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A theoretical analysis of the reaction between propargyl and molecular oxygen
Abstract
The temperature- and pressure-dependent kinetics of the reaction between propargyl and molecular oxygen have been studied with a combination of electronic structure theory, transition state theory, and the time-dependent master equation. The stationary points on the potential energy surface were located with B3LYP density functional theory. Approximate QCISD(T,Full)/6-311++G(3df,2pd) energies were obtained at these stationary points. At low temperatures the reaction is dominated by addition to the CH2 side of the propargyl radical followed by stabilization. However, addition to the CH side, which is followed by one of various possible internal rearrangements, becomes the dominant process at higher temperatures. These internal rearrangements involve a splitting of the O2 bond via the formation of 3-, 4- or 5-membered rings, with the apparent products being CH2CO + HCO. Rearrangement via the 3-membered ring is found to dominate the kinetics. Rearrangement from the CH2 addition product, via a 4-membered ring, would yield H2CO + HCCO, but the barrier to this rearrangement is too high to be kinetically significant. Other possible products require H transfers and, as a result, appear to be kinetically irrelevant. Modest variations in the energetics of a few key stationary points (most notably the entrance barrier heights) yield kinetic results that are in good agreement with the experimental results of Slagle and Gutman (I. R. Slagle and D. Gutman, Proc. Combust. Inst., 1986, 21, 875) and of Atkinson and Hudgens (D. B. Atkinson and J. W. Hudgens, J. Phys. Chem. A, 1999, 103, 4242).
... kcal·mol −1 /RT) cm −3 ·molecule −1 ·s −1 was implemented by Slagle and Gutman 16 using a tubular reactor coupled with a photoionization mass spectrometer. The RRKM/ME high-temperature rate constant k(T) = 2.83 × 10 −19 T 1.7 exp(−1500 kcal·mol −1 /RT) cm −3 ·molecule −1 ·s −1 (500 < T < 2000 K) was calculated by Hahn et al. 17 using the QCIST(T,full)/6-311++G(3df, 2pd) energies. At a hightemperature region, however, their data are found to be larger than the experimental values of Slagle and Gutman. ...
... In the temperature range of 380−430 K, the equilibrium constants for this reaction were measured, being 2.60 × 10 −16 −8.52 × 10 −16 cm 3 ·molecule −1 , which are found to be in good agreement with our calculated values, 2.48 × 10 −16 −8.36 × 10 −16 cm 3 ·molecule −1 , as shown in Figure 1. The predicted equilibrium constants in the study of Hahn et al. 17 were also in accord with the experimental data of Slagle and Gutman; 16 however, the energy of C 3 H 3 O 2 was adjusted to be 18.2 instead of 19.2 kcal·mol −1 calculated by the HL method (the high-level method described in detail in the study of Hahn et al. 17 ). In this study, there is no need to adjust the C 3 H 3 −O 2 bond energy because its value is 18.1 kcal·mol −1 , calculated at the CCSD(T)/CBS level of theory. ...
... In the temperature range of 380−430 K, the equilibrium constants for this reaction were measured, being 2.60 × 10 −16 −8.52 × 10 −16 cm 3 ·molecule −1 , which are found to be in good agreement with our calculated values, 2.48 × 10 −16 −8.36 × 10 −16 cm 3 ·molecule −1 , as shown in Figure 1. The predicted equilibrium constants in the study of Hahn et al. 17 were also in accord with the experimental data of Slagle and Gutman; 16 however, the energy of C 3 H 3 O 2 was adjusted to be 18.2 instead of 19.2 kcal·mol −1 calculated by the HL method (the high-level method described in detail in the study of Hahn et al. 17 ). In this study, there is no need to adjust the C 3 H 3 −O 2 bond energy because its value is 18.1 kcal·mol −1 , calculated at the CCSD(T)/CBS level of theory. ...
Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the C 3 H 3 O 2 potential energy surface. The temperature-and pressure-dependent reaction rate constants have been calculated using the Rice−Ramsperger−Kassel−Marcus Master Equation model. The calculated results indicate that the prevailing reaction channels lead to CH 3 CO + CO and CH 2 CO + HCO products. The branching ratios of CH 3 CO + CO and CH 2 CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300−2000 K, whereas CCCHO + H 2 O (0−10%) and CHCCO + H 2 O (0−17%) are significant minor products. The desirable products OH and H 2 O have been found for the first time. The individual rate constant of the C 3 H 3 + O 2 → CH 2 CO + HCO channel, 4.8 × 10 −14 exp[(−2.92 kcal·mol −1)/(RT)], is pressure independent; however, the total rate constant, 2.05 × 10 −14 T 0.33 exp[(−2.8 ± 0.03 kcal·mol −1)/(RT)], of the C 3 H 3 + O 2 reaction leading to the bimolecular products strongly depends on pressure. At P = 0.7− 5.56 Torr, the calculated rate constants of the reaction agree closely with the laboratory values measured by Slagle and Gutman [Symp. (Int.) Combust. 1988, 21, 875−883] with the uncertainty being less than 7.8%. At T < 500 K, the C 3 H 3 + O 2 reaction proceeds by simple addition, making an equilibrium of C 3 H 3 + O 2 ⇌ C 3 H 3 O 2. The calculated equilibrium constants, 2.60 × 10 −16 − 8.52 × 10 −16 cm 3 ·molecule −1 , were found to be in good agreement with the experimental data, being 2.48 × 10 −16 −8.36 × 10 −16 cm 3 ·molecule −1. The title reaction is concluded to play a substantial role in the oxidation of the five-member radicals and the present results corroborate the assertion that molecular oxygen is an efficient oxidizer of the propargyl radical.
... Of particular importance to soot formation are the resonantly stabilised radicals (RSR) such as propargyl, which is formed from hydrogen abstraction from allene/propyne or reactions between methylene radicals with acetylene [277], ...
... In valence bond theory the stabilisation can be said to arise from a resonance between two Kekulé structures [277], ...
... This spreads the spin density across the molecule reducing the reactivity [278] (see Figure 11). Propargyl is therefore unable to form strong bonds with O 2 in the flame (D 0 < 20 kcal mol −1 ) and is not susceptible to radical induced fragmentation from H [277]. This means propargyl and other delocalised π-radicals are stable enough to increase in concentration and are therefore important intermediates for soot formation [277,279,280]. ...
The route by which gas-phase molecules in hydrocarbon flames form condensed-phase carbonaceous nanoparticles (incipient soot) is reviewed. These products of incomplete combustion are introduced as particulates and materials revealing both their useful applications and unwanted impacts as pollutants. Significant advances in experimental techniques in the last decade have allowed the gas phase precursors and the transformation from molecules to nanoparticles to be directly observed. These measurements combined with computational techniques allow for various mechanisms known to date to be compared and explored. Questions remain surrounding the various mechanisms that lead to nanoparticle formation. Mechanisms combining physical and chemical routes, so-called physically stabilised soot inception, are highlighted as a possible “middle way”.
... Many of these R þ O 2 reactions are qualitatively different from reactions of saturated alkyl radicals, and do not display the ROO / QOOH pathways described above. For example, vinyl radicals [153e157], propargyl radicals [158,159], and aromatic radicals (phenyl [157,160], cyclopentadienyl [161]) tend to form CeC bond scission products, not HO 2 or OH. The oxidation of alkylated aromatics will of course display a competition between the effects of the aromatic ring and the aliphatic side chain [162]. ...
... In addition, the character of the transition state for the initial association of the radical with O 2 is changed because the resonance stabilization is lost during the formation of the ReOO bond. Hahn et al. [159] compared the reaction path energies of various small radicals at the B3LYP/6-31G(d) level of theory, as shown in Fig. 17. They found Fig. 17. ...
... They found Fig. 17. B3LYP/6-31G (d) minimum-energy pathways for the addition of molecular oxygen to 5 hydrocarbon radicals, taken from reference [159] e Reproduced by permission of The Royal Society of Chemistry. that the character of these curves transition from barrierless to having a saddle point, as a function of increasing resonance stabilization energy. ...
Advanced low-temperature combustion concepts that rely on compression ignition have placed new technological demands on the modeling of low-temperature oxidation in general and particularly on fuel effects in autoignition. Furthermore, the increasing use of alternative and non-traditional fuels presents new challenges for combustion modeling and demands accurate rate coefficients and branching fractions for a wider range of reactants. New experimental techniques, as well as modern variants on venerable methods, have recently been employed to investigate the fundamental reactions underlying autoignition in great detail. At the same time, improvements in theoretical kinetics and quantum chemistry have made theory an indispensible partner in reaction kinetics, particularly for complex reaction systems like the alkyl+O2 reactions. This review concentrates on recent developments in the study of elementary reaction kinetics in relation to the modeling and prediction of low-temperature combustion and autoignition, with specific focus placed on the emerging understanding of the critical alkylperoxy and hydroperoxyalkyl reactions. We especially highlight the power of cooperative theoretical and experimental efforts in establishing a rigorous mechanistic understanding of these fundamental reactions.
... 23−25 Due to the above, its reactions have attracted much attention and extensive computational investigations have been carried out to study the mechanisms and kinetics of the reactions between C3H3 and various species, including OH, O2, NO, CO, NH3, CH3, HCHO, C3H3, and C3H5. [26][27][28][29][30][31][32][33] Both C3H3 and HNCO can be produced during combustion and these species may interact at high temperatures or in the post-combustion environment, thus making the formation of C4H4NO and its subsequent reactivity a matter of interest in the modeling of such processes. The current scarcity of information concerning the formation and decomposition of C4H4NO has led us to carry out a comprehensive theoretical investigation of the mechanism and kinetics of the C4H4NO system. ...
... The reliability of the CCSD(T)/CBS(T,Q,5) level was also proved in several prior researches. 29,39,40 In addition, the multi-reference characters of the wavefunctions for all species in the system were checked by computing the T1 diagnostics 46,47 at the CCSD(T)/aug-cc-pVTZ level of theory. It should be mentioned here that all the quantumchemical calculations in the current study were carried out by the Gaussian 16 software 48 . ...
