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Rate constants for the H abstraction from alkanes (R–H) by R ′ O 2 radicals: A systematic study on the impact of R and R′
Abstract
A possible source of chain-branching in low temperature combustion is thermal decomposition of alkyl hydroperoxides (R′OOH). One way these species can be produced is via H atom abstraction reactions from alkanes (RH) by alkylperoxy radicals R′O2.An earlier study focussing on the abstraction from ethane by HO2, CH3O2 and C2H5O2 revealed that these reactions have a noticeable impact on calculated ignition times of ethane/O2 mixtures.Another outcome was that the abstraction rate constants for CH3O2 and C2H5O2 are virtually identical but smaller than that for HO2.The associated activation energies followed an Evans–Polanyi relationship while a common A-factor could be used to describe the kinetics of all three reactions within a factor of about 2–3.In this current study, we extend the investigation by (1) considering a set of alkanes(RH=CH4, C2H6, C3H8, C4H10) and (2) by including additional peroxy species (R′O2 with R′=H, CH3, C2H5, C3H7, C4H9, HCO, and CH3CO). We present rate constants for a total of 32 reactions and analyze the data with respect to systematic trends in the reactivity. The results reveal that the rate constants decrease in the order acylperoxy>HO2>alkylperoxy. The reactivity of different C–H bonds follows the bond strengths. Overall the heat of reaction is found to be the dominant but not the only rate constant controlling parameter. The accuracy of the calculations and implications of the results are discussed.
... In a modeling study of ethanol/gasoline blending, Cheng et al. 16 found that hydrogen abstraction reactions from fuels by methylperoxy radicals are important in the temperature range 700−1000 K. According to the comprehensive mechanism developed for iso-octane oxidation by Curran et al. 17 at high pressures the CH 3 Ȯ2 radicals stabilize, and hydrogen atom abstraction reactions by CH 3 Ȯ2 are important at 1000 K. 17 In the study of CH 4 /DME blends by Burke et al., 18 hydrogen atom abstraction by CH 3 Ȯ2 radicals was found to be important in the temperature range 706−1250 K. Carstensen et al. 19,20 calculated the rate constants for the abstraction of hydrogen atoms by CH 3 Ȯ2 radicals from ethane, propane, and iso-butane at the CBS-QB3//B3LYP/CBSB7 level of theory. Yang et al. ...
... At 2000 K the rate constant for abstraction from the tertiary carbon site is 5.2 times higher compared to the primary carbon site, and that for abstraction from the secondary carbon site is 3.6 times higher than that from the primary carbon site. In addition, the present work compares our rate constants (solid lines) for abstraction by CH 3 Ȯ2 radicals from ethane, propane, and iso-butane with those published previously by Carstensen et al. 20 (dashed lines). The comparisons in Figure 8a−c show that the rate constants calculated by Carstensen et al. are slightly higher than our calculations, mainly from the different computational methods used. ...
... In their calculations, the results were obtained at the CBS-QB3//B3LYP/CBSB7 level of theory. 20 The rate constants for abstraction reactions (on a per hydrogen atom basis) from monoalkenes are reported in Figure 9. The rate constants for abstraction from the α carbon sites (red lines) are highest, with the lowest being those for abstraction from the vinylic site (black lines). ...
The detailed kinetic properties of hydrogen atom abstraction by methylperoxy (CH3Ȯ2) radicals from alkanes, alkenes, dienes, alkynes, ethers, and ketones are systematically studied in this work. Geometry optimization, frequency analysis, and zero-point energy corrections were performed for all species at the M06-2X/6-311++G(d,p) level of theory. The intrinsic reaction coordinate calculation was consistently performed to ensure that the transition state connects the correct reactants and products, and one-dimensional hindered rotor scanning results were performed at the M06-2X/6-31G level of theory. The single-point energies of all reactants, transition states, and products were obtained at the QCISD(T)/CBS level of theory. High-pressure-limit rate constants of 61 reaction channels were calculated using conventional transition state theory with asymmetric Eckart tunneling corrections over the temperature range of 298.15-2000 K. Reaction rate rules for H atom abstraction by CH3Ȯ2 radicals from fuel molecules with different functional groups are constructed, which can be used in the development of combustion models of these fuels and fuel types. In addition, the influence of the functional groups on the internal rotation of the hindered rotor is also discussed.
... 17 Subsequently, they extended the investigation by considering more alkanes including methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), and n-butane (C 4 H 10 ) abstracted by additional peroxy species (RO 2 with R = H, CH 3 , C 2 H 5 , C 3 H 7 , C 4 H 9 , HCO, and CH 3 CO) by using the same computational methods. 18 It was concluded that the structure of fuels shows a large effect on the rate constants, indicating that systematic studies on this reaction class by including more fuel molecules with enough structural diversity are desired. Hashemi et al. performed high-level theoretical calculations for the abstraction reaction of CH 4 with CH 3 O 2 , which exhibits high sensitivity during high-pressure oxidation of CH 4 . ...
... Overall, the computed results for the studied reaction class reveal that the energy barriers at the same types of carbon atoms in the molecules are close to each other, and the reactivity trends are also in good consistent with previous studies. 17,18,48,52,53 3.3. Rate Constants. ...
... However, under high-temperature 19 Both the computed energy barrier and frequency results exhibit large influence on the computed results. The uncertainty of the energy information from CBS-QB3 tends to be larger than the other methods; thus, it is expected that a large uncertainty exists for the results by Carstensen et al. 18 It can be seen that the results for R4, i.e., abstraction reaction of ethane with CH 3 O 2 , by Carstensen et al. 18 are also significantly higher than those of the present work. Similar trends are also found for abstraction reactions of propane and n-butane as shown in Figure 2. ...
The hydrogen atom abstraction by the methyl peroxy radical (CH3O2) is an important reaction class in detailed chemical kinetic modeling of the autoignition properties of hydrocarbon fuels. Systematic theoretical studies are performed on this reaction class for H2/C1-C4 fuels, which is critical in the development of a base model for large fuels. The molecules include hydrogen, alkanes, alkenes, and alkynes with a carbon number from 1 to 4. The B2PLYP-D3/cc-pVTZ level of theory is employed to optimize the geometries of all of the reactants, transition states, and products and also the treatments of hindered rotation for lower frequency modes. Accurate benchmark calculations for abstraction reactions of hydrogen, methane, and ethylene with CH3O2 are performed by using the coupled cluster method with explicit inclusion of single and double electron excitations and perturbative inclusion of triple electron excitations (CCSD(T)), the domain-based local pair-natural orbital coupled cluster method (DLPNO-CCSD(T)), and the explicitly correlated CCSD(T)-F12 method with large basis sets. Reaction rate constants are computed via conventional transition state theory with quantum tunneling corrections. The computed rate constants are compared with literature values and those employed in detailed chemical kinetic mechanisms. The calculated rate constants are implemented into the recently developed NUIGMECH1.1 base model for kinetic modeling of ignition properties.
... Plot of the rate constant for C 3 H 8 + HO 2 → CH 3 CH 2 CH 2 + H 2 O (R4a). The current value is compared with experimental data from Handford-Styring and Walker [36] and predictions of Simmie and coworkers [37] and Deutschmann and coworkers [38] . Fig. 2. Plot of the rate constant for C 3 H 8 + HO 2 → CH 3 CHCH 3 + H 2 O (R4b). ...
... Fig. 2. Plot of the rate constant for C 3 H 8 + HO 2 → CH 3 CHCH 3 + H 2 O (R4b). The current value is compared with experimental data from Handford-Styring and Walker [36] and predictions of Simmie and coworkers [37] and Deutschman and coworkers [38] . based predictions of Deutschmann and coworkers [38] appear to substantially overestimate the abstraction rates. ...
... The current value is compared with experimental data from Handford-Styring and Walker [36] and predictions of Simmie and coworkers [37] and Deutschman and coworkers [38] . based predictions of Deutschmann and coworkers [38] appear to substantially overestimate the abstraction rates. ...
The oxidation properties of propane have been investigated by conducting experiments in a laminar flow reactor at a pressure of 100 bar and temperatures of 500-900 K. The onset temperature for reaction increased from 625 K under oxidizing conditions to 725 K under reducing conditions. A chemical kinetic model for high pressure propane oxidation was established, with particular emphasis on the peroxide chemistry. The rate constant for the important abstraction reaction C3H8 + HO2 was calculated theoretically. Modeling predictions were in satisfactory agreement with the present data as well as shock tube data (6-61 bar) and flame speeds (1-5 bar) from literature.
... Hydrogen-abstraction from ethane by HO 2 (R5) becomes increasingly important with decreasing temperature and increasing and reinterpreted in the present work with an updated value of k HO 2 +HO 2 [62] . Lines denote calculated rate constants from Carstensen and Dean [34] , Carstensen et al. [63] , and Aguilera-Iparraguirre et al. [64] . The rate constant from Carstensen et al. [63] was divided by a factor of 10 to correct for a typo in the A-factor. ...
... Lines denote calculated rate constants from Carstensen and Dean [34] , Carstensen et al. [63] , and Aguilera-Iparraguirre et al. [64] . The rate constant from Carstensen et al. [63] was divided by a factor of 10 to correct for a typo in the A-factor. ...
... The results are compared with theoretical rate constants for C 2 H 6 + HO 2 on Fig. 3 . Carstensen and coworkers calculated k 5 both from estimation rules [34] and from CBS-QB3 ab initio calculations [34,63] . More recently, Aguilera-Iparraguirre et al. [64] investigated reaction R5 at the CCSD(T) level but adjusted the results according to higher level benchmark calculations on CH 4 + HO 2 . ...
Ethane oxidation at intermediate temperatures and high pressures has been investigated in both a laminar flow reactor and a rapid compression machine (RCM). The flow-reactor measurements at 600–900 K and 20–100 bar showed an onset temperature for oxidation of ethane between 700 and 825 K, depending on pressure, stoichiometry, and residence time. Measured ignition delay times in the RCM at pressures of 10–80 bar and temperatures of 900–1025 K decreased with increasing pressure and/or temperature. A detailed chemical kinetic model was developed with particular attention to the peroxide chemistry. Rate constants for reactions on the C2H5O2 potential energy surface were adopted from the recent theoretical work of Klippenstein. In the present work, the internal H-abstraction in CH3CH2OO to form CH2CH2OOH was treated in detail. Modeling predictions were in good agreement with data from the present work as well as results at elevated pressure from literature. The experimental results and the modeling predictions do not support occurrence of NTC behavior in ethane oxidation. Even at the high-pressure conditions of the present work where the C2H5 + O2 reaction yields ethylperoxyl rather than C2H4 + HO2, the chain branching sequence branching is not competitive, because the internal H-atom transfer in CH3CH2OO to CH2CH2OOH is too slow compared to thermal dissociation to C2H4 and HO2.
... Consequently, the CVT/SCT/HR and Eckart/HR rates are very close one to another and their separate plots would be hardly distinguishable. The CVT/SCT/HR rate is slightly larger than those computed by Aguilera-Iparraguirre (Aguilera-Iparraguirre et al., 2008) as well as those recommended by Baulch et al. (Baulch et al., 1992), but is significantly smaller than those proposed by Carstensen et al. (Carstensen and Dean, 2007) in the whole temperature range. To the best of our knowledge, there are no direct measurements of the rates of reaction R 1 . ...
... This facilitates RC-TST as an effective tool to be utilized in the automated mechanism generation at a reasonable cost. The noticeable difference is observed only for rates of Carstensen et al. (Carstensen and Dean, 2007). This observation holds for all computations, thus their rates seem to be systematically overestimated. ...
A Reaction Class Transition State Theory (RC-TST) is applied to calculate thermal rate constants for hydrogen abstraction by OOH radical from alkanes in the temperature range of 300–2500 K. The rate constants for the reference reaction C2H6 + ∙OOH → ∙C2H5 + H2O2, is obtained with the Canonical Variational Transition State Theory (CVT) augmented with the Small Curvature Tunneling (SCT) correction. The necessary parameters were obtained from M06-2X/aug-cc-pVTZ data for a training set of 24 reactions. Depending on the approximation employed, only the reaction energy or no additional parameters are needed to predict the RC-TST rates for other class representatives. Although each of the reactions can in principle be investigated at higher levels of theory, the approach provides a nearly equally reliable rate constant at a fraction of the cost needed for larger and higher level calculations. The systematic error is smaller than 50% in comparison with high level computations. Satisfactory agreement with literature data, augmented by the lack of necessity of tedious and time consuming transition state calculations, facilitated the seamless application of the proposed methodology to the Automated Reaction Mechanism Generators (ARMGs) programs.
