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Hierarchical Study of the Reactions of Hydrogen Atoms with Alkenes: A Theoretical Study of the Reactions of Hydrogen Atoms with C₂-C₄ Alkenes.

ABSTRACT: The present study complements our previous studies of the reactions of hydrogen atoms with C₅ alkene species including 1- and 2-pentene and the branched isomers (2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene), by studying the reactions of hydrogen atoms with C₂-C₄ alkenes (ethylene, propene, 1- and 2-butene, and isobutene). The aim of the current work is to develop a hierarchical set of rate constants for Ḣ atom addition reactions to C₂-C₅ alkenes, both linear and branched, which can be used in the development of chemical kinetic models. High-pressure limiting and pressure-dependent rate constants are calculated using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and a one-dimensional master equation (ME). Rate constant recommendations for Ḣ atom addition and abstraction reactions in addition to alkyl radical decomposition reactions are also proposed and provide a useful tool for use in mechanisms of larger alkenes for which calculations do not exist. Additionally, validation of our theoretical results with single-pulse shock-tube pyrolysis experiments is carried out. An improvement in species mole fraction predictions for alkene pyrolysis is observed, showing the relevance of the present study.

SUBMITTER: Power J

PROVIDER: S-EPMC8279655 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Many of the radical-molecule reactions are nonelementary reactions with negative activation energies, which usually proceed through two steps. They exist extensively in the atmospheric chemistry and hydrocarbon fuel combustion, so they are extensively studied both theoretically and experimentally. At the same time, various models, such as a two transition state model, a steady-state model, an equilibrium-state model, and a direct elementary dynamics model are proposed to get the kinetic parameters for the overall reaction. In this paper, a conversion temperature T C1 is defined as the temperature at which the standard molar Gibbs free energy change of the formation of the reaction complex is equal to zero, and it is found that when T ? T C1, the direct elementary dynamics model with an inclusion of the tunneling correction of the second step reaction is applicable to calculate the overall reaction rate constants for this kind of reaction system. The reaction class of hydroxyl radical addition to alkenes is chosen as the objects of this study, five reactions are chosen as the representative for the reaction class, and their single-point energies are calculated using the method of CCSD(T)/CBS, and it is shown that the highest conversion temperature for the five reactions is 139.89 K, far below the usual initial low-temperature (550 K) oxidation chemistry of hydrocarbon fuels; therefore, the steady-state approximation method is applicable. All geometry optimizations are performed at the BH&HLYP/6-311+G(d,p) level, and the result shows that the geometric parameters in the reaction centers are conserved; hence, the isodesmic reaction method is applicable to this reaction class. To validate the accuracy of this scheme, a comparison of electronic energy difference at the BH&HLYP/6-311+G(d,p) level and the corrected electronic energy difference with the electronic energy difference at the CCSD(T)/CBS level is performed for the five representative reactions, and it is shown that the maximum absolute deviation of electronic energy difference can be reduced from 2.54 kcal·mol-1 before correction to 0.58 kcal·mol-1 after correction, indicating that the isodesmic reaction method is applicable for the accurate calculation of the kinetic parameters for large-size molecular systems with a negative activation energy reaction. The overall rate constants for 44 reactions of the reaction class of hydroxyl radical addition to alkenes are calculated using the transition-state theory in combination with the isodesmic correction scheme, and high-pressure limit rate rules for the reaction class are developed. In addition, the thermodynamic parameter is calculated and the results indicate that our dynamics model is applicable for our studied reaction class. A chemical kinetic modeling and sensitivity analysis using the calculated kinetic data is performed for the combustion of ethene, and the results indicate the studied reaction is important for the low-to-medium temperature combustion modeling of ethene.

Project description:Silicon-based organic precursors are widely applied in the vapor-fed flame synthesis of monocrystalline silicon, silicon dioxide, and silicon nitride. Due to the lack of kinetic investigations on reactions of silicon-based organic precursors, rate constants were usually analogized to those of their hydrocarbon counterparts. Investigations on the similarities and differences between the two types of compounds become necessary. This work reports a comparative theoretical investigation on H-abstraction reactions with H and CH3 attacking for silanes and their alkane counterparts, including silane and methane, disilane, methylsilane and ethane, dimethylsilane and propane, trimethylsilane and iso-butane, and tetramethylsilane and neo-pentane at the domain-based local pair natural orbital coupled cluster with perturbative triple excitations (DLPNO-CCSD(T))/cc-pVTZ//M06-2X/cc-pVTZ level. The rate constants were calculated using the conventional transition-state theory coupled with the asymmetric Eckart tunneling corrections over 600-2000 K. The calculated results show that dramatic discrepancies exist between H-abstraction from silicon sites in silanes and equivalent carbon sites in their alkane counterparts with H and CH3 attacking. The H-abstraction reactions from the primary carbon sites in silanes have generally lower barrier energies than the similar reactions in their alkane counterparts, while those in methylsilane and dimethylsilane with H attacking are the only two with higher barrier energies. Electrostatic potential mapped molecular van der Waals surfaces were adopted to provide insight into the calculated trends in barrier energies. The H-abstraction reactions from silicon sites in silanes have much higher rate constants than those from equivalent carbon sites in their alkane counterparts, especially under low-temperature conditions, while the rate constants of H-abstraction reactions from primary carbon sites in silanes and their alkane counterparts show relatively strong analogy.

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Hierarchical Study of the Reactions of Hydrogen Atoms with Alkenes: A Theoretical Study of the Reactions of Hydrogen Atoms with C₂-C₄ Alkenes.

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Hierarchical Study of the Reactions of Hydrogen Atoms with Alkenes: A Theoretical Study of the Reactions of Hydrogen Atoms with C2-C4 Alkenes.

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Hierarchical Study of the Reactions of Hydrogen Atoms with Alkenes: A Theoretical Study of the Reactions of Hydrogen Atoms with C₂-C₄ Alkenes.