Ab initio CCSD(T)/CBS(T,Q,5)//M06-2X/aug-cc-pVTZ calculations have been applied to figure out the C4H4NO (isopropyl aminocarbonyl) potential energy surface. The resulting energetics and molecular parameters of all species involved in the system have been then employed in the TST and RRKM/ME computations of temperature-and pressure-dependent rate coefficients and product yields. The calculated results show that rate constants of the abstraction reactions C3H3 + HNCO → C3H4 + NCO are pressure-independent while those of the addition reactions C3H3 + HNCO and the C4H4NO decomposition reactions were found to be pressure-dependent. The calculated enthalpies of formation for species involved in the system are in good ageement with the available literature data. All the results of this work may provide a useful database source for the further study of this kind of reactions.
... 1,2 The primary reaction under high temperature conditions is hydrogen atom abstraction on the alkyl side chain. However, the radicals formed in this step are resonance stabilized, 3 exhibit a low reactivity towards oxygen, 4 are thus long-lived in combustion engines and have a high tendency to form PAHs in bimolecular reactions. Especially the deactivation mechanism of such fuel radicals is of great interest, because it influences the sooting behavior. ...
... As visible the signal rise time is on the order of 10 ns, close to or shorter than the time resolution of the setup. 4. In the translational energy distributions (cf. ...
The photodissociation dynamics of the C8H9 isomers ortho- and para-xylyl are investigated in a free jet. The xylyl radicals are generated by flash pyrolysis from 2-(2-methylphenyl)- and 2-(4-methylphenyl) ethyl nitrite and are excited into the D3 state. REMPI- spectra show vibronic structure and the origin of the transition is identified at 32 291 cm⁻¹ for the para- and at 32 132 cm⁻¹ for the ortho-isomer. Photofragment H-atom action spectra show bands at the same energy and thus confirm H-atom loss from xylyl radicals. To gain further insight into the photodissociation dynamics, velocity map images of the hydrogen atom photofragments are recorded. Their angular distribution is isotropic and the translational energy release is in agreement with a dissociation to products in their electronic ground state. Photodissociation of para-xylyl leads to the formation of para-xylylene (C8H8), while the data for ortho-xylyl agree much better with the isomer benzocyclobutene as the dominant molecular fragment rather than ortho-xylylene. In computations we identified a new pathway for the reaction ortho-xylyl → benzocyclobutene + H with a barrier of 3.39 eV (27 340 cm⁻¹), which becomes accessible at the employed excitation energy. It proceeds via a combination of scissoring and rotational motion of the -CH2 and -CH3 groups. However, the observed rate constants measured by delaying the excitation and ionization laser with respect to each other are significantly faster than computed ones, indicating intrinsic non-RRKM behaviour. A comparably high value of around 30% of the excess energy is released as translation of the H-atom photofragment.
... All the products detected in oxidation were also observed in pyrolysis, except for CO 2, which was detected only in oxidation. The formation of benzene is slower under oxidative conditions compared to pyrolysis due to the interaction between benzene precursors, propargyl radicals, and molecular oxygen yielding ketene and formyl radical [69] . ...
The pyrolysis and oxidation of acetone were studied using three complementary experimental setups. Jet-stirred reactor experiments were performed at four equivalence ratios (ϕ = 0.5, 1, 2, and ∞), at pressure of 1.067 bar (800 Torr) and over the temperature range of 700–1200 K for pyrolysis and 600–1150 K for oxidation. The decomposition of acetone starts around 800 K with a conversion rate of 50% obtained around 1000 K in both pyrolysis and oxidation studies. The main stable products detected in both conditions are small hydrocarbons (methane, ethane, and ethylene), with also acetaldehyde, CO and CO2 for oxidation. Oscillation behavior was detected beyond 1000 K under oxidation conditions and the products were followed with on-line mass spectrometry. Ignition delay times were measured using a rapid compression machine at pressures of 20 and 40 bar under non-diluted stoichiometric conditions over the temperature range 850–1100 K. The ignition delay times measured in the present study, combined with shock tube data of literature, exhibit a slight inflexion to the Arrhenius behavior, but no negative temperature coefficient. Laminar burning velocities were measured using a flat flame burner at atmospheric pressure for three fresh gas temperatures: ambient temperature, 358 and 398 K. A detailed kinetic model of the combustion of acetone including 852 species and 3265 reactions was developed. This new kinetic mechanism predicts relatively well the experimental measurements of ignition delay times, the mole fraction of the products in the jet-stirred reactor including oscillations and laminar burning velocities. Related flow rate and sensitivity analysis are also presented providing new insights into the acetone reaction network.
... However, several new kinetic studies have been published recently on the molecular growth and oxidation of aromatic species since the publication of mechanism of Ref. [38] , and so the mechanism has been updated accordingly to incorporate the latest understanding of the chemistry. The major changes to the chemistry include a) oxidation of phenyl radical (C 6 H 5 ) by O 2 [54] , b) oxidation of resonantly stabilized C 9 H 7 radicals: indenyl and methyl-propargyl radicals [55][56][57][58] , c) oxidation of fulvenallenyl radical by O 2 and O, d) using a simplified description of the recombination reaction of cyclopentadienyl radicals [59] , e) ring-expansion reactions of methyl-radical with acenaphthyl radicals (C 12 H 7 ) to produce phenalene and its radical [60] , f) reactions of acenaphthyl radicals with acetylene [61] , g) reaction of phenyl radicals with phenylacetylene [62] , i) reactions of CH 3 with resonantly stabilized cyclohexadienyl radical, j) reactions of resonantly stabilized fluorenyl radical with CH 3 . Changes (a)-(g) pertain to adopting the new reactions channels and the associate rate parameters from ab-initio studies into the mechanism. ...
In a combined experimental and modeling effort, we investigated the molecular-growth pathways in propyne-doped low-pressure premixed flames of benzene and toluene. We determined the chemical structures of these two flames with flame-sampling molecular-beam mass spectrometry. The mole fraction profiles of the aromatic intermediates served as validation targets for two chemically detailed mechanisms that were independently developed at the German Aerospace Center (DLR) and at the Lawrence Livermore National Laboratory (LLNL). Reaction path analyses reveal the important pathways for indene, naphthalene, and phenanthrene. There is no appreciable fuel-structure effect and molecular growth was observed to be driven by radical-radical recombination reactions, and ring-closure and ring-enlargement reactions with little contribution from the classical HACA mechanism. Indene is formed in both flames through the reactions of the phenyl and propargyl radicals. The benzyl radical plays only a very minor role in the formation of indene through the reaction with acetylene. Reactions of the propargyl radical with fulvenallenyl, a C7H5 isomer, contribute significantly to naphthalene formation in both flames investigated here. Benzyl radicals contribute to naphthalene formation via reactions with propargyl radical through formation of phenyl-substituted butadienyl and vinylacetylene isomers. Ring-enlargement reactions converting indene's five-membered ring into naphthalene's six-membered ring also contribute in small amounts to naphthalene. Fulvenallenyl radicals also contribute substantially to phenanthrene formation.
... The reactions between Propargyl and OH (R int.1) is of great research interests, due to the fact that OH is the primary oxidizing agent for propargyl in rich and stoichiometric flames, since the oxidation by molecular oxygen is slow. [345,346] As stated by Hansen et al. [344], detailed analysis of the potential energy surface (PES), performed by Miller in his unpublished results, indicates clearly that the only significant products resulted from the addition of OH to C 3 H 3 are C 2 H 4 + CO and C 2 H 3 + HCO, which are both formed as a consequence of OH adding to the CH end of propargyl, followed by a 1,3-hydrogen transfer. Although this molecular study is present, accurate rate coefficients are still absent. ...
Reducing CO2 and pollutant emission is the essential challenge when dealing with climate change problems. In the transport sector, exhaust gas recirculation (EGR) technology is often used in turbocharged gasoline spark ignition (SI) engines to increase fuel economy, inhibit knock tendency, and reduce NOx emissions. However, high EGR ratios are still difficult to achieve, as they result in reduced heat release and engine stability. As increasing turbulence level and advance spark ignition systems could not bring sufficient improvements at such extreme conditions, growing interest is cast onto the combustion chemistry under high dilution. The present work aims to understand the combustion chemistry of highly-diluted gasoline premixed flames and to establish a detailed kinetic mechanism by multi-scale modeling to predict combustion characteristics with sufficient accuracy at highly-diluted conditions.This work adopts a multi-scale modeling approach, and targets on the laminar flame speed (SL) of a gasoline surrogate, which is named toluene reference fuel with ethanol addition (TRFE) and consist of isooctane, n-heptane, toluene, and ethanol. For micro-scale modeling, the reaction between ketene and hydroxyl radical, which might be important to the SL at highly-diluted conditions, is studied theoretically using ab initio electronic structure methods for the potential energy surface (PES) and Rice–Ramsperger–Kassel–Marcus Theory coupled with Master Equation (RRKM/ME) for the rate coefficients. Detailed PES is obtained, dominant pathways are identified, and their phenomenological rate coefficients are derived to be utilized in combustion modeling. For macro-scale modeling, firstly, important kinetic, thermodynamic, and transport parameters to the laminar flame speed at highly-diluted conditions, are firstly identified using sensitivity analysis based on a starting mechanism. Sensitive reactions are found to mostly involve HO2, C2--C3 species and fuel radicals. Secondly, in the sub-mechanisms where these reactions lies, diluted flames of the corresponding fuels are studied and chemical detail of the dilution effects are explored. The starting mechanism is updated by state-of-the-art kinetics parameters found in the literature for each sub-mechanisms. Finally, a detailed mechanism suitable for laminar flame speed calculations at highly-diluted conditions is established after validation. A mathematical SL correlation is generated for the use in computational fluid dynamic (CFD) simulations.
... Over the past several decades, C 3 H 3 was determined to be involved in combustion with an amount of abundance because it is very sustainable in the pyrolysis processes, 9−14 and its reaction with O 2 occurs slowly. 5 It is yielded as organic molecules, especially hydrocarbons, are cleaved into small species owing to the oxidation or temperature. 15,16 Moreover, this radical can be generated by the reaction of ketenyl radical or methylene with acetylene and by H-loss of propyne or allene. ...