... Figure 17 presents rate constants surveyed for R131. Rate constants of R131 were obtained experimentally by Baldwin et al. [91] and theoretically by Carstensen et al. (TST) [92] and Aguilera-Iparraguirre et al. (TST) [93] . The rate constant calculated by Aguilera-Iparraguirre et al. [93] was a factor of 3 lower than that calculated by Carstensen et al. [92] at 1500 K. ...
... Rate constants of R131 were obtained experimentally by Baldwin et al. [91] and theoretically by Carstensen et al. (TST) [92] and Aguilera-Iparraguirre et al. (TST) [93] . The rate constant calculated by Aguilera-Iparraguirre et al. [93] was a factor of 3 lower than that calculated by Carstensen et al. [92] at 1500 K. It agrees well with the experimentally obtained results reported by Baldwin et al. [91] . ...
To examine methane oxidation at intermediate temperatures (ca., 900–1200 K), chemiluminescence observation and laser-induced fluorescence (LIF) measurements for CH2O and OH were conducted for methane weak flames in a micro flow reactor with a controlled temperature profile (MFR) at atmospheric and elevated pressures. Locations of CH2O–LIF, chemiluminescence, and OH-LIF in MFR were arranged from the low temperature side at 1.0 and 5.0 bar. Spatial separation of methane oxidation was successfully demonstrated. One-dimensional computations with five detailed kinetic mechanisms were conducted. Computational profiles of CH2O molar concentration, heat release rate (HRR), and OH molar concentration normalized by their own peak values were compared with experimentally obtained intensity profiles of the CH2O–LIF, chemiluminescence, and OH-LIF. Computational results obtained with AramcoMech 1.3 showed better agreements with experimentally obtained results among the mechanisms employed. However, the flame position computed with AramcoMech 1.3 showed a slightly higher temperature region than the experimental flame position, indicating underprediction of methane reactivity. Sensitivity analysis identified a set of dominant reactions for weak flame positions. Rate constants of the identified reactions were modified within uncertainty to reproduce experimentally obtained weak flame positions. The modified mechanism also well predicted ignition delay times and flame speeds, and significant improvement of prediction was identified particularly for ignition delay times of lowest temperature and pressure investigated. Reaction path analysis highlighted the importance of intermediate-temperature oxidation chemistry for methane such as CH3→CH3O2→CH3O→CH2O reactions at higher pressures. Two-stage oxidation of methane was observed by chemiluminescence observation and numerical simulations at higher pressures (6.0–10.0 bar).
... The following reaction steps are dominated by oxygen addition, to form oxygenated species like alcohols, aldehydes, ethers etc., and further decomposition steps like abstraction of H atoms or hydrocarbons. H-abstraction reactions of different hydrocarbons were the focus of several theoretical [5][6][7][8][9][10] and experimental studies [11][12][13][14][15][16][17][18] . Khaled et al. have investigated the reactions of linear alkenes [11] and i-butene [12] with OH radicals in shock tubes and showed that the reaction rate coefficients are dependent on the chain length and the position of the double bond, that is, the constitutional isomer. ...
... Extensive studies of these reactions at different alkane H sites were performed by Carstensen et al . [7] and Badra et al . [8] . ...
The radicals produced by hydrogen abstraction in the initial fuel decomposition step are essential in combustion kinetics, but their experimental detection is very challenging. Imaging photoelectron photoion coincidence spectroscopy enables the detection and identification of even these isomeric radicals. Laminar low-pressure (40 mbar) hydrogen flames doped with different alkanes and alkenes are investigated systematically with the goal to identify the formation pathways and the fate of fuel radicals formed in hydrogen abstraction reactions. The abstraction reactions of primary, secondary, tertiary, and vinylic H atoms were never target of a systematic, direct semiquantitative investigation in a flame environment and this paper describes such a study for the first time. Performing the measurements at the vacuum ultraviolet beamline located at the Swiss Light Source enables isomer-selective detection of reactive radical species by imaging photoelectron photoion coincidence spectroscopy. For unambiguous identification of several isomeric radicals, threshold photoelectron spectra were compared with reference photoelectron spectra. H-abstraction ratios of isomeric radicals were determined and compared to literature reaction barriers and rate coefficients. In addition to the quantitative information, the peak positions of the profiles of radicals formed by hydrogen abstraction or addition to the fuel molecules as function of distance from the burner show faster H-abstraction for unbranched alkanes and alkenes than for branched fuels and faster H-addition than H-abstraction, respectively.
... For the transformation of keto-enol and enol-rotomar processes, rate coefficients were estimated in temperature range 273-333 K and calculated for the unimolecular reactions (k uni , in s −1 ) using the transition state theory (TST) [23][24][25] at the high pressure (HP) limits. TST was recently used in many advanced studies [26][27][28][29][30][31][32] and can be obtained from Eq. (1). ...
The tautomerizations mechanism of 4-(methylsulfanyl)-3[(1Z)-1-(2-phenylhydrazinylidene) ethyl] quinoline-2(1H)-one were inspected in the gas phase and ethanol using density function theory (DFT) M06-2X and B3LYP methods. Thermo-kinetic features of different conversion processes were estimated in temperature range 273-333 K using the Transition state theory(TST) accompanied with one dimensional Eckert tunneling correction(1D-Eck). Acidity and basicity were computed as well, and the computational results were compared against the experimental ones. Additionally, NMR, global descriptors, Fukui functions, NBO charges, and electrostatic potential (ESP) were discussed. From thermodynamics analysis, the keto form of 4-(methylsulfanyl)-3-[(1Z)-1-(2 phenylhydrazinylidene) quinoline-2(1H)-one is the most stable form in the gas phase and ethanol and the barrier heights required for tautomerization process were found to be high in the gas phase and ethanol 38.80 and 37.35 Kcal/mol, respectively. DFT methods were used for UV-Vis electronic spectra simulation and the time-dependent density functional theory solvation model (TDDFT-SMD) in acetonitrile compounds.
... 18,19 Generally, C-H bonds can be activated by the oxygencentered radicals present in the pool of reactive oxygen species; however, but these reactions are not selective. [20][21][22][23][24][25][26] A different concept to achieve C-H activation is by metal insertion. In organometallic chemistry, it is well known that C-H bonds can be activated intramolecularly by a strong electrophilic metal that is already bonded to another part of the organic molecule, forming an ''agostic'' bond, forming a three-center, two-electron bond. ...
Although C-H functionalization is one of the simplest reactions, it requires the use of highly active and selective catalysts. Recently, C-H-active transformations using porous materials such as crystalline metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) as well as amorphous porous-organic polymers (POPs) as new emerging heterogeneous catalysts have attracted significant attention due to their promising activity and potential material tunability. These porous solids offer exceptional structural uniformity, facile tunability and permanent porosity. In addition, tuning the catalytic selectivity of these porous materials can be achieved through engineering their site microenvironments, such as metal node substitution, linker changes, node/linker functionalization, and pore modification. The present review provides an overview of the current state of the art on MOFs, COFs and POPs as advanced catalysts for various C-H bond activation reactions, providing details about their chemo-, regio-, and stereo-selectivity control, comparing their performance with that of other catalysts, triggering additional research by showing the present limitations and challenges in this area, and providing a perspective for future developments.
... As shown in Fig. 10, with the increase of temperature up to 950 K, the reactions of H-atom abstraction from the nPCH by the HO2 radical show high sensitivity promoting reactivity, as these reactions consume HO2 radical to form H2O2, followed by its decomposition to produce two highly reactive OH radicals. Carstensen et al. [55] investigated rate constants for the H-atom abstractions from alkanes by a series of ROO radicals (R = H, CH3, C2H5, C3H7, C4H9, HC=O, and CH3C=O) at the CBS/QB3//B3LYP/CBSB7 level of theory systematically, and provided rate rules for primary/secondary/tertiary C-H abstractions by the HO2 radical. Aguilera-Iparraguirre et al. [56] carried out an accurate benchmark calculation of the reaction barrier height between HO2 radical and methane, which was used to correct those for HO2+alkanes CnH2n+2 with n = 2-4 calculated at the CCSD/cc-pVTZ level of theory. ...
n-Propylcyclohexane (nPCH) is an important surrogate component for jet fuel, gasoline, and diesel. To comprehensively understand its combustion properties and chemical kinetics, ignition delay time (IDT) measurements of nPCH/air mixtures were performed in a high-pressure shock tube (HPST) at fuel-rich conditions (φ = 2.0), pressures of 10‒40 bar and temperatures of 738‒1400 K. Also, low-temperature IDT measurements were carried out in a rapid compression machine (RCM) at a pressure of 10 bar, temperatures of 615‒750 K, and equivalence ratios of 0.5‒2.0. In addition, laminar flame speeds were measured at an initial temperature of 403 K, at pressures of 1.01 bar and 3.04 bar, and equivalence ratios ranging from 0.7 to 1.4. A detailed chemical kinetic mechanism has been developed in the current work to describe the oxidation of nPCH, including 10 high-temperature reaction classes and 24 low-to-intermediate temperature reaction classes. Important reactions were identified by sensitivity and flux analyses at different temperatures, pressures, and equivalence ratios. These reaction classes play a very important role in determining the fuel reactivity and the distribution of products. This model shows good agreement with the experiment measurements carried out in this work and the ones in the literature, including IDTs, species data from jet-stirred reactor and flow reactor experiments, and laminar flame speeds.
... The individual atomic enthalpies DH f X i ð Þ are extracted from the NIST WebBook. 43 Rate coefficients for the unimolecular decomposition (barrier reactions) of MePr are calculated using transition state theory (TST) [44][45][46][47][48][49] ...
This work reports on the thermochemistry and kinetics of methyl propanoate (MePr) initial pyrolysis using high ab initio multi-level composite W1 method over the temperature range 400-2000 K. Pyrolysis of MePr was simulated using ten complex bond fission reactions (R1-R10) and seven simple bond fission pathways (R11-R17). Rate coefficients of MePr decomposition have been estimated using transition state theory (TST) combined with tunneling through one-dimensional Eckart barrier (Eck). Statistical Rice–Ramsperger–Kassel–Marcus (RRKM) tight theory has been also used in a pressure range of 0.001-100 atm. Our estimated rate coefficients are in a strong agreement with previous literatures and strongly pressure-dependent especially at high temperatures. In addition, thermodynamic parameters for MePr and some species involved in its decomposition reactions have been computed and compared with previous literatures.
... While hydrogen abstraction by atomic hydrogen (H) [177,21] has been extensively studied because the high reaction rate constants and the abundance of H make it central in HACA pathways [52,53,57], quantum chemistry calculations were also carried out over decades for hydrogen abstraction reactions by other species, such as atomic oxygen (O) [24], oxygen molecule (O 2 ) [89], hydroxyl radical (OH) [122], hydroperoxyl radical (HO 2 ) [22,7], methyl radical (CH 3 ) [78], and vinyl radical (C 2 H 3 ) [134]. These reactions, as well as the hydrogen dissociation [119], are included in this mechanism to handle different gas-phase environments. ...
