atmospheric pressure using the relative rate method: k_exp(OH+EnPE) = (1.13 ± 0.09) × 10^-11 cm³ molecules^-1 s^-1. In addition, a parallel theoretical study was performed using the traditional transition state theory (TST) with a tunnelling effect correction in combination at M05-2X method with two basis sets, 6-311++G(d,p) and aug-cc-pVTZ. According to these calculations, H atom abstraction occurs more favorably from the methylene group adjacent to the -O- bond than from the other groups. The theoretical calculation of the total rate constant of the reaction of EnPE with OH radicals was consistent with the experimental values. The gas-phase products indicated that the major products observed were ethyl formate, ethyl propionate, propionic acid. Combined with the experimental and theoretical results, the possible reaction mechanisms were proposed and discussed. The atmospheric implications of the studied reaction are presented, and the lifetime of EnPE in the presence of OH radicals was evaluated to be approximately 1 day.

Reaction Mechanisms and Kinetics of the Hydrogen Abstraction Reactions of C₄⁻C₆ Alkenes with Hydroxyl Radical: A Theoretical Exploration

The reaction of alkenes with hydroxyl (OH) radical is of great importance to atmospheric and combustion chemistry. This work used a combined ab initio/transition state theory (TST) method to study the reaction mechanisms and kinetics for hydrogen abstraction reactions by OH radical on C₄–C₆ alkenes. The elementary abstraction reactions involved were divided into 10 reaction classes depending upon the type of carbon atoms in the reaction center. Geometry optimization was performed by using DFT M06-2X functional with the 6-311+G(d,p) basis set. The energies were computed at the high-level CCSD(T)/CBS level of theory. Linear correlation for the computed reaction barriers and enthalpies between M06-2X/6-311+G(d,p) and CCSD(T)/CBS methods were found. It was shown that the C=C double bond in long alkenes not only affected the related allylic reaction site, but also exhibited a large influence on the reaction sites nearby the allylic site due to steric effects. TST in conjunction with tunneling effects were employed to determine high-pressure limit rate constants of these abstraction reactions and the computed overall rate constants were compared with the available literature data.

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.

Reaction Kinetics of Hydrogen Atom Abstraction from C4-C6 Alkenes by the Hydrogen Atom and Methyl Radical

Alkenes are important ingredients of realistic fuels and are also critical intermediates during the combustion of a series of other fuels including alkanes, cycloalkanes, and biofuels. To provide insights into the combustion behavior of alkenes, detailed quantum chemical studies for crucial reactions are desired. Hydrogen abstractions of alkenes play a very important role in determining the reactivity of fuel molecules. This work is motivated by previous experimental and modeling evidence that current literature rate coefficients for the abstraction reactions of alkenes are still in need of refinement and/or redetermination. In light of this, this work reports a theoretical and kinetic study of hydrogen atom abstraction reactions from C4-C6 alkenes by the hydrogen (H) atom and methyl (CH₃) radical. A series of C4-C6 alkene molecules with enough structural diversity are taken into consideration. Geometry and vibrational properties are determined at the B3LYP/6-31G(2df,p) level implemented in the Gaussian-4 (G4) composite method. The G4 level of theory is used to calculate the electronic single point energies for all species to determine the energy barriers. Conventional transition state theory with Eckart tunneling corrections is used to determine the high-pressure-limit rate constants for 47 elementary reaction rate coefficients. To faciliate their applications in kinetic modeling, the obtained rate constants are given in the Arrhenius expression and rate coefficients for typical reaction classes are recommended. The overall rate coefficients for the reaction of H atom and CH₃ radical with all the studied alkenes are also compared. Branching ratios of these reaction channels for certain alkenes have also been analyzed.

Gas Phase Oxidation of Campholenic Aldehyde and Solution Phase Reactivity of its Epoxide Derivative

The rate constant for the OH reaction with campholenic aldehyde (CA) was measured using the flow tube-chemical ionization mass spectrometry method with a relative rate kinetics technique and was found to be (6.54 ± 0.52) × 10^-11 cm³ molecule^-1 s^-1 at 100 Torr pressure and 298 K. A mechanism for the formation of the observed products was developed for both NO-free and NO-present conditions. On the basis of measurements of the pressure dependent yields of the products, between 5 and 20% of the CA oxidation at atmospheric pressure is predicted to lead to campholenic aldehyde epoxide (CAE). The aqueous solution reaction rate constants for CAE were determined via NMR spectroscopy and were found to be (2.241 ± 0.036) × 10^-5 s^-1 for neutral conditions and 0.0989 ± 0.0053 M^-1 s^-1 for acid-catalyzed conditions at 298 K. The products of the CAE aqueous solution reaction were identified as an isomer of CAE and the aldehyde group hydrated form of this isomer. Unlike the isoprene-derived epoxide, IEPOX, a nucleophilic addition mechanism was not observed. On the basis of the rate constants determined for CA and CAE, it is likely that these species are reactive on atmospherically relevant time scales in the gas and aerosol phases, respectively. The results of the present study largely support a previous supposition that α-pinene-derived secondary organic aerosol may be influenced by the multiphase processing of various intermediate species, including those with epoxide functionality.