... This effect is particularly evident when considering the region before the formation of the first aromatic ring, because of the still very high concentration of molecular oxygen. Recurring to numerical simulations performed on sooting acetylene low-pressure flames, the oxidation of 2 3 C H has been found the critical branching point between carbon growth and carbon oxidation [18,38]. ...
Polycyclic Aromatic Hydrocarbons (PAHs) are important precursors of carbonaceous soot particles, thus influence the quantity and morphology of particulate emission of combustion processes. Furthermore, the PAHs adsorbed on the surface of the soot particles contribute to the carcinogenicity of the particles, so there is scientific interest in characterizing and quantifying those species to provide key information on the mechanism of soot formation and to understand their impact on environment and human health. An original setup based on the coupling of laser desorption, laser ionisation and time-of-flight mass spectrometry techniques has been dedicated to the analysis of PAHs desorbed from soot. Briefly, soot is sampled from flames using an extractive vacuum probe and deposed on porous filters. The samples are irradiated with a 532 nm laser beam to promote the desorption of neutral molecules. The ejecta are then ionized with a 266 nm laser beam, and the positive ions produced this way are mass analyzed in a TOF-MS. The complete characterization of the desorption and ionization process has been the first goal of this work. The acquired expertise allowed investigations on different flames in which the PAHs have been identified and studied at different level of their formation. Particularly, relevant differences in the mass spectra have been detected in the soot inception region. A new method based on the deposition of soot on different substrates is proposed in order to distinguish the PAHs belonging to the gas-phase during the sampling from those adsorbed on soot particles, hence the role of the heterogeneous reactions between gaseous PAHs and soot particles has been highlighted.
... Resistance to oxidation by molecular oxygen is a particularly important manifestation of RSFR stability. This point is treated in some detail by Miller et al. [5] and Hahn et al. [14] Briefly, the initial bond formed when O 2 adds to C 3 H 3 is only 18 or 19 kcal/mole, depending on whether the O 2 adds to the head (the CH 2 end ) or the tail (the CH end) of propargyl. These bonds are not sufficiently strong to support the rearrangement that is necessary to produce a fast reaction forming oxidized products. ...
This presentation summarizes our recent experimental and flame modeling studies focusing on understanding of the formation of small aromatic species, which potentially grow to polycyclic aromatic hydrocarbons (PAHs) and soot. In particular, we study premixed flames, which are stabilized on a flat-flame burner under a reduced pressure of ≈15–30 torr, to unravel the important chemical pathways to aromatics formation in flames fueled by small C3–C6 hydrocarbons. Flames of allene, propyne, 1,3-butadiene, cyclopentene, and C6H12 isomers 1-hexene, cyclohexane, 3,3-dimethyl-1-butene, and methylcyclopentane are analyzed by flame-sampling molecular-beam time-of-flight mass spectrometry. Isomer-specific experimental data and detailed modeling results reveal the dominant fuel-destruction pathways and the influence of different fuel structures on the formation of aromatic compounds and their commonly considered precursors. As a specific aspect, the role of resonance-stabilized free radical reactions is addressed for this large number of similar flames of structurally different fuels. While propargyl and allyl radicals dominate aromatics formation in most flames, contributions from reactions involving other resonance-stabilized radicals like i-C4H5 and C5H5 are revealed in flames of 1,3-butadiene, 3,3-dimethyl-1-butene, and methylcyclopentane. Dehydrogenation processes of the fuel are found to be important benzene formation steps in the cyclohexane flame and are likely to also contribute in methylcyclopentane flames.
... Detailed calculations were carried out for key reactions in both the cyclohexane and ethanol oxidations. Rate constants for these reactions were computed by solution of the time-dependent multiple-well master equation using the methodology developed by Miller and Klippenstein [20][21][22]. For barrierless channels such as the initial association of R with O 2 direct variable-reactioncoordinate transition-state theory (VRC-TST) [23][24] was required for accurate rate coefficients. ...
Autoignition chemistry is central to predictive modeling of many advanced engine designs that combine high efficiency and low inherent pollutant emissions. This chemistry, and especially its pressure dependence, is poorly known for fuels derived from heavy petroleum and for biofuels, both of which are becoming increasingly prominent in the nation's fuel stream. We have investigated the pressure dependence of key ignition reactions for a series of molecules representative of non-traditional and alternative fuels. These investigations combined experimental characterization of hydroxyl radical production in well-controlled photolytically initiated oxidation and a hybrid modeling strategy that linked detailed quantum chemistry and computational kinetics of critical reactions with rate-equation models of the global chemical system. Comprehensive mechanisms for autoignition generally ignore the pressure dependence of branching fractions in the important alkyl + O reaction systems; however we have demonstrated that pressure-dependent 'formally direct' pathways persist at in-cylinder pressures.
The reaction of the OH radical with cyclopentadiene (C5H6) was investigated at room temperature using multiplexed photoionization mass spectrometry. OH radicals in their ground electronic state were generated in the gas phase by 248 nm photolysis of H2O2 or 351 nm photolysis of HONO. Analysis of photoion spectra and temporal profiles reveal that at room temperature and over the 4-8 Torr pressure range, the resonance-stabilized 5-hydroxycyclopent-2-en-1-yl (C5H6OH) is the main observed reaction product. Abstraction products (C5H5) were not detected. The C5H6OH potential energy surface calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G** level of theory suggests that the resonance-stabilized radical product is formed through barrierless addition of the OH radical onto cyclopentadiene's π system to form a van der Waals complex. This weakly bound adduct isomerizes through a submerged energy barrier to the resonance-stabilized addition adduct. Master Equation calculations, including two OH-addition entrance pathways, predict that 5-hydroxycyclopent-2-en-1-yl remains the sole addition product up to 500 K. The detection of an OH-containing resonance-stabilized radical at room temperature further highlights their importance in carbon- and oxygen-rich environments such as combustion, planetary atmospheres, and the interstellar medium.
To reduce particulate emissions leading to a cleaner environment, it is important to understand how polycyclic-aromatic hydrocarbons (PAHs) and their precursors are formed during combustion. 2-butyne can decompose to propargyl and allyl radicals. These radicals can produce benzene and other PAHs, leading to the formation of soot. In the present study, pyrolysis, oxidation, and laminar flame speed experiments were performed for 2-butyne. The pyrolysis experiments were conducted in a single-pulse shock tube at 2 bar in the temperature range 1000 – 1500 K. Ignition delay times for 2-butyne/‘air’ mixtures were measured in the pressures range 1 – 50 bar, over the temperature range 660 – 1630 K, at equivalence ratios of 0.5, 1.0, and 2.0 using rapid compression machines and shock tubes. Moreover, laminar flame speed (LFS) experiments were performed at ambient temperature, at p = 1 – 3 atm, over an equivalence ratio range of 0.6 – 1.8. A new, detailed chemical kinetic model for 2-butyne has been developed and widely validated against the data measured in this study and those available in the literature. The significant reactions for 2-butyne pyrolysis, ignition, and oxidation are identified and discussed using flux and sensitivity analyses.
We have used laser-photolysis – photoionization mass spectrometry, quantum chemical calculations, and master equation simulations to investigate the kinetics of the reaction between ( E/Z )-pent-3-en-2-yl (CH 3 CHCHCHCH 3 ), a resonance-stabilised hydrocarbon radical, and...
We have investigated the reaction between 2-methylallyl radicals and oxygen molecules with experimental and computational methods. Kinetic experiments were conducted in a tubular laminar flow reactor using laser photolysis for radical production and photoionization mass spectrometry for detection. The reaction was investigated as a function of temperature (203-730 K) and pressure (0.2-9 torr) in helium and nitrogen bath gases. At low temperatures (T < 410 K), the reaction proceeds by a barrierless reaction to form 2-methylallylperoxyl. Equilibration of the peroxyl adduct and the reactants was observed between 350-410 K. Measurements were extended to even higher temperatures, up to 730 K, but no reaction could be observed. Master equation simulations of the reaction system were performed with the MESMER program. Kinetic parameters in the master equation model were optimized by direct fitting to time-resolved experimental 2-methylallyl traces. Trace fitting is a recently implemented novel feature in MESMER. The trace approach was compared with the more traditional approach where one uses experimental rate coefficients for parameter optimization. The optimized parameters yielded by the two approaches are very similar and do an excellent job at reproducing the experimental data. The optimized master equation model was then used to simulate the reaction under study over a wide temperature and pressure range, from 200 K and 0.01 bar to 1500 K and 100 bar. The simulations predict a small phenomenological rate coefficient under autoignition conditions; about 1 × 10-18 cm3 s-1 at 400 K and 5 × 10-16 cm3 s-1 at 1000 K. We provide modified Arrhenius expressions in PLOG format for the most important product channels to facilitate the use of our results in combustion models.
The C3H3• + CH3OH and C3H3• + C2H5OH reactions were characterised using MP2 and M06-2X theories. For the two reactions, a total of seven channels have been described, including the H-abstraction channels and OH-transfer channels. Potential energy diagrams for the named reactions have been evaluated at the CCSD(T)/CBS//MP2/cc-pVTZ level of theory. The kinetic study was conducted over the temperature range of 300–1500 K using CVT/SCT, CVT and TST methods. Note that the tunnelling effect was significant over low temperatures and the fitting parameters of Arrhensive formula were given. H-abstraction from the –CH3 group (CH3OH) and –CH2 group (C2H5OH) were identified as dominant channels less than 1000 K. While, the OH-transfer channel producing C3H4OH was an important channel higher than 1300 K.