Understanding the formation of polycyclic aromatic compounds (PACs) in combustion not only bridges the knowledge gap between the small gas-phase species and incipient soot particles, but may also help address the global emission issues of both PACs and soot. In this thesis, I present a kinetic mechanism utilizing reactive sites (i.e., the chemical and physical neighbourhoods) to describe the PAC growth in combustion. This kinetic mechanism was implemented for a stochastic modeling code (i.e., SNapS2) recently developed by the Violi Group. To address new experimental and computational discoveries, chemical reactions were gathered and categorized from various literature, while the reaction rate constants came from either literature or my own calculations to ensure full reversibility and thermodynamic consistency. These reactions were then implemented into SNapS2 kinetic mechanism with precise reactive site definitions to eliminate the possibility of steric hindrance and unrealistic reactions. Compared with the previous version of SNapS2, thanks to this new kinetic mechanism, the computational performance increased by an order of magnitude, enabling the simulation of complex two-dimensional flames. Some missing reaction pathways, which were identified from experimental evidence and simulations but not available in the literature, were explored and calculated using quantum chemistry methods. These newly discovered reactions were included in the SNapS2 kinetic mechanism as well, and some of them were already proven to be important under specific conditions. The characteristics of the PACs predicted with the kinetic mechanism were compared against different experimental measurements: mass spectra measured in a counterflow diffusion flame, the oxygen-to-carbon ratios obtained at different locations of a coflow diffusion flame, and the molecular structures observed in a premixed laminar flame. These successful validations demonstrate that the SNapS2 kinetic mechanism provides a high-fidelity, and yet generic, description of the PAC formation under various combustion conditions, making SNapS2 the first-of-its-kind to have such extensive flexibility and wealth of information. It greatly contributes to reveal the underlying chemical pathways to the experimental observations. Furthermore, SNapS2 code and the kinetic mechanism have shown its capability to provide valuable insights on the formation of aromatics beyond the limitation of diagnostics. For one application, spatial dependence of the PAC growth in an ethylene counterflow diffusion flame was characterized by SNapS2 simulations, revealing distinct PAC growth pathways for the streamlines starting from fuel side and oxidizer side. Given the fidelity of the SNapS2 predictions, it was also used to examine conditions that are impossible to test experimentally, like completely decoupling the effects of flame temperature when studying the effects of ethanol doping on the formation of aromatics, highlighting the chemical pathways that result in soot reduction. Both applications show the uniqueness and great potential of the model to obtain insights of the PAC formation when measurements are hard to obtain or experiments are difficult to control. Altogether, this dissertation lays a solid foundation that not only helps explain the experimental observations for the formation of soot precursors, but also provides a powerful tool for exploring the gas-phase nanoparticle growth that could drive the development of novel combustion technologies or the design of new nanomaterials.
... The chemistry is chiefly dominated by the formation and oxidations of alkylperoxy radicals, particularly, oxidation of HCs at low to intermediate temperature. 42 Guo and Wang 43 reported that more than The effects of SOI timing in the low temperature reaction species for DDC2 mode are shown in Figure 7. As discussed earlier, the progress of LTHR regime improves with advanced SOI timing and the complementary reduction has been noticed in the oxygen mass. ...
In this work, the influence of direct dual fuel injection on a compression ignition engine fueled with gasoline and diesel has been investigated. To do this, closed cycle combustion simulations have been performed. Gasoline has been supplied through port injection and early direct and late direct injection to achieve fuel stratification and emission reduction. Simulations have been done for various start of injection (SOI) timings of diesel fuel. A detailed discussion on a low temperature heat release (LTHR) mechanism has been done. Results revealed that the maximum gross indicated thermal efficiency (GITE) of 39% is obtained for port injection of gasoline mode. Direct dual fuel combustion (type 2) (DDC2) mode shows approximately 2% and 38% less GITE and oxides of nitrogen (NOx), respectively, and 40% more soot as compared to the port injection gasoline mode. DDC2 mode shows lower oxides of carbon and hydrocarbon emissions as compared to other dual fuel modes. More than 99% of combustion efficiency and less maximum pressure rise rate have been noticed in the DDC2 case. Strong LTHR and high temperature premixed combustion region have been found in advanced SOI timing cases (in DDC2). In-cylinder contours for the DDC2 case show that diesel and gasoline fuels combusted successively cause less in-cylinder temperature than that for the conventional dual fuel combustion case.
... Guided by the experimental studies [27,38,39] over temperature range 563-651 K, transition state theory (TST) was used to determine the unimolecular canonical high pressure limiting rate coefficients using the CBS-QB3 approach [70][71][72][73] ...
... Unimolecular reaction kinetics of compounds 1-4 were estimated in the high-pressure limit with the KiSThelP program [43] according to the transition state theory (TST). With this approach, kinetic studies on the decomposition reactions are given by [44,45]: ...
The gas-phase β-elimination kinetics of 2,2-difluoroethyltrifluorosilane (1), 2,2-difluoroethylmethyldifluorosilane (2), 2,2-difluoroethyldimethylfluorosilane (3), and 2,2-difluoroethyltrimethylsilane (4) have been investigated computationally using M06-2x exchange-correlation functional as well as the benchmark CBS-QB3 quantum chemical approach. The obtained energy profile has been enhanced with kinetic calculations using statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory and transition state theory (TST). The calculated results are in good agreement with the available experimental data which obtained by the CBS-QB3 approach. The comparison between all our calculations and experiments indicates that a thermodynamically-controlled reaction that gives more stable products derived from the compound 2 species will be the vinyl fluoride and methyltrifluorosilane species, whereas the elimination of compound 1 into the vinyl fluoride and silicon tetrafluoride species is favorable process from kinetic point of view. In proportion to rather larger barrier heights, pressures where P > 10―4 bar are insufficient to ensure a saturation of the calculated rate constant compared with the RRKM unimolecular rate kinetics (in high-pressure limit). Natural bond orbital analysis revealed that in accordance with an increase of barrier height from compounds 1 to 4, the HOMO-LUMO energy-gaps decreases. Furthermore, the obtained order of barrier heights could be explained by the number of electron-withdrawing fluorine atoms attached to the silicon atom. The occupancies of σC1―F3 bonding orbital for the studied compounds are as follows: 1>2>3>4 and those of σ*C1―F3 antibonding orbital increase in the opposite order (4>3>2>1) by NBO analysis. This fact explains a comparatively easier elimination of the σC1―F3 bond in compound 1 compared to the other compounds. The calculated data reveal that the polarization of the C1―F3 bond in the sense C1δ⁺–F3δ⁻ is the determining factor in the elimination reaction of the studied compounds.
... A computational study on the kinetics of unimolecular reactions can be performed by means of conventional transition state theory. With this approach the rate constants for decomposition reactions can be obtained as follows [34][35][36]: where k B , h and R are the Boltzmann's, Planck's and ideal gas constants, respectively. In this equation, r denotes a symmetry number, and j(T) is the Wigner's tunneling correction factor [37]. ...
The thermal decomposition kinetics of 2-chloroethylsilane and derivatives in the gas phase has been studied computationally using density functional theory, along with various exchange-correlation functionals (UM06-2x and ωB97XD) and the aug-cc-pVTZ basis set. The calculated energy profile has been supplemented with calculations of kinetic rate constants under atmospheric pressure and in the fall-off regime, using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Activation energies and rate constants obtained using the UM06-2x/aug-cc-pVTZ approach are in good agreement with the experimental data. The decomposition of 2-chloroethyltriethylsilane species into the related products [C2H4 + Et3SiCl] is characterized by 6 successive structural stability domains associated to the sequence of catastrophes C8H19SiCl: 6-C†FCC†[FF]-0: C6H15SiCl + C2H4. Breaking of Si–C bonds and formation of Si–Cl bonds occur in the vicinity of the transition state.
... This in turn indicates that, the values of H # strongly correlate with the estimates of BDH across the three abstraction radicals. Moreover, least-squares slopes are within the expected range of those of the Evans-Polanyi plots (0.0-1.0), and in accord with analogous values obtained for H abstraction from alkanes by RO 2 species, i.e., 0.60-0.65 [34]. Figure 6 compares Arrhenius plots for abstraction of secondary H by the three title radicals from EA, ethanol, and propane. ...
Ethylamine (EA) often serves as a surrogate species to represent aliphatic amines that occur in biofuels. This contribution reports, for the first time, the thermochemical and kinetic parameters for bimolecular reactions of EA with three prominent radicals that form in the initial stages of biomass decomposition; namely, H, CH 3 and NH 2. Abstraction of a methylene H atom from the EA molecule largely dominates H loss from the two other sites (i.e., methyl and amine hydrogens) for the three considered radicals. We demonstrate that, differences in bond dissociation enthalpies of methylene C–H bonds among EA, ethanol and propane reflect their corresponding HOMO/LUMO energy gaps. At low and intermediate temperatures, the rate of H abstraction from the methylene site in EA exceeds the corresponding values for propane and ethanol. As the temperature rises, matching entropic factors induce comparable rate constants for the three molecules.
... The β-scission and isomerization kinetics on the 2-butanoyl PES were predicted with a RRKM/Master Equation (ME) approach using the PAPER software package [19] . The energy transfer and the collision frequency were modeled according to Carstensen and Dean [20] with an average energy transfer of E down (T ) = 200 cm −1 (T / 300 K ) 0 . 85 , Lennard-Jones parameters from tert -C 4 H 9 Cl for 2-butanoyl and Argon as bath gas [21] . ...
2-butanone was recently identified as a promising gasoline biofuel. Its kinetic modeling requires high-level kinetic predictions of key reactions to reduce the uncertainty in the reaction model. The present work provides rate constants for hydrogen abstraction from 2-butanone by and . Subsequent unimolecular reactions on the 2-butanoyl radical potential energy surface were studied using RRKM/Master Equation. The updated rate constants deviate from previous predictions by up to two orders of magnitude.
We have employed automated mechanism generation tools to construct a detailed chemical kinetic model for combustion of n-pentane, as a step toward the generation of compact kinetic models for larger alkanes. Pentane is of particular interest as a prototype for combustion of alkanes and as the smallest paraffin employed as a hybrid rocket fuel, albeit at cryogenic conditions. A reaction mechanism for pentane combustion thus provides a foundation for modeling combustion of extra-large alkanes (paraffins) that are of more practical interest as hybrid rocket fuels, for which manual construction becomes infeasible. Here, an n-pentane combustion kinetic model is developed using the open-source software package Reaction Mechanism Generator (RMG). The model was generated and tested across a range of temperatures (650 to 1350 K) and equivalence ratios (0.5, 1.0, 2.0) at pressures of 1 and 10 atm. Available thermodynamic and kinetic databases were incorporated wherever possible. Predictions using the mechanism were validated against the published laminar burning velocities (Su) and ignition delay times (IDT) of n-pentane. To improve the model performance, a comprehensive analysis, including reaction path and sensitivity analyses of n-pentane oxidation, was performed, leading us to modify the thermochemistry and rate parameters for a few key species and reactions. These were combined as a separate data set, an RMG library, that was then used during mechanism generation. The resulting model predicted IDTs as accurately as the best manually constructed mechanisms, while remaining much more compact. It predicted flame speeds to within 10% of published experimental results. The degree of success of automated mechanism generation for this case suggests that it can be extended to construct reliable and compact models for combustion of larger n-alkanes, particularly when using this mechanism as a seed submodel.
The oxidation of n ‐butane at elevated pressures has been investigated by experiments in a laminar flow reactor at 100 bar and temperatures of 450–900 K. The onset temperature for reaction increased from 550 K under oxidizing conditions (Φ = 0.02) to 625 K under reducing conditions (Φ = 13). NTC behavior was observed at 600–650 K (Φ = 0.02) and 625–675 K (Φ = 1.0). A detailed chemical kinetic model for the oxidation of n ‐butane was established. The present model and those suggested in literature were evaluated against the present experimental results and literature data at elevated pressures. None of the tested models could accurately reproduce the NTC behavior of n ‐butane under stoichiometric conditions of the present study, but all evaluated models could reproduce experimental data from literature with different levels of accuracy.
The combustion chambers of direct‐fired supercritical CO 2 power plants operate at pressures of approximately 300 bar and CO 2 dilutions of up to 96%. The rate coefficients used in existing chemical kinetic mechanisms are validated for much lower pressures and much smaller concentrations of CO 2 . Recently, the UoS sCO 2 1.0 and UoS sCO 2 2.0 mechanisms have been developed to better predict ignition delay time (IDT) data from shock tube studies at pressures from 1 to 260 bar in various CO 2 ‐containing bath gas compositions. The chemistry of the methyldioxy radical (CH 3 O 2 ) has been identified as an essential combustion intermediate for methane combustion above 100 bar, where mechanisms missing this species begin to vastly overpredict the IDT. The current literature available on CH 3 O 2 is very limited and often concerned with atmospheric chemistry and low‐pressure, low‐temperature combustion. This means that the rate coefficients used in UoS sCO 2 2.0 are commonly determined at sub‐atmospheric pressures and temperatures below 1000 K with some rate coefficients being over 30 years old. In this work, the rate coefficients of new potential CH 3 O 2 reactions are added to the current mechanism to create UoS sCO 2 2.1 It is shown that the influence of CH 3 O 2 on the IDT is greatest at high pressures and low temperatures. It has also been demonstrated that CO 2 has very little effect on the chemistry of CH 3 O 2 at 300 bar meaning that CH 3 O 2 rate coefficients can be determined in other bath gases, reducing the impact of non‐ideal effects such as bifurcation when studying in a CO 2 bath gas. The updated UoS sCO 2 2.1 mechanism is then compared to high‐pressure IDT data and the most important reactions which require reinvestigation have been identified as the essential next steps in understanding high‐pressure methane combustion.