Theoretical Investigation on the Reaction between OH Radical and 4,4-Dimethyl-1-pentene in the Presence of O2

The atmospheric oxidation mechanism of 4,4-dimethyl-1-pentene (DMP441) initiated by OH radical has been theoretically investigated at the BH&HLYP/6-311++G(d,p) and CCSD(T)/6-31+G(d,p) levels of theory. HC(O)H and 3,3-dimethylbutanal [(CH3)3CCH2C(O)H] are identified in our calculations as major products in the OH-radical-initiated degradation of DMP441 in the presence of O2. However, the epoxide conformers and enols are expected to be minor products because of the high isomerization barriers involved. The calculated results are in qualitative accordance with experimental evidence. Conventional transition state theory has been used to calculate the rate constants of the initial addition channels of the OH + DMP441 reaction over the temperature range 220-500 K. The computed total rate constant at 298 K is 2.20 × 10(-11) cm(3) molecule(-1) s(-1), which is in very good agreement with the experimental value. Furthermore, it has been found that the calculated rate constant exhibits a weak non-Arrhenius behavior over the temperature range 220-500 K. The computed expression for the rate constant is k(OH+DMP441) = 1.22 × 10(-12) exp[(880 K)/T] cm(3) molecule(-1) s(-1).

Abstraction and addition kinetics of C2H radicals with CH4, C2H6, C3H8, C2H4, and C3H6: CVT/SCT/ISPE and hybrid meta-DFT methods

Rate coefficients for the reactions of C₂H radicals with methane (k₁), ethane (k₂), propane (k₃), ethylene (k₄), and propylene (k₅) were computed using canonical variational transition state theory (CVT) coupled with hybrid-meta density functional theory (DFT) over a wide range of temperatures from 150 to 5000 K.

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).

Ab initio kinetics for the decomposition of hydroxybutyl and butoxy radicals of n-butanol

The decomposition kinetics of the hydroxybutyl and butoxy radicals (C4H9O) arising via H abstraction from n-butanol were studied theoretically with ab initio transition-state-theory-based master equation analyses. Stationary points on the C4H9O potential energy surface were calculated at either the RQCISD(T)/CBS//B3LYP/6-311++G(d,p) level or the RQCISD(T)/CBS//CASPT2/aug-cc-pVDZ level. Unimolecular pressure- and temperature-dependent rate coefficients were calculated over broad ranges of temperature (300-2500 K) and pressure (1.3 × 10(-3) to 10(2) atm) by solving the time-dependent multiple-well master equation. The "well merging" phenomenon was observed and analyzed for its influence on the branching ratios and rate coefficients. The theoretical predictions were compared with the available experimental and theoretical data and any discrepancies were analyzed. The predicted rate coefficients are represented with forms that may readily be used in combustion modeling of n-butanol.

Gas-phase kinetics study of reaction of OH radical with CH3NHNH2 by second-order multireference perturbation theory

The gas-phase kinetics of H-abstraction reactions of monomethylhydrazine (MMH) by OH radical was investigated by second-order multireference perturbation theory and two-transition-state kinetic model. It was found that the abstractions of the central and terminal amine H atoms by the OH radical proceed through the formation of two hydrogen bonded preactivated complexes with energies of 6.16 and 5.90 kcal mol(-1) lower than that of the reactants, whereas the abstraction of methyl H atom is direct. Due to the multireference characters of the transition states, the geometries and ro-vibrational frequencies of the reactant, transition states, reactant complexes, and product complexes were optimized by the multireference CASPT2/aug-cc-pVTZ method, and the energies of the stationary points of the potential energy surface were refined at the QCISD(T)/CBS level via extrapolation of the QCISD(T)/cc-pVTZ and QCISD(T)/cc-pVQZ energies. It was found that the abstraction reactions of the central and two terminal amine H atoms of MMH have the submerged energy barriers with energies of 2.95, 2.12, and 1.24 kcal mol(-1) lower than that that of the reactants respectively, and the abstraction of methyl H atom has a real energy barrier of 3.09 kcal mol(-1). Furthermore, four MMH radical-H(2)O complexes were found to connect with product channels and the corresponding transition states. Consequently, the rate coefficients of MMH + OH for the H-abstraction of the amine H atoms were determined on the basis of a two-transition-state model, with the total energy E and angular momentum J conserved between the two transition-state regions. In units of cm(3) molecule(-1) s(-1), the rate coefficient was found to be k(1) = 3.37 × 10(-16)T(1.295) exp(1126.17/T) for the abstraction of the central amine H to form the CH(3)N(•)NH(2) radical, k(2) = 2.34 × 10(-17)T(1.907) exp(1052.26/T) for the abstraction of the terminal amine H to form the trans-CH(3)NHN(•)H radical, k(3) = 7.41 × 10(-20)T(2.428) exp(1343.20/T) for the abstraction of the terminal amine H to form the cis-CH(3)NHN(•)H radical, and k(4) = 9.13 × 10(-21)T(2.964) exp(-114.09/T) for the abstraction of the methyl H atom to form the C(•)H(2)NHNH(2) radical, respectively. Assuming that the rate coefficients are additive, the total rate coefficient of these theoretical predictions quantitatively agrees with the measured rate constant at temperatures of 200-650 K, with no adjustable parameters.