Understanding the details of pyrrole combustion chemistry in the O2/CO2 atmosphere contributes to developing the strategies for nitrogen oxides (NOx) control during the pressurized oxy-coal combustion (POCC). However, the existing kinetic models for pyrrole oxidation lack multi-dimensional validation, and the ignition delay times (IDTs) of pyrrole in the O2/CO2 atmosphere are still scarce. This study measured the IDTs of pyrrole in O2/CO2 atmospheres at an elevated pressure of 5.2 bar, temperatures from 1271 to 1645 K, and equivalence ratios Φ = 0.5, 1.0, and 2.0 in a shock tube. The results demonstrate that the pyrrole IDTs decrease with decreasing equivalence ratio and the pyrrole mixtures in O2/CO2 atmospheres have longer IDTs than those in O2/Ar atmospheres. A pyrrole oxidation kinetic model (HUST pyrrole model) has been developed by updating 19 reactions in our previous model (HUST pyridine model). HUST pyrrole model gives a satisfactory prediction of the IDTs of pyrrole in the O2/Ar atmosphere (measured by MacNamara et al.) and O2/CO2 atmosphere (measured in this study) and the profiles of pyrrole, HCN, and NO (measured by Yamamoto et al.). The comparison of the HUST pyrrole model with the Lumbreras model (2001) was conducted by the pyrrole IDTs and profiles of HCN using the sensitivity and flux analysis. The addition of HNCPROP and modification of R665 (C3H3 + O2 = CH2CO + HCO), R1273 (AC3H4CN + H2 => ALLYLCN + H), R1251 (PYRLNE = ALLYLCN), R1238 (PYRROLE + H = PYRLYL + H2), and R1240 (PYRROLE + OH = PYRLYL + H2O) in HUST pyrrole model significantly improve the prediction performance for the pyrrole oxidation. The effects of the equivalence ratio and CO2 on the IDTs for pyrrole oxidation have been analyzed.
We have used laser-photolysis/photoionization mass spectrometry to measure the kinetics of the reaction of 1-methylpropargyl (but-3-yn-2-yl, ) radicals with oxygen molecules as a function of temperature (T=200−685K) and bath gas density (1.2−15×1016cm−3). The low temperature (T ≤ 304 K) kinetics is dominated by oxygen addition to the carbon of the radical to form a peroxyl radical, and the measured bimolecular rate coefficient exhibits negative temperature dependence and depends on bath gas density. At slightly higher temperatures (335−396K), where the redissociation rate of the peroxyl is already observable, we measured the equilibrium constant as a function of temperature. At even higher temperatures (T=479−685K), the loss rate of 1-methylpropargyl is determined by the addition of oxygen to the terminal carbon and the reaction is observed to produce methylketene. The high-temperature bimolecular rate coefficient is independent of bath gas density and the temperature dependence is weakly positive. To explain our experimental findings, we performed quantum chemical calculations together with master equation simulations. By using our experimental data to constrain key parameters, the master equation model was able to reproduce experimental results reasonably well. We then extended the conditions of our simulations up to 2000 K and 100 bar. The results of these simulations are provided in ChemKin compatible PLOG format.
Cyclopentane is a suitable naphthene, or cycloalkane, in a palette for multi-component gasoline surrogate fuels due to its presence in market fuels and its relevance to alkyl substituted cyclopentanes also present. However, the previous oxidation studies of cyclopentane have primarily focused on neat mixtures. Blending cyclopentane with dimethyl ether in this work therefore serves to inform our understanding of, and improve predictive models for, multi-component mixtures. In this work, the auto-ignition of cyclopentane/dimethyl ether blends was studied in a high-pressure shock tube and in a rapid compression machine. A wide range of temperatures (650 – 1350 K) and elevated pressures of 20 and 40 bar were studied at equivalence ratios of 0.5, 1.0 and 2.0 in air for two blending ratios (30/70 and 70/30 mole% cyclopentane/di-methyl ether mixtures). A detailed kinetic model for cyclopentane was revised to capture the measured ignition delay times and apparent heat release rates in this study. Literature ignition delay time, jet-stirred reactor, and laminar burning velocity measurements of neat cyclopentane were used as additional validation. Improvements to the kinetic model were based on recent literature studies related to sub-models including cyclopentene and cyclopentadiene which allowed the removal of previous local rate-constant optimizations. Low temperature reactivity of cyclopentane was found to be controlled by the branching ratio between concerted elimination of HȮ2 and the strained formation of Q˙OOH radicals in agreement with previous studies. In this study, the low branching ratio of Q˙OOH formation increases the influence of a competing consumption pathway for cyclopentyl-peroxy (CPTȮ2J) radicals. The sensitivity of the simulated ignition delay times to the formation of cyclopentyl hydroperoxide (CPTO2H), from CPTȮ2J and HȮ2, is discussed. The current model is used to analyze the influence of dimethyl ether on the reactivity of cyclopentane in the context of previous literature studies of dimethyl ether binary blends with ethanol and toluene.
To improve our understanding of the combustion characteristics of propyne, new experimental data for ignition delay times (IDTs), pyrolysis speciation profiles and flame speed measurements are presented in this study. IDTs for propyne ignition were obtained at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10 and 30 bar, over a wide range of temperatures (690–1460 K) using a rapid compression machine and a high-pressure shock tube. Moreover, experiments were performed in a single-pulse shock tube to study propyne pyrolysis at 2 bar pressure and in the temperature range 1000–1600 K. In addition, laminar flame speeds of propyne were studied at an unburned gas temperature of 373 K and at 1 and 2 bar for a range of equivalence ratios. A detailed chemical kinetic model is provided to describe the pyrolytic and combustion characteristics of propyne across this wide-ranging set of experimental data. This new mechanism shows significant improvements in the predictions for the IDTs, fuel pyrolysis and flame speeds for propyne compared to AramcoMech3.0. The improvement in fuel reactivity predictions in the new mechanism is due to the inclusion of the propyne + HȮ2 reaction system along with ȮH radical addition to the triple bonds of propyne and subsequent reactions.
The kinetics of the i-C4H5 (buta-1,3-dien-2-yl) radical reaction with molecular oxygen has been measured over a wide temperature range (275–852 K) at low pressures (0.8–3 Torr) in direct, time-resolved experiments. The measurements were performed using a laminar flow reactor coupled to photoionization mass spectrometer (PIMS), and laser photolysis of either chloroprene (2-chlorobuta-1,3-diene) or isoprene was used to produce the resonantly stabilized i-C4H5 radical. Under the experimental conditions, the measured bimolecular rate coefficient of i-C4H5 + O2 reaction is independent of bath gas density and exhibits weak, negative temperature dependency, and can be described by the expression k3 = (1.45 ± 0.05) × 10⁻¹² × (T/298 K)−(0.13±0.05) cm³ s⁻¹. The measured bimolecular rate coefficient is surprisingly fast for a resonantly stabilized radical. Under combustion conditions, the reactions of i-C4H5 radical with ethylene and acetylene are believed to play an important role in forming the first aromatic ring. However, the current measurements show that i-C4H5 + O2 reaction is significantly faster under combustion conditions than previous estimations suggest and, consequently, inhibits the soot forming propensity of i-C4H5 radicals. The bimolecular rate coefficient estimates used for the i-C4H5 + O2 reaction in recent combustion simulations show significant variation and are up to two orders of magnitude slower than the current, measured value. All estimates, in contrast to our measurements, predict a positive temperature dependency. The observed products for the i-C4H5 + O2 reaction were formaldehyde and ketene. This is in agreement with the one theoretical study available for i-C4H5 + O2 reaction, which predicts the main bimolecular product channels to be H2CO + C2H3 + CO and H2CCO + CH2CHO.
Schwingungsspektroskopie ist eine vielseitige spektroskopische Methode, mit der Molekülstrukturen und inter-/intramolekulare Wechselwirkungen untersucht werden können. Sie ist deshalb ein hervorragendes Mittel für die Identifikation von Molekülen. Die vorliegende Arbeit umfasst drei Projekte, in denen Schwingungsspektroskopie angewandt wurde, um reaktive Moleküle und ihre Hochtemperatur-Reaktionsprodukte zu untersuchen: 1. Die Aufklärung der Entstehungsmechanismen von polycyclischen aromatischen Kohlenwasserstoffen (PAKs) in Verbrennungsprozessen ist eines der Hauptanliegen der Verbrennungschemie. In der vorliegenden Arbeit wurde IR/UV-Ion-Dip-Spektroskopie in Verbindung mit DFT-Frequenzrechnungen und FTIR-Messungen angewandt, um Produkte von Radikal-Radikal-Reaktionen in einem Mikroreaktor bei hohen Temperaturen zu identifizieren. Als IR-Laserquelle für die IR/UV-Ion-Dip-Experimente diente der Freie-Elektronen-Laser FELIX (Free-Electron Laser for Infrared eXperiments) in Nijmegen (Niederlande). In einem Teilprojekt wurde der A 1A´ (S1) <- X 1A´ (S0) Übergang in 1-(Phenylethinyl)naphthalin (1-PEN), einem mutmaßlich verbrennungsrelevanten Molekül, mit [1+1]-REMPI-Spektroskopie untersucht. 2. Die Identifikation von gasförmigen Reaktionsprodukten bei der thermischen Analyse (EGA: Emissionsgasanalyse) kann als komplementäre Methode zur DTA/TG zusätzliche Informationen für die Aufklärung von Reaktionsmechanismen liefern. Der Aufbau eines elementaren EGA/FTIR-Experiments, basierend auf einer heizbaren IR-Gaszelle, ermöglichte in der vorliegenden Arbeit die Durchführung dynamischer IR-Messungen, mit denen thermische Umsetzungen von Übergangsmetall-Precursorkomplexen zu Koordinationspolymeren untersucht wurden. 3. Die Synthese des ersten bei Raumtemperatur stabilen Diborins, einer Verbindung mit einer Bor-Bor-Dreifachbindung, stellte einen Meilenstein in der elementorganischen Chemie dar. Dies implizierte eine umfassende Untersuchung der Eigenschaften der BB-Bindung und hatte die Synthese einer Reihe ähnlicher Bor-Bor-Mehrfachbindungssysteme mit variierenden Bindungseigenschaften zur Folge. In der vorliegenden Arbeit wurde Raman-Spektroskopie in Verbindung mit DFT-Frequenzrechnungen angewandt, um für diese Bor-Bor-Systeme die strukturellen/elektronischen Eigenschaften der zentralen CBBC-Einheit zu untersuchen.
