Theoretical Kinetics Analysis for Ḣ Atom Addition to 1,3-Butadiene and Related Reactions on the Ċ4H7 Potential Energy Surface

Probing the Chemistry of Sulfurous Pollutants: Accurate Thermochemistry Determination of Extensive Sulfur-Containing Species

Sulfur-containing fuels, such as petroleum fuels, natural gas, and biofuels, produce SO2, SO3, and other highly toxic gases upon combustion, which are harmful to human health and the environment, making it essential to understand their thermochemical properties. This study used high-level quantum chemistry calculations to determine thermodynamic parameters, including entropy, enthalpy, and specific heat capacity for an extensive set of sulfur-containing species. The B3LYP/cc-pVTZ level of theory was used for geometry optimization, vibration frequency, and dihedral scan calculations. To determine an appropriate ab initio method for energy calculation, the Bland-Altman diagram, a statistical analysis method, was employed to visualize the 298 K enthalpy value between experimental data and three sets of ab initio methods: G3, CBS-QB3, and the average of G3 plus CBS-QB3. The CBS-QB3 method exhibited the highest accuracy and was eventually selected for the energy calculation in this study. Thermochemical property parameters were then calculated with the MultiWell program suite for all these sulfur-containing species, and the results were in good agreement with the thermochemical data of organic compounds and the National Institute of Standards and Technology Chemistry WebBook databases. The thermochemical property database established in this study is essential to studying sulfur-containing species in desulfurization.

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

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.

Photodissociation Dynamics of the Cyclohexyl Radical from the 3p Rydberg State at 248 nm

The photodissociation of jet-cooled cyclohexyl was studied by exciting the radicals to their 3p Rydberg state by using 248 nm laser light and detecting photoproducts by photofragment translational spectroscopy. Both H atom loss and dissociation to heavy fragment pairs are observed. The H atom loss channel exhibits a two-component translational energy distribution. The fast photoproduct component is attributed to impulsive cleavage directly from an excited state, likely the Rydberg 3s state, forming cyclohexene. The slow component is due to statistical decomposition of hot cyclohexyl radicals that internally convert to the ground electronic state prior to H atom loss. The fast and slow components are present in an ∼0.7:1 ratio, similar to findings in other alkyl radicals. Internal conversion to the ground state also leads to ring-opening followed by dissociation to 1-buten-4-yl + ethene in comparable yield to H-loss, with the C₄H₇ fragment containing enough internal energy to dissociate further to butadiene via H atom loss. A very minor ground-state C₅H₈ + CH₃ channel is observed, attributed predominantly to 1,3-pentadiene formation. The ground-state branching ratios agree well with RRKM calculations, which also predict C₄H₆ + C₂H₅ and C₃H₆ + C₃H₅ channels with similar yield to C₅H₈ + CH₃. If these channels were active, it was at levels too low to be observed.

Formation of polycyclic aromatic hydrocarbons, benzofuran, and dibenzofuran in fuel-rich oxidation of toluene using a flow reactor

Recently, polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) have been attracting considerable attention owing to their high toxicity. Understanding their formation mechanism during combustion processes is important to control their emission. However, there are few studies that have quantitatively investigated OPAH formation in the fuel-rich oxidation of hydrocarbons, despite the availability of several studies on PAH formation. In this study, benzofuran and dibenzofuran as OPAHs were quantified in the fuel-rich oxidation of toluene using a flow reactor at atmospheric pressure in a temperature range of 1050-1350 K at equivalence ratios from 3.0 to 12.0 and residence times from 0.2 to 1.5 s. In addition to benzofuran and dibenzofuran, 4 types of monocyclic aromatic hydrocarbons and 19 types of PAHs were also evaluated. The experimental data obtained in this study were compared with those of the ethylene oxidation performed in our previous study. The existing kinetic model for PAH growth was modified based on several theoretical studies to predict the behavior of OPAHs with furan structures. The modified model showed significant improvements in the prediction of benzofuran and dibenzofuran formation. Based on the rate of production and sensitivity analysis using the modified model, the dominant reaction pathways of benzofuran and dibenzofuran were investigated.