It remains a long-standing vacancy and challenge of predicting accurate reaction kinetics for H-atom abstractions by HO2 radical from cycloalkanes and cyclic alcohols, which plays a fundamental role in both...
Chemical kinetic studies of hydrogen atom abstraction reactions by hydroperoxyl (HȮ2) radical from six alkyl cyclohexanes of methyl cyclohexane (MCH), ethyl cyclohexane (ECH), n-propyl cyclohexane (nPCH), iso-propyl cyclohexane (iPCH), sec-butyl cyclohexane (sBCH), and iso-butyl cyclohexane (iBCH) are carried out systematically through high-level ab initio calculations. Geometry optimizations and frequency calculations for all species involved in the reactions are performed at the M06-2X/6-311++G(d,p) level of theory. Electronic single-point energy calculations are calculated at the UCCSD(T)-F12a/cc-pVDZ-F12 level of theory, with zero-point energy corrections. High-pressure limit rate constants for the reactions of alkyl cyclohexanes + HȮ2, in the temperature range of 500-2000 K, are calculated using conventional transition state theory taking asymmetric Eckart tunneling corrections and the one-dimensional hindered rotor approximation into consideration. Elementary reaction rate constants and branching ratios for each alkyl cyclohexane species were investigated, and rate constant rules of primary, secondary, and tertiary sites on the side-chain and the ring are provided here. Additionally, temperature-dependent thermochemical properties for reactants and products were also obtained in this work. The updated kinetics and thermochemistry data are used in the alkyl cyclohexane mechanisms to investigate their effects on ignition delay time predictions of shock tube and rapid compression machine data, and species concentrations from a jet-stirred reactor. It is found that these investigated reactions promote ignition delay times in the temperature range of 800-1200 K and also improve the prediction of cyclic olefin species formation which stems from the decomposition of fuel radicals.
Hydrogen atom abstraction by methyl peroxy (CH3OȮ) radicals can play an important role in gasoline/ethanol interacting chemistry for fuels that produce high concentrations of methyl radicals. Detailed kinetic reactions for hydrogen atom abstraction by CH3OȮ radicals from the components of FGF-LLNL (a gasoline surrogate) including cyclopentane, toluene, 1-hexene, n-heptane, and isooctane have been systematically studied in this work. The M06–2X/6–311++G(d,p) level of theory was used to obtain the optimized structure and vibrational frequency for all stationary points and the low-frequency torsional modes. The 1-D hindered rotor treatment for low-frequency torsional modes was treated at M06–2X/6–31G level of theory. The UCCSD(T)-F12a/cc-pVDZ-F12 and QCISD(T)/CBS level of theory were used to calculate single point energies for all species. High pressure limiting rate constants for all hydrogen atom abstraction channels were performed using conventional transition state theory with unsymmetric tunneling corrections. Individual rate constants are reported in the temperature range from 298.15 to 2000 K. Our computed results show that the abstraction of allylic hydrogen atoms from 1-hexene is the fastest at low temperatures. When the temperature increases, the hydrogen atom abstraction reaction channel at the primary alkyl site gradually becomes dominant. Thermodynamics properties for all stable species and high-pressure limiting rate constants for each reaction pathway obtained in this work were incorporated into the latest gasoline surrogate/ethanol model to investigate the influence of the rate constants calculated here on model predicted ignition delay times.
The gas-phase decomposition kinetics of isopropyl acetate (IPA) and its methyl, bromide and hydroxyl derivatives into the corresponding acid and propene were investigated using density functional theory (DFT) with the ωB97XD and M06–2x functionals, as well as the benchmark CBS-QB3 composite method. Transition state theory (TST) and RRKM theory calculations of rate constants under atmospheric pressure and in the fall-off regime were
used to supplement the measured energy profiles. The results show that the formation of propene and bromoacetic acid is the most dominant pathway at the CBS-QB3 method, both kinetically and thermodynamically. There was a good agreement with experimental results. Pressures greater than 0.01 bar, corresponding to larger barrier heights are insufficient to ensure saturation of the measured rate coefficient when compared to the RRKM kinetic rates.
Natural bond orbitals (NBO) charges, bond orders, bond indices, and synchronicity parameters all point to the considered pathways taking place via a homogenous, first-order concerted, as well as an asynchronous mechanism involving a non-planar cyclic six-membered transition state. The calculated data exhibit that the elongation of the Cα−O bond length and subsequent polarization of the Cα+δ…O−δ bond is the rate-determining step of the considered reactions in the cyclic transition state, which appears to be involved in this type of reaction.
Very scarce studies aiming at the low-temperature chemistry of multi-alkylated cycloalkanes were reported until now, which widely exist in real transportation fuels. Thus, this work presents the first study on the low-temperature oxidation characteristics of 1,3,5-trimethylcyclohexane (T135MCH) in an atmospheric jet-stirred reactor (JSR) combined with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at fuel-lean (ϕ = 0.5), stoichiometric (ϕ = 1.0) and fuel-rich (ϕ = 1.5) conditions. Abundant quantitative information of species is acquired including reactants, major products, reactive hydroperoxides, fuel-specific cycloalkenes and other C1–C5 oxygenated intermediates. Pronounced three-stage oxidation behavior was observed including two fuel decomposition processes and one negative-temperature-coefficient (NTC) window. A detailed model incorporating both high and low-temperature reaction scheme was proposed and validated against the measured data with reasonable agreement. Representative large oxygenated intermediates were detected in this work, such as cyclic ether (CE), cyclohexylhydroperoxide (ROOH), ketohydroperoxide (KHP), olefinic hydroperoxide (OFHP) and C9 acyclic/cyclic bi-carbonyl species whose signal profiles were further compared with the simulated mole fraction profiles justifying the occurrence of specific low-temperature chemistry. Based on the modeling analysis, the competition between the chain-branching processes forming OH and chain-propagating processes forming less reactive HO2 or CE was found to determine the distinct oxidation behavior at different temperature regions. As temperature rises, chain-branching processes are inhibited and the accumulation of HO2 is accelerated, which lead to the occurrence of self-combination forming H2O2, a chain-termination process, resulting in the NTC behavior. With the arrival of high-temperature region, H2O2 becomes thermally unstable and tends to dissociate to double OH via a chain-branching process, therefore triggering the second fuel decomposition via H-atom abstractions.
The extending material durability of poly(lactic acid) (PLA)/styrene-isoprene-styrene copolymer (SIS) blend with 30% of SIS by the presence of acrylic acid (AA) and two stabilizers, caffeic acid or vanillic acid, is evaluated by γ-irradiation at technological doses (25 or 50 kGy) applied for sterilization or for the manufacturing of several commodities, medical, and packaging items, respectively, where an advanced exposure would affect the post-irradiation oxidation stability. The chemiluminescence investigations provide the onset oxidation temperature values from which the activation energies of degradation were calculated. Differential scanning calorimetry and infrared spectroscopy complete the evaluation of improvement efficiencies that characterize the thermal behavior of these materials. The effects of γ-exposure on the material properties are discussed relative to the radiolysis modifications occurred during radiation processing. The relevance of oxidation protectors is analyzed based on the efficient delay of ageing. The results are the qualification information for the packaging and medical products subjected to the radiation processing and radiation sterilization.
This paper discusses a brief history of chemical kinetic modeling, with some emphasis on the development of chemical kinetic mechanisms describing fuel oxidation. At high temperatures, the important reactions tend to be those associated with the H2/O2 and C1–C2 sub-mechanisms, particularly for non-aromatic fuels. At low temperatures, and for aromatic fuels, the reactions that dominate and control the reaction kinetics are those associated with the parent fuel and its daughter radicals. Strategies used to develop and optimize chemical kinetic mechanisms are discussed and some reference is made to lumped and reduced mechanisms. The importance of accurate thermodynamic parameters for the species involved is also highlighted, as is the little-studied importance of collider efficiencies of different third bodies involved in pressure-dependent reactions.
Our understanding of fuel oxidation has improved with rigorous experimental and theoretical investigations being performed in recent years. As investigation methods evolve, our understanding of fundamental fuel chemistry advances. This process allows us to revisit and improve our existing chemical kinetic models. Propane and propene have been studied in various facilities at different conditions; however, the interaction of these two species has not been explored well. These two species play a crucial role in the oxidation of larger hydrocarbons and constitute a significant fraction of liquefied petroleum gas. The current study involves an experimental investigation of ignition delay time measurements for neat propene and propane/propene (50%/50%) mixtures in a rapid compression machine for a range of pressures (20–80 bar). These auto-ignition experiments are complemented by the measurement of stable intermediate species mole fraction profiles at 750 K for the non-diluted stoichiometric condition at 40 bar and 50 bar. The experimental output from this study has contributed to the development of NUIGMech1.0 at high-pressure conditions for mixtures that are relevant to engine applications. NUIGMech1.0 is utilized for the kinetic analysis, and its performance is also compared with two other relevant mechanisms. The kinetic analysis is used to understand the fundamental chemistry controlling fuel oxidation and provide updates of the chemical kinetic mechanism. Additionally, the critical reaction pathways and sensitive reactions that lead to the intermediate species that control reactivity are explained in detail. It is found that cross-reactions from both the propane and propene sub-mechanisms play a crucial role in controlling the reactivity of the mixtures. NUIGMech1.0 captures the reactivity and speciation data for the neat components and shows good predictions of the mixtures at the conditions studied.
Impact of water (H2O) vitiation on auto-ignition characteristics of kerosene/air mixtures was investigated behind the reflected shock waves covering pressures of 0.45–7.5 atm and temperatures of 900–1450 K. Arrhenius-type expressions were fitted for both mixtures with and without H2O using multiple linear regression method. Temperature- and pressure-dependences of ignition delay times were experimentally observed to be in-line with conventional hydrocarbons for all the test mixtures, but stronger pressure-dependence exhibited when presence of H2O. A four-component surrogate model fuel (25.7% n-tetradecane/23.0% 2-methylundecane/42.1% n-butylcyclohexane/9.2% n-butylbenzene) was proposed based on similarity criterion of function group. A surrogate mechanism was subsequently assembled by incorporating the modified rate rule of certain reaction class. The proposed kinetic model reproduces well the experimental observations at the entire conditions. Thermal and kinetic effects of H2O on the RP-3 reactivity were distinguished by factitiously inducing a weak collision H2O* and an inert H2O**. Results reveal that the kinetic-based promotion of H2O outweighs the thermal-based inhibition on the RP-3 reactivity at high pressures due to higher collision efficiency, which facilitates the reaction H2O2 (+M) ⟺ OH + OH (+M). However, the RP-3 reactivity is inhibited only by thermodynamics without kinetics at low pressures.
The kinetics of the thermal isomerization of trimethylsilylcyclopropane in the temperature range of 689.5–751.1 K have been theoretically studied using Rice-Ramsperger-Kassel-Marcus (RRKM) theory and transition state theory (TST) in conjugation with CBS-QB3 calculations. Three possible reaction pathways are identified. Among them, the three-membered ring opening and hydrogen atom transfer to the carbon atom bonded to the SiMe3 group and formation of the allyltrimethylsilane is the main reaction. Our calculated kinetic rate constants appear to be in excellent agreement with the available experimental data. The results show that the most abundant product derived from Trimethylsilylcyclopropane will be the (Z)-1-propenyltrimethylsilane under thermodynamic control, while the most favorable process is isomerization reaction of that reactant into the allyltrimethylsilane from a kinetic viewpoint. The regioselectivity of the reaction decreases with decreasing pressures and increasing temperatures. In proportion to greater barrier heights, pressures P > 10−4 bar are in general enough for confirming saturation of the calculated rate coefficients compared with the high-pressure limit of the RRKM rates.