The reaction of nitric oxide (NO) with propargyl radical (C3H3) was investigated at the CCSD(T)/cc-pVTZ//B3LYP/6–311++G(df, pd) level of theory. The rate coefficients of the system were determined using RRKM – CVT method with Eckart tunneling correction over a temperature range of 200 - 800 K and a pressure range of 1.0 × 10-4 – 10.0 bar. Eight channels proceeding via the barrierless formation of excited intermediate ONCH2CCH or CH2CCHNO at the first step were explored. Three favorable channels (i.e. channels producing adduct of ONCH2CCH and CH2CCHNO and products of HCN and H2CCO) were confirmed. The rate coefficients of channels producing adduct of ONCH2CCH and CH2CCHNO are comparable and have weak negative temperature dependence and positive pressure dependence. Channel producing products of HCN and H2CCO is more important at low pressure and high temperature and less important after pressure greater than 1.0 × 10-2 bar (with a branching ratio less than 6% at 0.1 bar).
With the rise in production of natural gas, there is increased interest in homogeneous partial oxidation (POX) to convert methane to syngas (CO+H2), ethene (C2H4) and acetylene (C2H2). In POX,...
Ortho-benzyne, a Kekulé-type biradical is considered to be a key intermediate in the formation of polycyclic aromatic hydrocarbons (PAH) and soot. In the present work we study the ortho-benzyne self-reactions in a hot micro-reactor and identify the high-temperature products by IR/UV spectroscopy and by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) in a free jet. Ms-TPES confirms formation of ortho-benzyne as generated from benzocyclobutendione, as well as benzene, biphenylene, diacetylene and acetylene, originating from the reaction o-C6H4 HCC-CCH + C2H2 , and CH3. PAH molecules like naphthalene, 2-ethynylnaphthalene, fluorene, phenanthrene and triphenylene are identified based on their IR/UV spectra. By comparison with recent computations their formation starting from ortho-benzyne can be readily understood and supports the importance of the biradical addition (1,4-cycloaddition followed by fargmenation) pathway to PAH molecules, recently proposed by Comandini et al.
A computational investigation into the kinetics of the NO + CH2CCH reaction is presented. The stationary points on the C3H3N1O1 potential energy surface are analyzed using the compound method ANL0, with key regions of the potential energy surface computed using multi-reference methods. The temperature- and pressure-dependent rate constants are computed using the RRKM/Master Equation. The dominant bimolecular products are HCN + CH2CO, CH2CNH + CO, and CH3CN + CO. Additional calculations for the thermal decomposition of an unimolecular intermediate, isoxazole, are in excellent agreement with the available experimental data. The new rate constants are implemented in a detailed chemical kinetic mechanism for the oxidation of C2H4 by O2 + NO. Analysis of a constant temperature, constant pressure batch reaction suggests that NO + CH2CCH could be an important pathway for both NO reduction and CH2CCH oxidation in reburn chemistry.
The addition of acetylene (C2H2) to the propargyl radical (C3H3) initiates a cascade of molecular weight growth reactions that result in the production of polycyclic aromatic hydrocarbons (PAHs) in flames. Although it is well-established that the first reaction step produces the cyclic C5H5 radical cyclopentadienyl (c-C5H5), recent studies have also detected significant quantities of the open chain form, 1-vinylpropargyl (l-C5H5). This work presents a mechanism for the C3H3 + C2H2 reaction from ab initio calculations, which includes pathways for the formation of both the open and shut isomers as well as for their interconversion. Formation of both isomers proceeds from the initial HCCCH2CHCH• reaction adduct with similar barriers, both well-below the entrance channel energy. Subsequent isomerization of l-C5H5 with c-C5H5 also transpires at below the energy of the reactants, although this process connects two deep wells (being resonance stabilized radicals), and must compete with collisional energy transfer. An RRKM theory / master equation model is developed for the reported C5H5 reaction mechanism. Master equation simulations suggest that both cyclic and open-chain isomers are expected to form from the C3H3 + C2H2 reaction across a range of temperatures, although the lifetime of l-C5H5 is relatively short for rearrangement to c-C5H5.
The propargyl radical is considered to be of key importance in the formation of the first aromatic ring in combustion processes. Here we study the bimolecular (self-) reactions of propargyl in a high-temperature pyrolysis flow reactor. The reaction products are identified by IR/UV ion dip spectroscopy, using the free electron laser FELIX as mid-infrared source. This technique combines mass selectivity with structural sensitivity. We identified several reaction products based on their infrared spectra, among them benzene, naphthalene, phenanthrene, indene, biphenyl and surprisingly a number of aromatic compounds with acetylenic (ethynyl) side chains. The observation of benzene confirms that propargyl is involved in the formation of the first aromatic ring. The observation of compounds with acetylenic side chains shows that in addition to a propargyl- and phenyl-based mechanism the HACA (hydrogen abstraction C2H2 addition) mechanism of polycyclic aromatic hydrocarbons formation is present, although no acetylene was used as a reactant. Based on the experimental results we suggest a mechanism that connects the two pathways.
Matrix isolation spectroscopy is a viable tool for the isolation and characterization of free radicals. In inert media (in particular, rare gases such as argon or neon) at cryogenic temperatures (below 30 K), radicals are immobilized and kinetically stabilized. In some cases, it was also possible to investigate bimolecular reactions of matrix-isolated radicals with small molecules. The spectroscopic characterization of simple alkyl radicals such as methyl and ethyl, of unsaturated radicals such as allyl and vinyl, and of aromatic radicals is described here.Keywords:matrix isolation;spectroscopy;infrared;EPR;radicals;methyl;phenyl
Alkylperoxy radicals play critical roles in both combustion and in atmospheric chemistry. This review focuses on what electronic structure calculations, particularly those involving ab initio composite methods and density functional theory, have revealed about the mechanisms of important reactions in combustion and atmospheric systems. The formation of alkylperoxy radicals by addition of O2 to alkyl radicals usually proceeds without an energy barrier, and the barriers that do exist often must be treated by multireference electronic structure methods to be predicted accurately. In contrast, peroxy radical well depths can be predicted accurately by single-reference methods, provided that electron correlation is treated at the CCSD(T) or QCISD(T) level with basis sets containing multiple polarization functions. The composite CBS-QB3 method does particularly well as judged against experimental measurements of carbon-peroxy bond dissociation enthalpies. CBS-QB3 also describes atmospheric reactions of alkylperoxy radicals with HO2 more accurately than the B3LYP method of density functional theory. Finally, the same single-reference methods that proved accurate for peroxy radical well depths can also be employed in accurate statistical rate theory models of the unimolecular reactions involved in low-temperature combustion. Multiple variants of density functional theory have proven considerably less accurate in this regard.
The reaction C2H5 + O2 (+ M → C2H5O2 (+ M) was studied at 298 K at pressures of the bath gas M = Ar between 100 and 1000 bar. The transition from the falloff curve of an energy transfer mechanism to a high pressure range with contributions from the radical complex mechanism was observed. Further experiments were done between 188 and 298 K in the bath gas M = He at pressures in the range 0.7 - 2.0 Torr. The available data are analyzed in terms of unimolecular rate theory. An improved analytical representation of the temperature and pressure dependence of the rate constant is given for conditions where the chemical activation process C2H5 + O2 (+ M) → C2H4 + HO2 (+ M) is only of minor importance.
The pulsed laser photolysis/cw laser absorption technique is used to investigate the reaction of vinyl (C2H3) with NO in the temperature range from 295 to 700 K and pressures from 10 to 320 Torr (1.33 to 42.6 kPa). Vinyl radicals are generated by photolysis of vinyl iodide at 266 nm and detected by visible laser absorption in a vibronic band of the (Ã←) transition near 403 nm. The potential energy surface is explored with both quadratic configuration interaction and multi-reference configuration interaction ab initio calculations. These ab initio predictions are employed in RRKM theory based master equation simulations of the temperature and pressure dependent kinetics. At room temperature, the overall rate constant for removal of vinyl radical by NO is measured to be 1.6±0.4×10−11 cm3 molecule−1 s−1, with negligible pressure dependence from 10 Torr (1.33 kPa) to 160 Torr (21.3 kPa) of helium. At constant pressure the rate constant decreases rapidly with temperature. At higher temperatures, a falloff of the rate constant to lower pressure is observed. The ab initio characterizations suggest a significant contribution from HCN+CH2O formation, with both isomerization transition states for the pathway leading to this product lying ∼15 kcal mol−1 (63 kJ mol−1) below the entrance channel. The master equation analysis provides a reasonably satisfactory reproduction of the observed kinetic data. The HCN+CH2O bimolecular channel, which proceeds from the addition complex through tight ring forming and opening transition states, has a negative temperature dependence and is the dominant channel for pressures of about 50 Torr (6.7 kPa) and lower. The theoretically predicted zero pressure rate coefficient is reproduced by the modified Arrhenius expression 5.02×10−11(T/298)−3.382exp(−516.3/T) cm3 molecule−1 s−1 (with T in K).
The low-pressure-limit unimolecular decomposition of methane, CH4 (+ M) → CH3 + H (+ M), is characterized via low-order moments of the total energy, E, and angular momentum, J, transferred due to collisions. The low-order moments are calculated using ensembles of classical trajectories, with new direct dynamics results for M = H2O and new results for M = O2 compared with previous results for several typical atomic (M = He, Ne, Ar, Kr) and diatomic (M = H2 and N2) bath gases and one polyatomic bath gas, M = CH4. The calculated moments are used to parameterize three different models of the energy transfer function, from which low-pressure-limit rate coefficients for dissociation, k0, are calculated. Both one-dimensional and two-dimensional collisional energy transfer models are considered. The collision efficiency for M = H2O relative to the other bath gases (defined as the ratio of low-pressure limit rate coefficients) is found to depend on temperature, with, e.g., k0(H2O)/k0(Ar) = 7 at 2000 K but only 3 at 300 K. We also consider the rotational collision efficiency of the various baths. Water is the only bath gas found to fully equilibrate rotations, and only at temperatures below 1000 K. At elevated temperatures, the kinetic effect of "weak-collider-in-J" collisions is found to be small. At room temperature, however, the use of an explicitly two-dimensional master equation model that includes weak-collider-in-J effects predicts smaller rate coefficients by 50% relative to the use of a statistical model for rotations. The accuracies of several methods for predicting relative collision efficiencies that do not require solving the master equation and that are based on the calculated low-order moments are tested. Troe's weak collider efficiency, βc, includes the effect of saturation of collision outcomes above threshold and accurately predicts the relative collision efficiencies of the nine baths. Finally, a brief discussion is presented of mechanistic details of the energy transfer process, as inferred from the trajectories.