Hindered rotor benchmarks for the transition states of free radical additions to unsaturated hydrocarbons

The hindered internal rotors of 32 transition states (TSs) formed through four free radicals, namely methyl, vinyl, ethyl, methoxy (ĊH3, Ċ2H3, Ċ2H5, CH3) additions to acetylene, ethylene, allene, propyne, and propene (C2H2/C2H4/C3H4-a/C3H4-p/C3H6) are studied. To validate the uncertainties of rate constants that stem from the use of different electronic structure methods to treat hindered rotors, the rotations of the newly formed C-C and/or C-O rotors in the transition states are calculated using commonly used DFT methods (B3LYP, M06-2X, ωB97X-D and B2PLYP-D3 with two Pople basis sets (6-31+G(d,p), 6-311++G(d,p)) and cc-pVTZ). The hindrance potential energies V(χ) calculated using the M06-2X/6-311++G(d,p) method are benchmarked at the CCSD(T), CCSD(T)-F12, DLPNO-CCSD(T) levels of theory with cc-pVTZ-F12 and cc-pVXZ (X = T, Q) basis sets and are extrapolated to the complete basis set (CBS) limit. The DLPNO-CCSD(T)/CBS method is proven to reproduce the CCSD(T)/CBS energies within 0.5 kJ mol-1 and this method is selected as the benchmark for all of the rotors in this study. Rotational constants B(χ) are computed for each method based on the optimized geometries for the hindrance potential via the I(2,3) approximation. Thereafter, the V(χ) and B(χ) values are used to compute hindered internal rotation partition functions, QHR, as a function of temperature. The uncertainties in the V(χ), B(χ) and QHR calculations stem from the use of different DFT methods for the internal rotor treatment are discussed for these newly formed rotors. For rotors formed by + C2 alkenes/alkynes, the V(χ) and QHR values calculated using DFT methods are compared with the DLPNO-CCSD(T)/CBS results and analysed according to reaction types. Based on comparisons of the DFT methods with the benchmarking method, reliable DFT methods are recommended for the treatment of internal rotors for different reaction types considering both accuracy and computational cost. This work, to the authors' knowledge, is the first to systematically benchmark hindrance potentials which can be used to estimate uncertainties in theoretically derived rate constants arising from the choice of different electronic structure methods.

A Systematic Theoretical Kinetics Analysis for the Waddington Mechanism in the Low-Temperature Oxidation of Butene and Butanol Isomers

The Waddington mechanism, or the Waddington-type reaction pathway, is crucial for low-temperature oxidation of both alkenes and alcohols. In this study, the Waddington mechanism in the oxidation chemistry of butene and butanol isomers was systematically investigated. Fundamental quantum chemical calculations were conducted for the rate constants and thermodynamic properties of the reactions and species in this mechanism. Calculations were performed using two different ab initio solvers: Gaussian 09 and Orca 4.0.0, and two different kinetic solvers: PAPR and MultiWell, comprehensively. Temperature- and pressure-dependent rate constants were performed based on the transition state theory, associated with the Rice Ramsperger Kassel Marcus and master equation theories. Temperature-dependent thermochemistry (enthalpies of formation, entropy, and heat capacity) of all major species was also conducted, based on the statistical thermodynamics. Of the two types of reaction, dissociation reactions were significantly faster than isomerization reactions, while the rate constants of both reactions converged toward higher temperatures. In comparison, between two ab initio solvers, the barrier height difference among all isomerization and dissociation reactions was about 2 and 0.5 kcal/mol, respectively, resulting in less than 50%, and a factor of 2–10 differences for the predicted rate coefficients of the two reaction types, respectively. Comparing the two kinetic solvers, the rate constants of the isomerization reactions showed less than a 32% difference, while the rate of one dissociation reaction (P1 ↔ WDT12) exhibited 1–2 orders of magnitude discrepancy. Compared with results from the literature, both reaction rate coefficients (R4 and R5 reaction systems) and species’ thermochemistry (all closed shell molecules and open shell radicals R4 and R5) showed good agreement with the corresponding values obtained from the literature. All calculated results can be directly used for the chemical kinetic model development of butene and butanol isomer oxidation.