Uncertainty analysis is a useful tool for inspecting and improving detailed kinetic mechanisms because it can identify the greatest sources of model output error. Owing to the very nonlinear relationship between kinetic and thermodynamic parameters and computed concentrations, model predictions can be extremely sensitive to uncertainties in some parameters while uncertainties in other parameters can be irrelevant. Error propagation becomes even more convoluted in automatically generated kinetic models, where input uncertainties are correlated through kinetic rate rules and thermodynamic group values. Local and global uncertainty analyses were implemented and used to analyze error propagation in Reaction Mechanism Generator (RMG), an open‐source software for generating kinetic models. A framework for automatically assigning parameter uncertainties to estimated thermodynamics and kinetics was created, enabling tracking of correlated uncertainties. Local first‐order uncertainty propagation was implemented using sensitivities computed natively within RMG. Global uncertainty analysis was implemented using adaptive Smolyak pseudospectral approximations as implemented in the MIT Uncertainty Quantification Library to efficiently compute and construct polynomial chaos expansions to approximate the dependence of outputs on a subset of uncertain inputs. Cantera was used as a backend for simulating the reactor system in the global analysis. Analyses were performed for a phenyldodecane pyrolysis model. Local and global methods demonstrated similar trends; however, many uncertainties were significantly overestimated by the local analysis. Both local and global analyses show that correlated uncertainties based on kinetic rate rules and thermochemical groups drastically reduce a model's degrees of freedom and have a large impact on the determination of the most influential input parameters. These results highlight the necessity of incorporating uncertainty analysis in the mechanism generation workflow.
Kinetic rate constants for the oxidation reaction of the hydroxyl radical with CH 3 SH, C 2 H 5 SH, n-C 3 H 7 SH, and iso-C 3 H 7 SH under inert (Ar) conditions over the temperature range 252–430 K have been studied theoretically using density functional theory along with various exchange–correlation functionals as well as the benchmark CBS-QB3 quantum chemical approach.
Bimolecular rate constants were estimated using transition state theory and the statistical Rice–Ramsperger–Kassel–Marcus theory. Comparison with experiment confirms that in the OH addition reaction pathways leading to the related products, the first bimolecular reaction steps have effective negative activation energy barriers. Effective rate constants have been calculated according to a steady-state analysis of a two-step model reaction mechanism. As a consequence of the negative activation energies, pressures higher than 10 ⁴ bar are required to reach the high-pressure limit. Both from thermodynamic and kinetic viewpoints, the most favorable process here is the oxidation reaction of hydroxyl radicals with n-C 3 H 7 SH.
The pyrolysis and oxidation of dimethyl ether (DME) and its mixture with methane were investigated at high pressure (50 and 100 bar) and intermediate temperature (450–900 K). Mixtures highly diluted in nitrogen with different fuel–air equivalence ratios (Φ=∞, 20, 1, 0.06) were studied in a laminar flow reactor. At 50 bar, the DME pyrolysis started at 825 K and the major products were CH4, CH2O, and CO. For the DME oxidation at 50 bar, the onset temperature of reaction was 525 K, independent of fuel–air equivalence ratio. The DME oxidation was characterized by a negative temperature coefficient (NTC) zone which was found sensitive to changes in the mixture stoichiometry but always occurring at temperatures of 575–625 K. The oxidation of methane doped by DME was studied in the flow reactor at 100 bar. The fuel–air equivalence ratio (Φ) was varied from 0.06 to 20, and the DME to CH4 ratio changed over 1.8–3.6%. Addition of DME had a considerable promoting effect on methane ignition as the onset of reaction shifted to lower temperatures by 25–150 K. A detailed chemical kinetic model was developed by adding a DME reaction subset to a model developed in previous high-pressure work. The model was evaluated against the present data as well as data from literature. Additional work is required to reconcile experimental and theoretical work on reactions on the CH3OCH2OO PES with ignition delay measurements in the NTC region for DME.
The recently proposed natural orbital functional second-order Møller–Plesset (NOF-MP2) method is capable of achieving both dynamic and static correlations even for those systems with a significant multiconfigurational character. We test its reliability to describe the electron correlation in radical formation reactions, namely in the homolytic X–H bond cleavage of LiH, BH, CH4,NH3,H2O and HF molecules. Our results are compared with CASSCF and CASPT2 wavefunction calculations and the experimental data. For a dataset of 20 organic molecules, the thermodynamics of C–H homolytic bond cleavage, in which the C–H bond is broken in the presence of different chemical environments, is presented. The radical stabilization energies obtained for such general dataset are compared with the experimental data. It is observed that NOF-MP2 is able to give a quantitative agreement for dissociation energies, with a performance comparable to that of the accurate CASPT2 method.
Laminar burning velocities of fuel mixtures of methane/ethane/propane with the compositions 100/0/0, 80/20/0, 80/0/20 and 80/10/10 vol% burning with air were determined experimentally using the heat flux method at 1 atm and initial gas temperatures 298, 318 and 338 K. The mixtures were selected as surrogates for natural gas, with the aim to investigate the effect of heavier hydrocarbons on the laminar burning velocity of the main component, methane. It was found, in agreement with the literature data, that the heavier hydrocarbons increase laminar burning velocity compared to that of methane + air flames. A common correlation for the temperature dependence of the burning velocity S L = S L0 (T/T 0 ) α , where T 0 is a reference temperature and S L0 is the laminar burning velocity at this temperature, was used to interpret new measurements. The power exponents, α, were derived from the experimental data for methane and three surrogates for natural gas. It was found that the temperature dependence of the burning velocities is practically identical for all mixtures studied. The measurements have been compared with the modelling using two kinetic schemes: recent version of the Aramco mech 2.0 and an updated version of a model developed by the authors. Both kinetic mechanisms show systematic trends in slight over- and under-prediction of the burning velocities, respectively, for all fuel blends. However, the temperature dependence of the burning velocities is accurately reproduced by these two models. Further analysis indicates that even though rate constants of the reactions determining flame propagation are somewhat different for the two mechanisms, the power exponents α are not sensitive to the differences. This indicates that, detailed kinetic schemes capable in predicting burning velocities at a specific initial mixture temperature are able to operate at higher temperatures as well, at least from lean to moderately rich natural gas mixtures.
Kinetic rate constants for the oxidation reactions of OH radicals with CH3SH (1), C2H5SH (2), n-C3H7SH (3) and iso-C3H7SH (4) under inert conditions (Ar) over the temperature range 252−430 K have been studied using the CBS-QB3 composite method. Kinetic rate constants under atmospheric pressure and in the fall-off regime have been estimated using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Comparison with experiment confirms that in the OH-addition pathways 1−4 leading to the related products, the first bimolecular reaction step has effective negative activation energies around −2.61 to 3.70 kcal mol⁻¹. Effective rate coefficients have been calculated according to a steady-state analysis of a two-step model reaction mechanism. As a result of the negative activation energies, pressures larger than 10⁴ bar would be required to restore to some extent the validity of this approximation for all the channels. By comparison with experimental data, all our calculations for both the OH-addition and H-abstraction reaction pathways indicate that from a kinetic viewpoint and in line with the computed reaction energy barriers, the most favourable process is the OH-addition pathway to n-C3H7SH to yield the [n-C3H7SH−OH]• species, whereas under thermodynamic control of the bimolecular reactions (R−SH+OH•), the most abundant product derived from the H-abstraction pathway will be the [n-C3H7 S•+H2O] species.
The mechanism and kinetics of the reaction of hydrogen sulphide (H2S(¹A1)) with hydroperoxyl radical (HO2(²A″)) on the lowest doublet potential energy surface have been theoretically studied. The potential energy surface for possible pathways has been investigated by employing Complete Basis Set (CBS), DFT, and CCSD(T) methods. Three possible pathways are suggested for the title reaction. The most probable entrance channel consists of formation of a hydrogen-bonded pre-reaction complex (vdw1) and two energised intermediates. Multichannel RRKM-Steady State Approximation and CVT calculations have been carried out to compute the rate constants over a broad range of temperature from 200 K to 3000 K to cover the atmospheric and combustion conditions and pressure from 0.1 to 2000 Torr. No sign of pressure dependence was observed for the title reaction over the stated range of pressure. We have shown that the major products of the title reaction are H2O2 and SH while at higher temperatures, formation of the other products such as H2O, HOS, HSOH and OH are feasible, too. Our calculated overall rate constant is in agreement with the reported experimental data in the literature.
Based on the recently constructed neural-network potential energy surface [Chen et al., J. Chem. Phys. 138, 221104 (2013)], ring-polymer molecular dynamics (RPMD) calculations are performed to compute rate coefficients of the barrierless OH + CO system at T ≤ 500 K. To recover the barrierless feature, a Lindemann-Hinshelwood-type mechanism and hence a reduced rate coefficient are used to approximate the overall rate coefficient. An agreement between RPMD and experimental rate coefficients can be found. These RPMD results reproduce correctly the temperature-independence of the overall rate coefficient. Finally, potential sources of errors in the present RPMD calculations are discussed.
An ab initio and direct dynamic study on the reactions of CH3O2 + CH3OH and CH3O2 + CH2OH has been carried out over the temperature range of 300 – 1500 K. All stationary points were calculated at MP2/Aug-cc-pVTZ level of theory for CH3O2 + CH3OH or at M06-2X/MG3S level of theory for CH3O2 + CH2OH and identified for local minimum. The energetic parameters were refined at QCISD (T)/cc-pVTZ and CCSD (T)/aug-cc-pVTZ levels of theory. For the reaction of CH3OO + CH3OH, two hydrogen abstraction channels producing CH3OOH+CH2OH (R1) and CH3OOH+CH3O (R2) were confirmed. These two channels consist of the same reversible first step involving the formation of a prereactive complex in the entrance channel. The rate constants of these two channels have been calculated by canonical transition station theory (TST) and canonical variational transition station theory (VTST) with Eckart tunneling correction, and compared with the available literature data. The positive temperature dependence of the rate constants was observed. The tunneling effect is important at low temperature and decreases with the increase of the temperature. The contribution of R1 to the total rate constant is dominant with branching ratios of 0.93 at 500 K and 0.67 at 1000 K, although the branching ratio for R2 increases dramatically with the increase of the temperature from 500 K. For the reaction of CH3OO + CH2OH, eight channels were explored on the lowest singlet and triplet surfaces and an excited intermediate was found to be formed on the singlet surface. A channel proceeding through the formation of excited intermediate followed by its impulsive dissociation was confirmed as the dominant channels with the branching ratio more than 0.99 at the temperature range of 300 – 1500 K, where products of CH3O and OCH2OH were given. The rate constant of the dominant channel calculated by multichannel RRKM-VTST is comparable with the available literature data.
The thermal decomposition kinetics of allyl methyl amine, allyl methyl ether, and allyl methyl sulfide in the gas phase has been studied theoretically using the M06-2x/aug-cc-pVTZ quantum chemical approach. The observed activation parameters are consistent with a concerted unimolecular mechanism involving a non-planar cyclic six-membered transition state. Based on the optimized ground state geometries, a natural bond orbital analysis of donor–acceptor interactions reveals that the stabilization energies corresponding to the electronic delocalization from the lone-pair (LP) non-bonding orbitals on the heteroatom to the neighboring \(\sigma_{{{\text{C2}} - {\text{C3}}}}^{*}\) antibonding orbitals decrease from allyl methyl amine to allyl methyl sulfide. This delocalization fairly explains the increase of occupancies of LP orbitals on the heteroatom from allyl methyl sulfide to allyl methyl amine. The results also suggest that the kinetics of the thermolysis of the studied compounds are dominated by \({\text{LP}}\, \to \,\sigma^{*}\) electronic delocalization effects. Analysis of bond order, bond indices, and synchronicity parameters demonstrates that these reactions proceed through a concerted and slightly asynchronous mechanism.
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The β-elimination kinetics of 2,2-dihaloethyltrihalosilanes in the gas phase has been studied computationally using density functional theory (DFT) along with the M06-2x exchange–correlation functional and the aug-cc-pVTZ basis set. The calculated energy profiles have been supplemented with calculations of rate constants under atmospheric pressure and in the fall-off regime, by means of transition state theory (TST), variational transition state theory (VTST), and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Activation energies and rate constants obtained using the M06-2x/aug-cc-pVTZ approaches are in good agreement with the available experimental data. Analysis of bond order, natural bond orbitals, and synchronicity parameters suggests that the β-elimination of the studied compounds can be described as concerted and slightly asynchronous. The transition states of these reactions correspond to four-membered cyclic structures. Based on the optimized ground state geometries, a natural bond orbital (NBO) analysis of donor–acceptor interactions also show that the resonance energies related to the electronic delocalization from \(\sigma_{{{\text{C}}_{ 1} {-}{\text{C}}_{ 2} }}\) bonding orbitals to \(\sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}\) antibonding orbitals, increase from 2,2-difluoroethyltrifluorosilane to 2,2-dichloroethyltrichlorosilane and then to 2,2-dibromoethyltriboromosilane. The decrease of \(\sigma_{{{\text{C}}_{ 1} {-}{\text{C}}_{ 2} }}\) bonding orbitals occupancies and increase of the \(\sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}\) antibonding orbitals occupancies through \(\sigma_{{{\text{C}}_{ 1} - {\text{C}}_{ 2} }} \to \sigma^{*}_{{{\text{C}}_{ 2} - {\text{Si}}_{ 3} }}\) delocalizations could facilitate the β-elimination of the 2,2-difluoroethyltrifluorosilane compound, compared to 2,2-dichloroethyltrichlorosilane and 2,2-dibromoethyltriboromosilane.