A combination of liquid He droplet experiments and multireference electronic structure calculations are used to probe the potential energy surface for the reaction between propargyl radical and O2. Infrared laser spectroscopy is used to probe the outcome of the low temperature, liquid He-mediated reaction. Bands in the spectrum are assigned to the acetylenic CH stretch (ν2), the symmetric CH2 stretch (ν2) and the antisymmetric CH2 stretch (ν2) of the acetylenic-trans propargyl peroxy radical ((.)OO-CH2-C≡CH). The observed band origins are in excellent agreement with previously reported anharmonic frequency computations for this species [Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Flowers, B. A.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2010, 114, 1498.]. The Stark spectrum of the ν1 band provides further evidence that the reaction leads only to the acetylenic-trans species. There are no other bands in the CH2 stretching region that can be attributed to any of the other three propargyl peroxy isomers/conformers that are predicted to be minimum energy structures (acetylenic-gauche-, allenic-cis- and allenic-trans-). There is also no evidence for the kinetic stabilization of a van der Waals complex between propargyl and O2. A combination of multireference and coupled-cluster electronic structure calculations are used to probe the potential energy surface in the neighborhood of the transition state connecting reactants with the acetylenic adduct. The multi-reference based evaluation of the doublet-quartet splitting added to the coupled-cluster calculated quartet state energies yields what are likely the most accurate predictions for the doublet potential curve. This calculation suggests that there is no saddle point for the addition process, in agreement with the experimental observations. Other calculations suggest the possible presence of a small submerged barrier.
This paper describes the kinetic study of a number of gas-phase reactions of iron oxides and hydroxides with O, H and O3. These reactions are important for characterising the chemistry of meteor-ablated iron in the earth's upper mesosphere. Pulses of atomic Fe were produced in the upstream section of a fast flow tube by the pulsed laser ablation of a pure Fe rod, and detected at the downstream end by LIF at 248.3 nm Fe(x5F05←a5D4). The Fe-containing reactant species FeO and FeO2 were produced by sequential reaction of Fe with NO2; FeO3 by the reaction of metastable excited Fe atoms with O2 to form FeO, followed by addition of O2; and Fe(OH)2 by the addition of H2O to FeO. Atomic O or H was produced by the microwave discharge of N2
(with addition of NO) or H2, respectively, and their absolute concentrations determined by conventional titration with NO2. Rate coefficients were essentially measured relative to absolute rate coefficients for Fe and FeO determined previously [J. M. C. Plane and R. J. Rollason, Phys. Chem. Chem. Phys, 1999, 1, 1843; R. J. Rollason and J. M. C. Plane, ibid., 2000, 2, 2335], but were extracted using a full kinetic model including diffusive loss on the flow tube walls of the relevant species. The following results were obtained (units: cm3 molecule−1 s−1; quoted uncertainty is 2σ): k(FeO+O→Fe+O2, 209–381 K)=4.6+2.6−1.6×10−10 e−(350±130)/T; k(FeO2+O→FeO+O2, 209–381 K)=1.4+0.8−0.5×10−10 e−(580±120)/T; k(FeO3+O→FeO2+O2, 610 K)=8+10−5×10−12; k(FeO2+O3→FeO3+O2, 224–298 K)=4.4+6.4−2.6×10−10 e−(170±230)/T; k(FeO3+H→FeOH+O2, 294 K)=(2.0+1.2−0.6)×10−11, and k(FeOH+H→products, 294 K)=(1.3±0.3)×10−11 . Theoretical calculations at the B3LYP/6-311+g(2d,p) level were used to identify the stationary points on the relevant potential energy surfaces for most of these reactions, before applying statistical theories to model the kinetics. The implications of these results for the chemistry of iron in flames and the upper atmosphere are then discussed.
The cyclopentadiene/cyclopentadienyl system forms a critical part in the oxidation chemistry of aromatic fuel components used in surrogate fuels and the importance of the cyclopentadienyl radical in poly-aromatic hydrocarbon (PAH) growth has also been noted due to its site dependent reactivity. The latter aspect has been subject to a number of studies along with the initial pyrolysis steps. By contrast, few studies have been performed of the corresponding oxidation chemistry under conditions of relevance to combustion applications. Thermochemical data for oxidation reactions featuring the cyclopentadienyl radical with O, OH, HO2 and O2 were determined at the G3B3 and G4/G4MP2 levels in combination with an analysis of internal rotations using density functional theory and with the Jahn–Teller effect treated as a pseudo-rotation. The calculated potential energy surfaces were subsequently used in a consistent manner for the determination of pressure dependent reaction rate parameters through the Rice–Ramsperger–Kassel–Marcus/master-equation approach with Eckart quantum tunnelling corrections applied to reactions involving hydrogen transfers. The accuracy of the method was further investigated by comparisons of computed rate parameters for pyrolysis reactions with alternative determinations. The resulting chemistry was incorporated into an evaluation framework for the study of cyclopentadiene oxidation using recent experimental flow reactor data and principal uncertainties in reaction pathways assessed.
Reactions of α-hydroxyethyl (CH3CHOH) and β-hydroxyethyl (CH2CH2OH) radicals with oxygen are of key importance in ethanol combustion. High-level ab initio calculations of the potential energy surfaces of these two reactions were coupled with master equation methods to compute rate coefficients and product branching ratios for temperatures of 250–1000K. The α-hydroxyethyl+O2 reaction is controlled by the barrierless entrance channel and shows negligible pressure dependence; in contrast, the reaction of the β isomer displays pronounced pressure dependence. The high pressure limit rate coefficients of both reactions are about the same at the temperatures investigated. Products of the reactions were monitored experimentally at 4Torr and 300–600K using tunable synchrotron photoionization mass spectrometry. Hydroxyethyl radicals were produced from the reaction of ethanol with chlorine atoms and the β isomer was also selectively produced by the addition reaction C2H4+OH→CH2CH2OH. Formaldehyde, acetaldehyde, vinyl alcohol and H2O2 products were detected, in qualitative agreement with the theoretical predictions.
The kinetics of the C3H3+C3H3 reaction was investigated behind incident shock waves at temperatures ranging from 995 to 1440K and at pressures between 600 and 1000mbar with argon as bath gas. The C3H3 radicals were generated by thermal decomposition of propargyl iodide with initial concentrations between 3×1015 and 6×1015cm−3 corresponding to initial mole fractions between 750 and 800ppm. Measurement of the UV absorption at 332.5nm was used to monitor the C3H3 concentration, and the rate coefficients for the C3H3+C3H3 reaction were determined by fitting a second-order rate law to the absorption–time profiles. Values between 2×10−11cm3s−1 near 1000K and 7.5×10−12cm3s−1 near 1400K were obtained with no significant pressure dependence. The negative temperature dependence can be expressed in the form k1=5.8×10−13exp(3534K/T) cm3s−1 with an estimated maximum error of ±30% in k1. The reaction channel leading to C6H5+H is estimated to contribute with less than 10% to the overall reaction at temperatures between 1000 and 1300K and, pressures ranging from 80 to 500mbar.
This review of the role of reaction kinetics in combustion chemistry traces the historical evolution and present state of qualitative and quantitative understanding of a number of reaction systems. Starting from the H2–O2 system, in particular from the reaction between H and O2, mechanisms and key reactions for soot formation, for the appearance of NOx, and for processes of peroxy radicals in hydrocarbon oxidation are illustrated. The struggle for precise rate constants on the experimental and theoretical side is demonstrated for the example of the reaction H+O2→OH+O. The intrinsic complexity of complex-forming bimolecular reactions, such as observed even in this reaction, also dominates most other key reactions of the systems considered and can be unravelled only with the help of quantum-chemical methods. The multi-channel character of these reactions often also requires the combination with master equation codes. Although kinetics provides an already impressive database for quantitative modelling of simple combustion systems, considerable effort is still required to quantitatively account for the complexities of more complicated fuel oxidation processes.
This article reviews recent crossed-beams and ab-initio studies of reactions of ground-state carbon atoms C( 3 Pj) with unsaturated hydrocarbons and their radicals. All reactions have no entrance barrier and are initiated via an addition of the carbon atom to the p system. With the exception of the carbon atom reaction with acetylene, which also shows a signi®cant fraction of molecular hydrogen loss, these bimolecular collisions are dominated by an atomic carbon versus hydrogen atom exchange mechanism. In some systems, homolytic cleavages of carbon±carbon bonds present additional decomposition routes of typically a few per cent at the most. The impact-parameter-dependent chemical dynamics are interpreted in terms of statistical and non-statistical decomposition of reaction intermediates involved. The polyatomic reaction products are highly hydrogen- de®cient resonance-stabilized free radicals. The latter are strongly suggested as suitable precursors to form the ®rst (substituted) aromatic ring molecule, polycyclic aromatic hydrocarbon-like species and carbonaceous nanoparticles in combustion processes, chemical vapour deposition and the outows of carbon stars.
The kinetics of the C3H3+C3H3 reaction was investigated by using dipropargyl oxalate (DPO) as a new, halogen-free photolytic source for propargyl radicals in the gas phase. After laser-flash photolysis of DPO at 193 nm, the initial absorbance was determined at different wavelengths, and the results were compared with values obtained in analogous experiments using propargyl halides as precursors. A satisfactory agreement of the absorbances was found between 295 and 355 nm but differences were observed near 242 nm. The latter wavelength has also been proposed for C3H3 detection. Our results, however, indicate that this absorption is probably due to halogen-containing species. The rate coefficient of the C3H3+C3H3 reaction was then determined from time-resolved absorption measurements at 332.5 nm with DPO as precursor. Values of (2.7±0.6)×10−11 cm3 molecule−1 s−1 at 373 K, (2.8±0.6)×10−11 cm3 molecule−1 s−1 at 425 K, (3.5±0.8)×10−11 cm3 molecule−1 s−1 at 500 K, and (4.1±0.8)×10−11 cm3 molecule−1 s−1 at 520 K were obtained with no significant pressure dependence between 1 and ca. 100 bar (140 bar for T=373 K).