An ab Initio/Transition State Theory Study of the Reactions of Ċ5H9 Species of Relevance to 1,3-Pentadiene, Part II: Pressure Dependent Rate Constants and Implications for Combustion Modeling

The temperature- and pressure-dependence of rate constants for several radicals and unsaturated hydrocarbons reactions (1,3-C₅H₈/1,4-C₅H₈/cyC₅H₈ + Ḣ, C₂H₄ + Ċ₃H₅-a, C₃H₆ + Ċ₂H₃) are analyzed in this paper. The abstraction reactions of these systems are also calculated and compared with available literature data. Ċ₅H₉ radicals can be produced via Ḣ atom addition reactions to the pentadiene isomers and cyclopentene, and also by H-atom abstraction reactions from 1- and 2-pentene and cyclopentane. Comprehensive Ċ₅H₉ potential energy surface (PES) analyses and high-pressure limiting rate constants for related reactions have been explored in part I of this work ( J. Phys. Chem. A 2019, 123 (22), 9019-9052). In this work, a chemical kinetic model is constructed based on the computed thermochemistry and high-pressure limiting rate constants from part I, to further understand the chemistry of different C₅H₈ molecules. The most important channels for these addition reactions are discussed in the present work based on reaction pathway analyses. The dominant reaction pathways for these five systems are combined together to generate a simplified Ċ₅H₉ PES including nine reactants, 25 transition states (TSs), and nine products. Spin-restricted single point energies are calculated for radicals and TSs on the simplified PES at the ROCCSD(T)/aug-cc-pVTZ level of theory with basis set corrections from MP2/aug-cc-pVXZ (where X = T and Q). Temperature- and pressure-dependent rate constants are calculated using RRKM theory with a Master Equation analysis, with restricted energies used for minima on the simplified Ċ₅H₉ PES and unrestricted energies for other species, over a temperature range of 300-2000 K and in the pressure range 0.01-100 atm. The rate constants calculated are in good agreement with those in the literature. The chemical kinetic model is updated with pressure-dependent rate constants and is used to simulate the species concentration profiles for Ḣ atom addition to cyclopentane and cyclopentene. Through detailed analyses and comparisons, this model can reproduce the experimental measurements of species qualitatively and quantitatively with reasonably good agreement.

Ab Initio/Transition-State Theory Study of the Reactions of Ċ5H9 Species of Relevance to 1,3-Pentadiene, Part I: Potential Energy Surfaces, Thermochemistry, and High-Pressure Limiting Rate Constants

In this study, the reactions of Ċ₅H₉ radicals are theoretically investigated, with a particular emphasis on hydrogen atom addition reactions to 1,3-pentadiene (C₅H₈) to form Ċ₅H₉ radicals, although the subsequent isomerization and decomposition reactions of the Ċ₅H₉ radicals are also of direct relevance to the radicals formed from the pyrolysis and oxidation of species including pentene and cyclopentane. Moreover, H-atom abstraction reactions by hydrogen atoms from 1,3-pentadiene are also investigated. The geometries and frequencies of 63 potential energy surface (PES) minima and 88 transition states are optimized at the ωB97XD/aug-cc-pVTZ level of theory. Spin-unrestricted open-shell single-point energies for all the species are calculated at the CCSD(T)/aug-cc-pVTZ level of theory with basis set corrections from MP2/aug-cc-pVXZ (where X = T and Q). A one-dimensional hindered rotor treatment is employed for torsional modes, with the M06-2X/6-311++G(D,P) method used to compute the potential energy as a function of the dihedral angle. The high-pressure limiting rate constants and the thermochemical properties for C₅ species are calculated using the Master Equation System Solver (MESS) with conventional transition-state theory and comparisons made with existing available literature data. A hydrogen atom can add to the terminal carbon atom of 1,3-pentadiene to form the 2,4-Ċ₅H₉ radical and/or the internal carbon atoms to form 2,5-Ċ₅H₉, 1,4-Ċ₅H₉, and 1,3-Ċ₅H₉ radicals. Among the four entrance channels for Ḣ atom addition reactions, the formation of 2,4-Ċ₅H₉ and 1,3-Ċ₅H₉ radicals is more exothermic in comparison to the other Ċ₅H₉ isomers (2,5-Ċ₅H₉, 1,4-Ċ₅H₉) because of the resonantly stabilized allylic structure. Consequently, the formation of the former is generally dominant in terms of barrier heights. Ḣ atom addition reactions to 1,3-pentadiene are compared to available C₃-C₅ alkenes and dienes, with external addition calculated to be kinetically favored over internal addition. However, the correlation between heats of formation and energy barriers for Ḣ atom addition to 1,2-dienes is different from that for 1,3- and 1,4-dienes. Hydrogen atom addition and abstraction rate constants are also compared for 1,3-pentadiene, with addition found to be dominant. The subsequent unimolecular reactions on the Ċ₅H₉ PES are found to be highly complex with reactions taking place on a multiple-well multiple-channel PES. For clarity, the reaction mechanism and kinetics of each Ċ₅H₉ radical are discussed individually in terms of the computed enthalpy of the reaction and activation, the transition-state structure/reaction class, and also in terms of the combustion species for which the reactions are of potential importance. The reactions on the Ċ₅H₉ PES are divided into three reaction classes (H-shift isomerization, cycloaddition, and β-scission reactions), and the reactivity-structure-based estimation rules for energy barriers are derived for these three reaction classes and compared to literature results for alkyl radicals.