A scarcity of known chemical kinetic parameters leads to the use of many reaction rate estimates, which are not always sufficiently accurate, in the construction of detailed kinetic models. To reduce the reliance on these estimates and improve the accuracy of predictive kinetic models, we have developed a high-throughput, fully automated, reaction rate calculation method, AutoTST. The algorithm integrates automated saddle-point geometry search methods and a canonical transition state theory kinetics calculator. The automatically calculated reaction rates compare favorably to existing estimated rates. Comparison against high level theoretical calculations show the new automated method performs better than rate estimates when the estimate is made by a poor analogy. The method will improve by accounting for internal rotor contributions and by improving methods to determine molecular symmetry.
This contribution investigates thermokinetic parameters of bimolecular gas-phase reactions involving the amine (NH2) radical and a large number of saturated and unsaturated hydrocarbons. These reactions play an important role in combustion and pyrolysis of nitrogen-rich fuels, most notably biomass. Computations performed at the CBS-QB3 level and based on the conventional transition-state theory yield potential-energy surfaces and reaction rate constants, accounting for tunnelling effects and the presence of hindered rotors. In an analogy to other H abstraction systems, we demonstrate only a small influence of variational effects on the rate constants for selected reaction. The studied reactions cover the abstraction of hydrogen atoms by the NH2 radical from the C‒H bonds in C1‒C4 species, and four C5 hydrocarbons of 2-methylbutane, 2-methyl-1-butene, 3-methyl-1-butene, 3-methyl-2-butene, and 3-methyl-1-butyne. For the abstraction of H from methane, in the temperature window of 300-500 K and 1600-2000 K, the calculated reaction rate constants concur with the available experimental measurements i.e., kcalculated/kexperimetal = 0.3-2.5 and 1.1-1.4 and the previous theoretical estimates. Abstraction of H atom from ethane attains the ratio of kcalculated/kexperimetal equal to 0.10-1.2 and 1.3-1.5 over the temperature windows of available experimental measurements; i.e., 300-900 K and 1500-2000 K, respectively. For the remaining alkanes (propane and n-butane), the average kexperimental/kcalculated ratio remains 2.6 and 1.3 over the temperature range of experimental data. Also comparing the calculated standard enthalpy of reaction (ΔrHº298) with the available experimental measurements for alkanes, we found the mean unsigned error of computations as 3.7 kJ mol-1. This agreement provides an accuracy benchmark of our methodology, affording the estimation of the unreported kinetic parameters for H abstractions from alkenes and alkynes. Based on the Evans-Polanyi plots, calculated bond dissociation enthalpies (BDHs) correlate linearly with the standard enthalpy of activation (Δ#Hº298), allowing estimation of the enthalpy barrier for reaction of NH2 with other hydrocarbons in future work. Finally, we develop six sets of the generalised Arrhenius rate parameters for H abstractions from different C-H bond types. These parameters extend the application of the present results to any non-cyclic hydrocarbon interacting with the NH2 radical.
Density functional theory, along with the ωB97XD and UM06-2x exchange-correlation functional, has been used to study the reaction mechanisms and kinetics of the atmospheric oxidation of the main (kinetically dominant) thiophene-OH adduct [C4H4S-OH]• (R1) by molecular oxygen in its triplet electronic ground state. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been calculated by means of transition state theory (TST), variational transition state theory (VTST) and statistical Rice−Ramsperger−Kassel−Marcus (RRKM) theory. In line with the computed energy profiles, the dominant process under both the thermodynamic and kinetic control of the reaction is O2 addition at the C5 position in syn mode. The computed branching ratios indicate that the regioselectivity of the reaction decreases with increasing temperature and decreasing pressure.
The thermal decomposition kinetics of 2,3-epoxy-2,3-dimethylbutane have been studied computationally using density functional theory, along with various exchange-correlation functionals and an extremely large basis set. The calculated energy profiles have been supplemented with calculations of kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been supplied, using transition state theory (TST) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been supplied, using transition state and RRKM theories. By comparison with experiment, all our calculations indicate that, from a kinetic viewpoint, the most favorable process is thermal decomposition of 2,3-epoxy-2,3-dimethylbutane into the 2,3-dimethylbut-3-en-2-ol, whereas under thermodynamic control of the reactions, the most abundant product derived from the 2,3-epoxy-2,3-dimethylbutane species will be the 3,3-dimethylbutan-2-one species. The regioselectivity of the decomposition decreases with increasing temperatures and decreasing pressures. In line with rather larger energy barriers, pressures larger than 10⁻⁶ bar are in general sufficient for ensuring a saturation of the computed unimolecular kinetic rate constants compared with the high-pressure limit (TST) of the RRKM unimolecular rate constants. The bonding evolution theory indicated that thermal decomposition of 2,3-epoxy-2,3-dimethylbutane into the 2,3-dimethylbut-3-en-2-ol takes place along three differentiated successive structural stability domains after passing the reactant from the associated transition state.
Growing awareness of climate change and the risks associated with our society's dependence on fossil fuels has motivated global initiatives to develop economically viable, renewable energy sources. However, the transportation sector remains a major hurdle. Although electric vehicles are becoming more mainstream, the transportation sector is expected to continue relying heavily on combustion engines, particularly in the freight and airline industries. Therefore, research efforts to develop cleaner combustion must continue. This includes the development of more efficient combustion engines, identification of compatible alternative fuels, and the streamlining of existing petroleum resources. These dynamic systems have complex chemistry and are often difficult and expensive to probe experimentally, making detailed chemical kinetic modeling an attractive option for simulating and predicting macroscopic observables such as ignition delay or CO₂ concentrations. This thesis presents several methods and applications towards high fidelity predictive modeling using Reaction Mechanism Generator (RMG), an open source software package which automatically constructs kinetic mechanisms. Several sources contribute to model error during automatic mechanism generation, including incomplete or incorrect handling of chemistry, poor estimation of thermodynamic and kinetics parameters, and uncertainty propagation. First, an overview of RMG is presented along with algorithmic changes for handling incomplete or incorrect chemistry. Completeness of chemistry is often limited by CPU speed and memory in the combinational problem of generating reactions for large molecules. A method for filtering reactions is presented for efficiently and accurately building models for larger systems. An extensible species representation was also implemented based on chemical graph theory, allowing chemistry to be extended to lone pairs, charges, and variable valencies. Several chemistries are explored in this thesis through modeling three combustion related processes. Ketone and cyclic ether chemistry are explored in the study of diisoproyl ketone and cineole, biofuel candidates produced by fungi in the decomposition of cellulosic biomass. Detailed kinetic modeling in conjunction with engine experiments and metabolic engineering form a collaborative feedback loop that efficiently screens biofuel candidates for use in novel engine technologies. Next, the challenge of modeling constrained cyclic geometries is tackled in generating a combustion model of JP-10, a synthetic jet fuel used in propulsion technologies. The model is validated against experimental and literature data and succeeds in capturing key product distributions, including aromatic compounds, which are precursors to polyaromatic hydrocarbons (PAHs) and soot. Finally, oil-to-gas cracking processes under geological conditions are studied through modeling the low temperature pyrolysis of the heavy oil analog phenyldodecane in the presence of diethyldisulfide. This system is used to gather mechanistic insight on the observation that sulfur-rich kerogens have accelerated oil-to-gas decomposition, a topic relevant to petroleum reservoir modeling. The model shows that free radical timescales matter in low temperature systems where alkylaromatics are relatively stable. Local and global uncertainty propagation methods are used to analyze error in automatically generated kinetic models. A framework for local uncertainty analysis was implemented using Cantera as a backend. Global uncertainty analysis was implemented using adaptive Smolyak pscudospcctral approximations to efficiently compute and construct polynomial chaos expansions (PCE) to approximate the dependence of outputs on a subset of uncertain inputs. Both local and global methods provide similar qualitative insights towards identifying the most influential input parameters in a model. The analysis shows that correlated uncertainties based on kinetics rate rules and group additivity estimates of thermochemistry drastically reduce a model's degrees of freedom and can have a large impact on model outputs. These results highlight the necessity of uncertainty analysis in the mechanism generation workflow. This thesis demonstrates that predictive chemical kinetics can aid in the mechanistic understanding of complex chemical processes and contributes new methods for refining and building high fidelity models in the automatic mechanism generation workflow. These contributions are available to the kinetics community through the RMG software package.
This study reports the autoignition delay times of methyl butanoate in argon–air (i.e. Ar/O2 = 3.76 by mole) mixtures under thermodynamic conditions relevant to compression ignition engines, using a rapid compression machine (RCM). The ignition delay times were obtained for the compressed temperature range of 833–1112 K and compressed pressures corresponding to 15, 30, 45, and 75 bar. In addition, the effect of fuel mole fraction on ignition delay times were experimentally studied by covering equivalence ratios corresponding to 0.25, 0.5, and 1.0 under realistic fuel loadings without additional dilution. For the range of conditions investigated, the ignition delay times exhibit an Arrhenius dependence on temperature and an inverse relationship with the compressed pressure. No evidence of two-stage ignition was observed in the current experiments, nor was a negative temperature coefficient trend seen in the ignition delay times. The experimental results were compared to zero-dimensional simulations taking into account the full compression stroke and the heat loss characteristics of the RCM, using chemical kinetic models reported in the literature. The literature models were found to predict significantly higher reactivity when compared to the current experiments. Chemical kinetic analysis was then conducted to identify the reactions responsible for the mismatch between the experiments and the simulated results. Key reactions that could help obtain a better match between the experimental and simulated results were identified using both brute-force and global sensitivity analyses. In view of the large uncertainties associated with the low-temperature chemistry of methyl butanoate, further studies are needed to update the kinetic parameters of the key reactions in order to improve the model comprehensiveness.
The chemistry of formyl radicals plays an important role in the kinetic modeling of oxygenated hydrocarbons. Here, the fate of rovibrationally excited formic acid produced via HC(●)O + (●)OH is evaluated in a RRKM / Master Equation study. For that purpose, the HC(●)O + (●)OH potential energy surface is studied theoretically using high-level quantum mechanics. Direct reaction from HC(●)O + (●)OH to the bimolecular products is found to dominate for most relevant conditions due to formic acid well-skipping. The kinetics of these well-skipping reactions can only be evaluated when including the unimolecular intermediate, formic acid. Further, hydrogen abstraction from rovibrationally excited formic acid is found to be important at low-temperature conditions and for high radical concentrations.
The kinetics of the chemically activated reaction between the ethyl radical and molecular oxygen are analyzed using quantum Rice-Ramsperger-Kassel (QRRK) theory for k(E) with both a master equation analysis and a modified strong-collision approach to account for collisional deactivation. Thermodynamic properties of species and transition states are determined by ab initio methods at the G2 and CBS-Q//B3LYP/6-31G(d,p) levels of theory and isodesmic reaction analysis. Rate coefficients for reactions of the energized adducts are obtained from canonical transition state theory. The reaction of C2H5 with O2 forms an energized peroxy adduct with a calculated well depth of 35.3 kcal mol-1 at the CBS-Q//B3LYP/6-31G(d,p) level of theory. The calculated (VTST) high-pressure limit bimolecular addition reaction rate constant for C2H5 + O2 is 2.94 ¥ 1013T-0.44. Predictions of the chemically activated branching ratios using both collisional deactivation models are similar. All of the product formation pathways of ethyl radical with O2, except the direct HO2 elimination from the CH3CH2OO adduct, involve barriers that are above the energy of the reactants. As a result, formation of the stabilized CH3CH2OO adduct is important at low to moderate temperatures; subsequent reactions of this adduct should be included in kinetic mechanisms. The temperature and pressure dependent rate coefficients for both the chemically activated reactions of the energized adducts and the thermally activated reactions of the stabilized adducts are assembled into a reaction mechanism. Comparisons of predictions using this mechanism to experiment demonstrate the necessity of including dissociation of the stabilized ethylperoxy adduct. Two channels are particularly important, direct HO2 elimination and reverse reaction to C2H5 + O2, where the ratio of these rates is a function of temperature and pressure. The predictions, using unadjusted rate coefficients, are consistent with literature observations over extended temperature and pressure ranges. Comparison of a mechanism using 7 ¥ 3 Chebyshev polynomials to represent k(T,P) to a conventional mechanism which used k(T) only (different values for k(T) at different pressures) showed good agreement. The kinetic implications for low-temperature ignition due to the direct formation of ethylene and HO2 from ethylperoxy are discussed.