The kinetics of the reactions of vinyl (C2H3) and propargyl (C3H3) radicals with NO2 have been studied in direct measurements at temperatures between 220 and 340 K, using a tubular flow reactor coupled to a photoionization mass spectrometer. The vinyl and propargyl radicals have been homogeneously generated at 193 nm by the pulsed laser photolysis of methyl vinyl ketone (vinyl bromide) and propargyl chloride, respectively. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of NO2 being in large excess over radical concentrations. The bimolecular rate coefficients of both reactions are independent of the bath gas (He or N2) and pressure within the experimental range (1−7 Torr) and are found to depend on temperature as follows: k(C2H3 + NO2) = [(4.19 ± 0.05) × 10-11](T/300 K)-0.60 ± 0.07 cm3 molecule-1 s-1 and k(C3H3 + NO2) = [(2.55 ± 0.05) × 10-11](T/300 K)-1.06 ± 0.10 cm3 molecule-1 s-1, with the uncertainties given as 1 standard deviation. The photolysis of propargyl chloride has also been observed to produce C3H3Cl2 radicals rapidly under the experimental conditions, thus enabling us to measure the bimolecular reaction rate coefficient of the C3H3Cl2 radical with NO2 at room temperature: k(C3H3Cl2 + NO2) = (2.37 ± 0.05) × 10-11 cm3 molecule-1 s-1. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are about ±20%. The only reaction product observed for the vinyl radical reaction with NO2 is NO. The experimental findings have been compared with the results of ab initio calculations, which give insight into possible reaction pathways.
Using various forms of electronic-structure theory to characterize the important features of the potential energy surface, RRKM theory to calculate microcanonical rate coeffients, and several formulations of the master equation to predict phenomenological rate coefficients, we have studied a number of reactions that occur on the C3H4 potential. We discuss the results in some detail and compare them with experiment when possible. Generally, the agreement with experiment is excellent. “Multiple-well effects” are emphasized throughout the discussion. We cast our results in the form of modified Arrhenius functions for use in chemical kinetics modeling.
In the present study, the reaction mechanism of NCS+NO2 on the singlet potential energy surface is studied using the MP2/6-311G(d,p) level of theory. It is shown that the pathway (1) NCS+NO2⇌O(O)NNCS→O(NNC(S)O)→N2O+OCS is the major pathway, and the pathways (2) NCS+NO2⇌O(O)NSCN→ONS+OCN and (3) NCS+NO2⇌cis‐cis‐ONONCS→NO+SCNO are the minor pathways in the singlet potential energy surface. The major pathway (1), involving the barrierless entrance to the first adduct isomer O(O)NNCS and tight transition states to the products N2O and OCS, is in good agreement with the experimentally observed negative temperature dependence of rate constants. The energies of the stationary points are refined using a multi-level method.
The A∼-X∼ electronic absorption spectrum of allyl peroxy radical has been recorded at room temperature by cavity ringdown spectroscopy. Photolysis of an allyl bromide/O2 mixture at 193nm or 248nm yields allyl peroxy and HO2 radicals, with 248nm photolysis more strongly favoring allyl peroxy formation. Utilizing electronic structure calculations (including computed OOCC torsional potentials), the experimental spectrum is assigned in terms of the three conformers predicted to have the greatest intensity. This work represents an extension of recent progress in measuring A∼-X∼ absorption spectra of saturated organic peroxy radicals to unsaturated species.
The crossed molecular beams method has been applied to produce the 1-butene-3-yne-2-yl radical, i-C4H3(X2A') under single collision conditions via the reaction of dicarbon molecules with ethylene. We recorded time-of-flight spectra of the radical at the center-of-mass angle (28.0°) of the parent ion (m/z = 51; C4H3+) and of the fragments at m/z = 50 (C4H2+), m/z = 49 (C4H+), m/z = 48 (C4+), m/z = 39 (C3H3+), m/z = 38 (C3H2+), m/z = 37 (C3H+), and m/z = 36 (C3+). This yielded relative intensity ratios of I(m/z = 51):I(m/z = 50):I(m/z = 49):I(m/z = 48):I(m/z = 39):I(m/z = 38):I(m/z = 37):I(m/z = 36) = 0.47 +/- 0.01:0.94 +/- 0.01:1.0:0.07 +/- 0.02:0.31 +/- 0.01:0.23 +/- 0.02:0.24 +/- 0.01:0.12 +/- 0.01 at 70 eV electron impact energy. Upper limits at mass-to-charge ratios between 27 and m/z = 24 and m/z = 14-12 were derived to be 0.02 +/- 0.01. Note that the intensity of the 13C isotopic peak of the 1-butene-3-yne-2-yl radical at m/z = 52 (13C12C3H3+) is about 0.04 +/- 0.01 relative to m/z = 51. Employing linear scaling methods, the absolute electron impact ionization cross section of the 1-butene-3-yne-2-yl radical was computed to be 7.8 +/- 1.6 × 10-16 cm2. These data can be employed to monitor the 1-butene-3-yne-2-yl radical in oxygen-poor combustion flames and in the framework of prospective explorations of planetary atmospheres (Jupiter, Saturn, Uranus, Neptune, Pluto) and of their moons (Titan, Triton, Oberon) in situ via matrix interval arithmetic assisted mass spectrometry.
Complex chemical reactions in the gas phase can be decomposed into a network of elementary (e.g., unimolecular and bimolecular) steps which may involve multiple reactant channels, multiple intermediates, and multiple products. The modeling of such reactions involves describing the molecular species and their transformation by reaction at a detailed level. Here we focus on a detailed modeling of the C(3P)+allene(C3H4) reaction, for which molecular beam experiments and theoretical calculations have previously been performed. In our previous calculations, product branching ratios for a nonrotating isomerizing unimolecular system were predicted. We extend the previous calculations to predict absolute unimolecular rate coefficients and branching ratios using microcanonical variational transition state theory (μ-VTST) with full energy and angular momentum resolution. Our calculation of the initial capture rate is facilitated by systematic ab initio potential energy surface calculations that describe the interaction potential between carbon and allene as a function of the angle of attack. Furthermore, the chemical kinetic scheme is enhanced to explicitly treat the entrance channels in terms of a predicted overall input flux and also to allow for the possibility of redissociation via the entrance channels. Thus, the computation of total bimolecular reaction rates and partial capture rates is now possible.
The geometries, energies, and vibrational frequencies of the reactants, transition states, intermediates, and products of the reaction of ethyl radical with the oxygen molecule have been examined using density functional theory (DFT). Rather different theoretical predictions are obtained from the BLYP, B3LYP, and BHLYP methods. Comparisons with experimental deductions and high-level coupled cluster results suggest that the B3LYP method is superior for the C2H5+O2 problem. Using the B3LYP method with a triple-zeta plus double-polarization plus f function (TZ2Pf) basis set, a transition state between the ethylperoxy radical and products is discovered which lies 3.3 kcal mol−1below reactants. This transition-state energy is consistent with the observed high yields of ethylene in the high-temperature reaction and is in good agreement with the height of the barrier estimated via modeling of the experimental kinetic data. However, this transition state (TS1) corresponds not to the internal proton transfer leading to the hydroperoxyethyl radical C2H4OOH but to the concerted elimination of ethylene. For the reverse reaction C2H4+HO2↠C2H4OOH, the TZ2Pf UB3LYP classical barrier is 11.2 kcal mol−1.
Gaussian-3 theory (G3 theory) for the calculation of molecular energies of compounds containing first (Li–F) and second row (Na–Cl) atoms is presented. This new theoretical procedure, which is based on ab initio molecular-orbital theory, modifies G2 theory [J. Chem. Phys. 94, 7221 (1991)] in several ways including a new sequence of single point energy calculations using different basis sets, a new formulation of the higher level correction, a spin–orbit correction for atoms, and a correction for core correlation. G3 theory is assessed using 299 energies from the G2/97 test set including enthalpies of formation, ionization potentials, electron affinities, and proton affinities. This new procedure corrects many of the deficiencies of G2 theory. There is a large improvement for nonhydrogen systems such as SiF4 and CF4, substituted hydrocarbons, and unsaturated cyclic species. Core-related correlation is found to be a significant factor, especially for species with unsaturated rings. The average absolute deviation from experiment for the 148 calculated enthalpies of formation is reduced to under one kcal/mol, from 1.56 kcal/mol for G2 theory to 0.94 kcal/mol for G3 theory. Significant improvement is also found for ionization potentials and electron affinities. The overall average absolute deviation of G3 theory from experiment for the 299 energies is 1.02 kcal/mol compared to 1.48 kcal/mol for G2 theory. © 1998 American Institute of Physics.
By using 193 nm laser photolysis and cavity ring-down spectroscopy to produce and monitor the propargyl radical (CH2CCH), the self-reaction and oxygen termolecular association rate coefficients for the propargyl radical were measured at 295 K between total pressures of 300 Pa and 13300 Pa (2.25 and 100 Torr) in Ar, He, and N-2 buffer gases. The rate coefficients obtained by simple second-order fits to the decay data were observed to vary with the photolytic precursors: allene, propargyl chloride, and propargyl bromide. By using a numerical fitting routine and more comprehensive mechanisms, a distinct rate coefficient for the self-reaction was determined, k(infinity)(C3H3+C3H3) = (4.3 +/- 0.6) x 10(-11) cm(3) molecule(-1) s(-1) at 295 K. This rate coefficient which is a factor of 2.8 times slower than reported previously, was independent of total pressure and buffer choice over the entire pressure range. Other rate coefficients derived during the modeling included k(C3H3+H 665 Pa He) = (2.5 +/- 1.1) x 10(-10) cm(3) molecule(-1) s(-1), k(C3H3+C3H3Cl2) = (7 +/- 4) x 10(-11) cm(3) molecule(-1) s(-1), and k(C3H3+C3H3Br2) = (2.4 +/- 2) x 10(-11) cm(3) molecule(-1) s(-1). The association reaction C3H3+O-2 was found to lie in the falloff region between linear and saturated pressure dependence for each buffer gas (Ar, He, and N-2) between 300 Pa and 13300 Pa. A fit of these data derived the high-pressure limiting rate coefficient k(infinity)(C3H3+O-2) = (2.3 +/- 0.5) x 10(-13) cm(3) molecule(-1) s(-1). Three measurements of the propargyl radical-absorption cross-section obtained sigma(332.5) = (413 +/- 60) x 10(-20) cm(2) molecule(-1) at 332.5 nm. Stated uncertainties are two standard deviations and include the uncertainty of the absorption cross section, where appropriate.