Theoretical, Experimental, and Modeling Study of the Reaction of Hydrogen Atoms with 1- and 2-Pentene

Alkyl radicals are prominent in combustion chemistry as they are formed by hydrocarbon decomposition or from a radical attack on hydrocarbons. Accurate determinations of the thermochemistry and kinetics of their unimolecular isomerization and decomposition reactions and related addition reactions of alkenes are therefore important in simulating the combustion chemistry of virtually all hydrocarbon fuels. In this work, a comprehensive potential energy surface (PES) for Ḣ-atom addition to and abstraction from 1- and 2-pentene, and the subsequent C-C and C-H β-scission reactions, and H-atom transfer reactions has been considered. Thermochemical values for the species on the Ċ₅H₁₁ PES were calculated as a function of temperature (298-2000 K), with enthalpies of formation determined using a network of isodesmic reactions. High-pressure limiting and pressure-dependent rate constants were calculated using the Rice-Ramsperger-Kassel-Marcus theory coupled with a one-dimensional master equation. As a validation of our theoretical results, hydrogen atomic resonance absorption spectrometry experiments were performed on the Ḣ-atom addition and abstraction reactions of 1- and 2-pentene. By incorporating our calculations into a detailed chemical kinetic model (AramcoMech 3.0), excellent agreement with these experiments is observed. The theoretical results are further validated via a comprehensive series of simulations of literature data. Our a priori model is found to reproduce important absolute species concentrations and product ratios reported therein.

Insights into the Reactions of Hydroxyl Radical with Diolefins from Atmospheric to Combustion Environments

Hydroxyl radicals and olefins are quite important from a combustion and an atmospheric chemistry standpoint. Large amounts of olefinic compounds are emitted into the earth's atmosphere from both biogenic and anthropogenic sources. Olefins make a significant share in transportation fuels (e.g., up to 20% by volume in gasoline), and they appear as important intermediates during hydrocarbon oxidation. As olefins inhibit low-temperature heat release, they have caught some attention for their applicability in future advanced combustion engine technology. Despite their importance, the literature data for the reactions of olefins are quite scarce. In this work, we have measured the rate coefficients for the reaction of hydroxyl radicals (OH) with several diolefins, namely 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, and 1,4-pentadiene, over a wide range of experimental conditions ( T = 294-468 K and p ∼ 53 mbar; T = 881-1348 K and p ∼ 1-2.5 bar). We obtained the low- T data in a flow reactor using laser flash photolysis and laser-induced fluorescence (LPFR/LIF), and the high- T data were obtained with a shock tube and UV laser-absorption (ST/LA). At low temperatures, we observed differences in the reactivity of cis- and trans-1,3-pentadiene, but these molecules exhibited similar reactivity at high temperatures. Similar to monoolefins + OH reactions, we observed negative temperature dependence for dienes + OH reactions at low temperatures-revealing that OH-addition channels prevail at low temperatures. Except for the 1,4-pentadiene + OH reaction, which shows evidence of significant H-abstraction reactions even at low-temperatures, other diolefins studied here almost exclusively undergo addition reaction with OH radicals at the low-temperature end of our experiments; whereas the reactions mainly switch to hydrogen abstraction at high temperatures. These reactions show complex Arrhenius behavior as a result of many possible chemical pathways in such a convoluted system.