A potential barrier of the kind studied by Fowler and others may be represented by the analytic function V (Eq. (1)). The Schrödinger equation associated to this potential is soluble in terms of hypergeometric functions, and the coefficient of reflection for electrons approaching the barrier with energy W is calculable (Eq. (15)). The approximate formula, 1-ρ=exp{-∫4πh(2m(V-W))12dx} is shown to agree very well with the exact formula when the width of the barrier is great compared to the de Broglie wave-length of the incident electron, and W<Vmax.
Third-law gas-phase statistical entropies are computed for a variety of closed-shell singlet state species using standard formulae based upon canonical partition functions. Molecular parameters are determined ab initio, and sensitivity analyses are performed to determine expected accuracies. Several choices for the canonical partition function are examined for internal rotations. Three general utility procedures for calculating the entropies are developed and designated E1, E2, and E3 in order of increased accuracy. The E1 procedure adheres to the harmonic oscillator approximation for all vibrational degrees of freedom other than for very low barrier internal rotations, these being treated as free rotations, and yields entropies to an accuracy of better than 1 J mol−1 K−1 for molecules with no internal rotations. For molecules with internal rotations, errors of up to 1.8 J mol−1 K−1 per internal rotation are observed. Our E2 procedure, which treats each individual internal rotation explicitly with a simple cosine potential, yields total entropies to an accuracy of better than 1 J mol−1 K−1 for species with zero or one internal rotation, and better than 2 J mol−1K−1 for species with two internal rotation modes. Rotor–rotor coupling is found to contribute on the order of 1 J mol−1 K−1 for a third-law entropy. Our E3 procedure takes this into account and, with the aid of new ab initio two-dimensional torsional potential energy surfaces of state-of-the-art accuracy, improves the accuracy of the predicted entropy for species with two internal rotation modes to approximately 1 J mol−1 K−1.
Relative rate constants have been determined for the reactions of H atoms and HO2 radicals with toluene and ethylbenzene by adding traces of these compounds to mixtures of H2 + O2 at 773 K. The values k24t = (5.5 ± 1.5) × 104 and k24e = (1.65 ± 0.63) × 105 dm3 mol−1 s−1 are the first reliable kinetic data obtained for the abstraction of an H atom from any aromatic compound by HO2 radicals. It is shown that the values are significantly lower than expected on the grounds of enthalpy of reaction, and it is concluded the explanation lies in a combination of a lower A factor than observed with alkanes (because of the loss of entropy of activation in the emerging electron-delocalized radicals), and a slightly higher activation energy. A comprehensive database for HO2 abstraction reactions from alkanes, aromatics, alkenes, and related compounds has been assembled and recommended for use over the temperature range 600 to 1200 K. (24t)HO2+C6H5CH3 → H2O2+C6H5CH2(24e)HO2+C6H5C2H5 → H2O2+C6H5CHCH3/C6H5CH2CH2 The overall rate constants k22t=(5.0 ± 1.0) × 108 and k22e = (1.07 ± 0.25) × 109 dm3 mol−1 s−1 are obtained for H attack on toluene and ethylbenzene at 773 K. As found for the HO2 reactions with toluene and ethylbenzene, the values are lower than expected, despite the fact that at least one other pathway besides abstraction is known to occur. (22t)H+C6H5CH3 → products(22e)H+C6H5C2H5 → products
Studies have been made of the addition of CH4 to mixtures of tetramethylbutane (TMB)+ O2 at 443 °C in KCl-coated Pyrex vessels. From measurements of isobutene and formaldehyde, formed virtually as sole products from TMB and CH4, respectively, a value of k10/k1/27= 1.00×10–2(dm3 mol–1 s–1)1/2 has been obtained. HO2+ CH4→ H2O2+ CH3(10), HO2+ HO2→ H2O2+ O2(7). Computer analysis shows that this value is very sensitive to the value of k18/k9 which controls the proportion of CH4 removed by OH attack, and the optimum conditions for studying reaction (10) were carefully selected. Use of A7= 109.48 dm3 mol–1 s–1 and E7= 5.8 kJ mol–1, obtained recently, gives k10=(3.39 ± 0.78)×102 dm3mol–1s–1 at 443 °C. No other experimental determinations of this rate constant have been reported. Based on the reasonable assumption of equal A factors per C—H bond for HO2+ CH4 and HO2+ C2H6, then Arrhenius parameters of A10= 10.05 ± 0.3 dm3mol–1s–1 and E10=103.1 ± 3.5 kJ mol–1 are suggested for the temperature range 350–1000 °C. OH + TMB → H2O + C8H17(9), OH + CH4→ H2O + CH3(18)
Studies have been made of the addition of C2H6 to mixtures of tetramethylbutane (TMB)+ O2 over the temperature range 400–520 °C in both boric acid-coated and KCl-coated Pyrex vessels. From measurements of the relative yields of i-C4H8 and C2H4, formed from TMB and C2H6, respectively, values of k10/k½7 have been determined: HO2+ C2H6→ H2O2+ C2H5(10), HO2+ HO2→ H2O2+ O2. (7) By use of low total pressures (ca. 15 Torr), the contribution of OH attack to the overall formation of C2H4 has been reduced to a very low level. At temperatures above 470 °C, the values of k10/k½7 in KCl-coated vessels were noticeably above those obtained from the boric acid-coated vessels, and this has been attributed to possible gas-phase regeneration of radicals when H2O2 is removed at the KCl surface. Values of E10–½E7= 82.7 ± 1.5 kJ mol–1 and A10/A½7= 105.49 ± 0.11(dm3 mol–1 s–1)½ were obtained.
The reactions of neopentyl radicals in an oxidising environment have been studied by adding neopentane to slowly reacting mixtures of H2+ O2 over the temperature range 380–520 °C. Over a wide range of mixture composition, the only detectable initial products at these temperatures are 3,3-dimethyloxetan (DMO), acetone, i-butene, methane, and formaldehyde. A relatively simple mechanism involving the formation of neopentylhydroperoxide (QOOH) radicals gives a quantitative interpretation of the product yields. Although the major source of i-butene is the C—C homolysis of neopentyl radicals, a significant proportion is formed in reaction (6)(CH3)2C(CH2OOH)CH2→(CH3)2CCH2+ HCHO + OH. (6)
An analytical study is made of the products formed in the negative temperature coefficient zone for slow oxidation of alkanes. A clear increase in the formation of carbon oxides and olefins is obtained; these products are thought to be obtained from RCO3•, RCO•, and RO2• radicals. The results are explained by competition between two different mechanisms, namely L1 and L2. This competition explains the ratio of products obtained with various ratios. The formation of olefins is shown to be assocated with the isomerization of RO2• radicals; the disproportionation reaction R• + O2 → olefin + HO2• is likely to be of a negligible importance, except in the case of t-C4H9 radicals.
We employ Born−Oppenheimer molecular dynamics (BOMD), with forces derived from spin-polarized density functional theory using the B3LYP hybrid exchange-correlation functional, to explore the dynamics of oxidation of ethyl radical to produce ethylene, along the concerted-elimination path CH3CH2 + O2 → CH3CH2OO → CH2CH2 + HOO. The transition state connecting CH3CH2OO to CH2CH2 and HOO has a planar, five-membered-ring structure ···C−C−H−O−O··· known as TS1. The electronic nature of this saddle point has been the subject of controversy. Recent ab initio calculations have indicated that TS1 has a 2A‘ ‘ electronic ground state within Cs symmetry. In this state, intramolecular neutral hydrogen transfer from the methyl group of the intermediate ethylperoxy radical (CH3CH2OO·) to the terminal oxygen is hindered by the lack of overlap between the 1s orbital of the in-plane hydrogen atom and the singly-occupied 2p (a‘ ‘) orbital of the terminal oxygen. Previous explanations invoked proton transfer, a rather unpalatable process for an alkylperoxy radical. Two other possibilities that both facilitate neutral H-transfer are explored in the present work, namely: (i) an O2 π*-resonance mechanism and (ii) 2A‘−2A‘ ‘ state mixing. First, we show that the structure of TS1 is a “late,” loose transition state, consistent with a loosely coupled O2 that can shift π*-electrons to aid neutral hydrogen atom transfer. Second, our BOMD trajectories reveal that torsional motion in the ethylperoxy radical and at the transition state causes symmetry-breaking and 2A‘−2A‘ ‘ state mixing. The low-lying 2A‘ excited state, with its in-plane, singly occupied oxygen 2p orbital, can easily transfer a neutral H atom. Not only is vibrationally-induced symmetry-breaking present near (and after crossing) TS1, but also in the CH3CH2 and O2 entrance channel, which again exhibits torsional motion that allows both the 2A‘ ‘ ground state and the excited 2A‘ state to be accessed while forming the ethylperoxy radical. Thus we propose that vibronic state mixing is a key feature of the reaction dynamics of ethane combustion, helping to facilitate dehydrogenation.
Ab initio and density functional calculations are performed to determine thermochemical and kinetic parameters in analysis of the 2 hydroperoxy-ethyl radical association with O2. The system serves as an initial model for O2 association with higher molecular weight alkyl-hydroperoxide radicals and is an important component in the well-studied ethyl radical plus O2 reaction system. The CBS-Q//B3LYP/6-31G(d,p) and G3(MP2) composite methods are utilized to calculate energies. The well depth is determined as 35 kcal/mol and transition state results show two low energy paths (barriers below the entrance channel) for reaction to new products: (i) a HO2 molecular elimination and (ii) a hydrogen shift path. Intramolecular hydrogen transfer (five-member ring) leads to 2 hydroperoxide acetadehyde + OH, where the barrier is ca. 7 kcal/mol lower than previously estimated. The HOOCH2CH(O) formed here is chemically activated and a significant fraction dissociates to OH + formyl-methoxy radical, before stabilization. The barrier for hydrogen transfer is several kcal/mole lower than the corresponding reaction in a conventional hydrocarbon for this five-member ring transition state because the weak C−H bond on the hydroperoxide carbon. The second path is unimolecular HO2 elimination leading to a vinyl hydroperoxide + HO2. The vinyl hydroperoxide has a weak (22.5 kcal/mol) CH2CHO−OH bond and rapidly dissociates to formyl methyl plus OH radicals; a second low energy chain branching path in low-temperature HC oxidation. Kinetic analysis with falloff on chemical activation and unimolecular dissociation, illustrate that both low energy paths are competing. Results also show significant formation of a diradical, •OCH2CH2OO• + OH, an additional new path to chain branching, which results from the chemical activation reaction. The HO2 molecular elimination plus vinyl hydroperoxide dominates the H transfer by a factor of 1.8 at low temperatures, a result of its small entropy advantage. At high temperatures, dissociation to the higher energy, but loose transition state, hydroperoxide ethyl radical + O2 (back to reactants) is the dominant path.
The C2H5• + O2 reaction, central to ethane oxidation and thus of fundamental importance to hydrocarbon combustion chemistry, has been examined in detail via highly sophisticated electronic structure methods. The geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products for the reaction of the ethyl radical (X̃ 2A‘) with O2 (X 3 , a 1Δg) have been investigated using the CCSD and CCSD(T) ab initio methods with basis sets ranging in quality from double-zeta plus polarization (DZP) to triple-zeta plus double polarization with f functions (TZ2Pf). Five mechanisms (M1−M5) involving the ground-state reactants are introduced within the context of previous experimental and theoretical studies. In this work, each mechanism is systematically explored, giving the following overall 0 K activation energies with respect to ground-state reactants, Ea(0 K), at our best level of theory: (M1) direct hydrogen abstraction from the ethyl radical by O2 to give ethylene + HO2•, Ea(0 K) = +15.1 kcal mol-1; (M2) ethylperoxy β-hydrogen transfer with O−O bond rupture to yield oxirane + •OH, Ea(0 K) = +5.3 kcal mol-l; (M3) ethylperoxy α-hydrogen transfer with O−O bond rupture to yield acetaldehyde + •OH, Ea(0 K) = +11.5 kcal mol-1; (M4) ethylperoxy β-hydrogen transfer with C−O bond rupture to yield ethylene + HO2•, Ea(0 K) = +5.3 kcal mol-1, the C−O bond rupture barrier lying 1.2 kcal mol-1 above the O−O bond rupture barrier of M2; (M5) concerted elimination of HO2• from the ethylperoxy radical to give ethylene + HO2•, Ea(0 K) = −0.9 kcal mol-1. We show that M5 is energetically preferred and is also the only mechanism consistent with experimental observations of a negative temperature coefficient. The reverse reaction (C2H4 + HO2• → •C2H4OOH) has a zero-point-corrected barrier of 14.4 kcal mol-1.
This publication contains evaluated and estimated data on the kinetics of reactions involving isobutane, t-butyl radical and isobutyl radical and various small inorganic and organic species which are of importance for the proper understanding of isobutane combustion and pyrolysis. It is meant to be used in conjunction with the kinetic data given in earlier publications, which is of direct pertinence to the understanding of methane, ethane, methanol and propane pyrolysis and combustion, but which also contains a large volume of data that are applicable to the isobutane system. The temperature range covered is 300–2500 K and the density range 1×1016 to 1×1021 molecules cm−3.
This document contains evaluated data on the kinetics and thermodynamic properties of species that are of importance in methane pyrolysis and combustion. Specifically, the substances considered include H, H2, O, O2, OH, HO2, H2O2, H2O, CH4, C2H6, HCHO, CO2, CO, HCO, CH3, C2H5, C2H4, C2H3, C2H2, C2H, CH3CO, CH3O2, CH3O, singlet CH2, and triplet CH2. All possible reactions are considered. In arriving at recommended values, first preference is given to experimental measurements. Where data do not exist, a best possible estimate is given. In making extrapolations, extensive use is made RRKM calculations for the pressure dependence of unimolecular processes and the BEBO method for hydrogen transfer reactions. In the total absence of data, recourse is made to the principle of detailed balancing, thermokinetic estimates, or comparisons with analogous reactions. The temperature range covered is 300–2500 K and the density range 1×1016–1×1021 molecules/cm3. This data base forms a subset of the chemical kinetic data base for all combustion chemistry processes. Additions and revisions will be issued periodically.
Studies have been made of the addition of cyclohexane (CHX) to O2
+ tetramethylbutane (TMB) mixtures between 673 and 773 K in aged boric-acid-coated Pyrex vessels to obtain kinetic data for the reaction of HO2 radicals with CHX. The contribution by OH radicals to the removal of CHX is minimised by the use of a total pressure of 15 Torr, and by product analysis well within 5% consumption of CHX and TMB. From measurements of the relative rate of consumption of CHX and TMB, values of k12/k71/2 were obtained at 673, 713, 753 and 773 K which were relatively insensitive to any other parameter associated with the mechanism.
This publication contains evaluated and estimated data on the kinetics of reactions involving propane, isopropyl radical, n-propyl radical, and various small inorganic and organic species which are of importance for proper small inorganic and organic species which are of importance for proper understanding of propane pyrolysis and combustion. It is meant to be used in conjunction with the kinetic data given in earlier publications which are of direct pertinence to the understanding of methane pyrolysis and combustion, but which also contain a large volume of data that are applicable to the propane system. The temperature range covered is 300–2500 K and the density range 1×1016 to 1×1021 molecules cm−3.
Studies have been made of the separate addition of cyclopentane (c-C5H10) and propane (C3H8) to mixtures of O2 and tetramethylbutane (TMB) between 673 and 783 K in aged boric-acid-coated vessels to obtain kinetic data for the reaction of HO2 radicals with each of the additives. The contribution by OH radicals to the removal of c-C5H10 and C3H8 has been minimised by use of a total pressure of 15 Torr and by measurements well within 5% consumption of TMB and the additives. A full product analysis was carried out for each kinetic data point which shows that at least 85% of the radicals produced from c-C5H10 and C3H8 give conjugate alkene + HO2 and which permits a precise correction for the small percentage of OH radicals formed. From measurements of the relative rates
of consumption of TMB and the additives, values of k12p/k71/2 and k12c/k71/2 were obtained at each temperature used and were shown by sensitivity analysis to be relatively insensitive to any other parameter associated with the mechanism.
A method is given for calculating the elements of the kinetic energy matrix for rotation for any molecule. The treatment includes the effects due to any number of linked rotating groups, balanced or unbalanced. In a simple case these equations reduce to the simpler ones of the two previous papers of this series. This rotational matrix is then converted into the matrix of the internal rotations. The reduced moments of inertia that form the latter are then used with the methods of the previous papers of this series to calculate energy levels and thermodynamic functions.
The recently introduced complete basis set, CBS-Q, model chemistry is modified to use B3LYP hybrid density functional geometries and frequencies, which give both improved reliability (maximum error for the G2 test set reduced from 3.9 to 2.8 kcal/mol) and increased accuracy (mean absolute error reduced from 0.98 to 0.87 kcal/mol), with little penalty in computational speed. The use of a common method for geometries and frequencies makes the modified model applicable to transition states for chemical reactions. © 1999 American Institute of Physics.
The general oxidation chemistry of alkanes and of alkyl radicals is well understood, although many of the detailed aspects are still speculative. This chapter provides an overview of a detailed account of the reactions, thermochemistry, and detailed mechanisms involved in the gas-phase chemistry of hydrocarbon oxidation. Alkane oxidation proceeds through many intermediate compounds prior to the formation of the final products CO2 and H2O. The intermediates, which include O-heterocyclic compounds, aldehydes, ketones, other oxygenated species, alkenes, peroxides, and CO, can play an important role in determining the path of the oxidation and, particularly in influencing the extent of pollution from practical combustion units. However, for many of these compounds, both mechanistic information and kinetic data on the elementary reactions involved are very limited. The chapter also discusses the characteristics of low-temperature combustion, experimental methods for establishing mechanisms and determining rate constants, primary initiation reactions, and homolysis of alkyl radicals. There are a number of key reactions that, under some circumstances, have enormous influence on oxidation phenomena, but for a significant proportion of these reactions, vital information is still not available.
The reaction C2H5 + O2 has been studied by steady-state photolysis of mixtures containing Cl2, C2H6, and O2 over the temperature range 298−680 K at a constant density of 6.8 × 1018 molecules cm-3. Limited experiments were also performed as a function of pressure (200−1300 Torr) at four temperatures. After UV irradiation, the mixtures were analyzed by GC/MS to determine the product yields. The yield of C2H4 increases slowly between 298 and 450 K (Ea 1 kcal mol-1) and then increases sharply (Ea 25 kcal mol-1) reaching a yield of 100(±10)% of the O2 reaction channel by 630 K. For T < 450 K, the C2H4 yield depends on the inverse of the pressure, indicating that the ethylene is formed via a chemically activated (C2H5O2*) radical as has been observed previously. Above 500 K, the C2H4 yield is independent of pressure indicating that a new channel has opened. This is confirmed by the observation that the ratio β (= C2H4/C2H5Cl) increases sharply (from 0.8 to 3.5) between 450 and 500 K. If the C2H5 radical remained the sole source of C2H4 (via C2H5O2*) throughout the entire temperature range, no sharp break in β would occur. The very small yield of ethylene oxide (2.5% at 660 K) and the excellent carbon balance between C2H6 consumed and products formed above 530 K support the formation of C2H4 at elevated temperature via both the thermally activated, concerted path (C2H5O2 → C2H4 + HO2) proposed recently and the chemically activated (C2H5O2*) path. The former reaction occurs from a stabilized ethylperoxy radical without passing through a stable hydroperoxyethyl radical. Phenomenological rate constants at the experimental density are presented for the chemically and thermally activated paths to C2H4 formation. The data also indicate that the formation of C2H4 by direct H-atom abstraction is negligible under the conditions of these experiments [kabs(700 K) < 1 × 10-13 cm3 molecule-1 s-1].
The present investigation is a rather substantial extension and elaboration of our previous work on the same reaction, In this article we accomplish four primary objectives: 1. We show quantitatively how sensitive the high-temperature rate coefficient k(T) is to E-02, the threshold energy of the transition state for direct molecular elimination of HO2 from ethylperoxy radical (C2H5O2), thus deducing a value of E-02 = -3.0 kcal/mol (measured from reactants), 2. We derive the result that k(0)(T) approximate to k ' (infinity)(T) in the high-temperature regime, where k(0)(T) is the zero-pressure rate coefficient, and k ' (infinity)(T) is the infinite-pressure rate coefficient for the bimolecular channel. 3. Most importantly. we discuss the three different regimes of the reaction (low-temperature, transition, and high-temperature) in terms of the eigenvectors and eigenvalues of G, the transition matrix of the master equation, The transition regime is shown to be a region of avoided crossing between the two chemically significant eigenvalue curves in which the thermal rate coefficient kj,p) jumps from one eigenvalue to the other. This jump is accompanied by a "mixing" of the corresponding eigenvectors, through which both eigenvectors deplete the reactant. The onset of the high-temperature regime is triggered by reaching the "stabilization limit" of the ethylperoxy adduct, a limit that is induced by a shift in equilibrium of the stabilization reaction, our identification of the prompt and secondary HO2 formed by the reaction with these eigenvalue/eigenvector pairs leads to good agreement between theory and the experiments of Clifford et al. (J Phys Chem A 2000, 104, 11549-11560). 4. Lastly, we describe the master equation results in terms of a set of elementary reactions and phenomenological rate coefficients. (C) 2001 John Wiley & Sons, Inc.
The thermal dissociation of hydroperoxides to form alkoxy and hydroxyl radicals is thought to play an important role in the low temperature oxidation of alkanes. One possible source of these species is the hydrogen abstraction from alkanes by alkylperoxy radicals. Motivated by a special interest in the oxidation chemistry of ethane, we performed transition state (TST) calculations with tunneling corrections of the rate constants for the reactions of HO2, CH3O2, and C2H5O2 with C2H6. The required input data were obtained from CBS-QB3 ab initio calculations. Complementary to these calculations we obtained rate constants for the same reactions based on empirical estimation rules. The agreement between both sets of rate constants is within a factor of 2 for the ‘Negative Temperature Coefficient’ region. Based on the TST results, we recommend By performing simulations of a 50:50 C2H6/O2 mixture using a low temperature ethane oxidation mechanism with and without the C2H5O2 + C2H6 reaction included, we demonstrate that this reaction has a significant impact on predicted ignition times.
Modeling of low-temperature ethane oxidation requires an accurate description of the reaction of C(2)H(5) + O(2), because its multiple reaction channels either accelerate the oxidation process via chain branching, or inhibit it by forming stable, less reactive products. We have used a steady-state chemical-activation analysis to generate pressure and temperature dependent rate coefficients for the various channels of this system. Input parameters for this analysis were obtained from ab initio calculations at the CBS-QB3 level of theory with bond-additivity corrections, followed by transition state theory calculations with Wigner tunneling corrections. The chemical-activation analysis used QRRK theory to determine k(E) and the modified strong collision (MSC) model to account for collisional deactivation. This procedure resulted in a C(2)H(5) + O(2) submechanism which was either used directly (possibly augmented with a few C(2)H(5) generating and consuming reactions) or as part of a larger extended mechanism to predict the temperature and pressure dependencies of the overall loss of ethyl and of the yields of ethylene, ethylene oxide, HO(2), and OH. A comparison of the predictions using both mechanisms allowed an assessment of the sensitivity of the experimental data to secondary reactions. Except for the time dependent OH profiles, the predictions using the extended mechanism were in good agreement with the observations. By replacing the MSC model with master equation approaches, both steady-state and time dependent, it was confirmed that the MSC assumption is adequate for the analysis of the C(2)H(5) + O(2) reaction. The good overall performance of the C(2)H(5) + O(2) submechanism developed in this study suggests that it provides a good building block for an ethane oxidation mechanism.
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