The stochastic model relates the rate of a chemical reaction to the underlying transition probabilities.
VODE is a new initial value ODE solver for stiff and nonstiff systems. It uses variable-coefficient Adams-Moulton and Backward Differentiation Formula (BDF) methods in Nordsieck form, as taken from the older solvers EPISODE and EPISODEB, treating the Jacobian as full or banded. Unlike the older codes, VODE has a highly flexible user interface that is nearly identical to that of the ODEPACK solver LSODE.
In the process, several algorithmic improvements have been made in VODE, aside from the new user interface. First, a change in stepsize and/or order that is decided upon at the end of one successful step is not implemented until the start of the next step, so that interpolations performed between steps use the more correct data. Second, a new algorithm for setting the initial stepsize has been included, which iterates briefly to estimate the required second derivative vector. Efficiency is often greatly enhanced by an added algorithm for saving and reusing the Jacobian matrix J, as it occurs in the Newton matrix, under certain conditions. As an option, this Jacobian-saving feature can be suppressed if the required extra storage is prohibitive. Finally, the modified Newton iteration is relaxed by a scalar factor in the stiff case, as a partial correction for the fact that the scalar coefficient in the Newton matrix may be out of date.
The fixed-leading-coefficient form of the BDF methods has been studied independently, and a version of VODE that incorporates it has been developed. This version does show better performance on some problems, but further tuning and testing are needed to make a final evaluation of it.
Like its predecessors, VODE demonstrates that multistep methods with fully variable stepsizes and coefficients can outperform fixed-step-interpolatory methods on problems with widely different active time scales. In one comparison test, on a one-dimensional diurnal kinetics-transport problem with a banded internal Jacobian, the run time for VODE was 36 percent lower than that of LSODE without the J-saving algorithm and 49 percent lower with it. The fixed-leading-coefficient version ran slightly faster, by another 12 percent without J-saving and 5 percent with it. All of the runs achieved about the same accuracy.
The reaction of the ethynyl radical with molecular oxygen has been examined using density functional theory. Two major reaction routes are open to the chemically activated HCCOO adduct : dissociation to HCCO+O and formation of the thermodynamically most stable products HCO + CO : HCCOO → dioxirenyl → oxyrenyloxy → oxo-ketene → HCO + CO → H + 2CO . The CCSD(T)/6-311++G(d,p)// B3LYP/6-311++G(d,p) energies of the respective rate controlling transition states, and , indicate that the route leading to H + CO + CO should dominate. Several other [C2,H,O2] isomers and other, minor pathways have also been characterised. The present study reveals this reaction to be a capture-limited association-elimination reaction with a high and pressure-independent rate coefficient.
We have studied the effects of adding allene (C3H4) to a rich (φ = 1.67) C2H2/O2/Ar flame. Temperatures were measured by thermocouple and by OH laser-induced fluorescence. Stable species profiles were determined from mass spectrometer measurements using a quartz microprobe, and OH and CH concentrations were determined using LIF. The experiments were analyzed with the aid of a chemical kinetic model. The most noteworthy result of our experiments is that significant quantities of benzene appear in the flame with allene, whereas there is no detectable benzene in the pure acetylene flame. This result lends support to the theory that the reaction between two propargyl radicals is an important “cyclization step” in flames. Various aspects of the flame chemistry are discussed in depth.
We have studied the reaction between vinyl and acetylene theoretically using electronic structure theory (DFT-B3LYP and a G2-like method) to calculate properties of stationary points on the potential, RRKM theory to compute microcanonical rate coefficients, and solutions to the time-dependent, multiple-well master equation to extract information about the thermal rate coefficient and product distribution as a function of temperature and pressure. For the temperature range, 300 K ≤ T ≤ 700 K, both the total rate coefficient k1(T,p) and the products are functions of pressure. For 700 K ≤ T ≤ 900 K, k1(T,p) is not always well defined in that the reactants can exhibit nonexponential decays in time. At sufficiently high pressure, the dominant product of the reaction changes from n-C4H5 to c-C4H5 (a four-numbered ring) to C4H4 + H, where C4H4 is vinyl acetylene, as the temperature is increased from 600 K to 900 K. For T > 900 K, the reaction can be written as an elementary step, C2H3 + C2H2 → C4H4 + H (R1), with a rate coefficient, k1 = 2.19 × 10-12T0.163 exp(−8312/RT) cm3/(molecule·s), independent of pressure, even though the intermediate collision complex may suffer numerous collisions. We interpret our results in terms of the eigenvalues and eigenvectors of the G matrix, i.e., the relaxation/reaction matrix of the master equation. For T > 900 K, k1(T,p) always corresponds to the largest eigenvalue of G, which in turn corresponds to the zero-pressure-limit rate coefficient k0(T). The situation is more complicated at lower temperatures. Our predictions are in good agreement with the limited amount of experimental information available on the reaction. The quantum chemistry calculations indicate that both c-C4H5 and i-C4H5 are more stable than n-C4H5. The G2-like method gives results for the ΔHf(0)(0 K) of c-C4H5 and i-C4H5 that are lower that that of n-C4H5 by 9.5 and 11.2 kcal/mol, respectively. The DFT-B3LYP results show similar differences of 6.0 and 13.7 kcal/mol, respectively.
A potential energy surface for the reaction of vinyl radical with molecular oxygen has been studied using the ab initio G2M(RCC, MP2) method. The most favorable reaction pathway leading to the major CHO+CHâO products is described. The CâHâO+O products can be formed by elimination of the oxygen atom from CâHâOO via TS 23, which is by 7.8 kcal/mol lower in energy than the reactants, but by 6.5 kcal/mol higher than TS 9`. The hydrogen migration in 1` gives rise to another significant product channel: CâHâ+Oâ â 1` â TS 25` â CâHâ+OâH, with TS 25` lying below CâHâ+Oâ by 3.5 kcal/mol. Multichannel RRKM calculations have been carried out for the total and individual rate constants for various channels using the G2M(RCC, MP2) energetics and molecular parameters of the intermediates and transition states. The computed low pressure reaction rate constant is in quantitative agreement with experiment. At atmospheric pressure, the title reaction is dominated by the stabilization of vinylperoxy radical CâHâOO at room temperature. In the 500-900 K temperature range, the CHO+CHâO channel has the highest rate constant, and at T >= 900 K, CâHâO+O are the major products. At very high temperatures, the channel producing CâHâ + OâH becomes competitive. 15 refs., 3 figs., 4 tabs.
The propargyl radical, C3H3, is thought to be an important precursor to the formation of aromatic
compounds and of soot in combustion systems. These radicals are produced during combustion by the reaction of 1CH2 with acetylene, which proceeds via a three well mechanism. A master equation model of this system is constructed with the aim of determining the branching ratio for formation of the propargyl radical as a function of temperature and pressure. The rate limiting step is the initial formation of cyclopropene from the reactants and a knowledge of the rate of this reaction is necessary for accurate modelling. The rate coefficient for the overall reaction was measured, as a function of temperature, using laser flash photolysis of a ketene–acetylene mixture. The reaction was monitored by laser induced fluorescence of 1CH2. Experimental results are presented and used in the master equation model, which shows that the yield, γ, of dissociation products H+C3H3 decreases with increasing pressure and that the onset of the decrease shifts to higher pressures as the temperature increases. At higher pressures and temperatures, there is an overlap in the timescales of dissociation of thermalised C3H4 and of the nascent C3H4* formed from 1CH2+C2H2,
so that a simple
description through time independent rate coefficients is no longer possible.
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.
Complete active space SCF/internally contracted configuration interaction calculations using large atomic natural orbital basis sets are reported for CH3+O2. Two potential energy surfaces are found to be important in the CH3+O2 reaction. In Cs symmetry, the lower 2A″ surface correlates with CH3+O2(3Σ−g) and connects to a bound CH3OO species with no barrier, but leads only to CH3O+O products. A higher surface of 2A′ symmetry correlates with CH3+O2(1Δ−g and leads to CH2+OH with a computed barrier of 13.7 kcal/mol (with respect to CH3+O2(3Σ−g)). Even in lower symmetry, two surfaces are involved leading to a more complex model for this reaction than had been previously considered.
The authors have studied the NHâ + NO reaction theoretically in order to try to deduce a theoretical model that will accurately reproduce both the total rate coefficient k{sub T}(T) and the branching fraction α(T) of the reaction NHâ + NO â Nâ + HâO (a), NHâ + NO â NNH + OH (b), and NHâ + NO â NâO + Hâ (c), where k{sub T} = k{sub a} + k{sub b} + k{sub c} and α = k{sub b}/k{sub T}. The analysis, which makes the RRKM assumption and utilizes conventional transition-state theory for the internal-rearrangement transition states and microcanconical/fixed-J variational transition-state theory for the bond fissions, is discussed at length. The results of the analysis show clearly that k{sub T}(T) is determined almost exclusively by the transition state for the 1,3 hydrogen transfer connecting the initial NHâNO complex to HNNOH. The branching fraction is sensitive to several features of the potential energy surface, most of them associated with the fragmentation of the various HNNOH complexes into NNH + OH. By adjusting properties of the potential energy surface, a constructed theoretical model that predicts results for both k{sub T}(T) and α(T) that are in good agreement with experiment. A variety of sensitivity analyses for the branching fraction indicate that reaction b is most likely thermoneutral to within {+-}1 kcal/mol. The authors prediction of k{sub c}(T) is between 2 and 3 orders of magnitude smaller than values deduced from experiment suggesting that the experiment may have detected the existence of a fourth channel, HNNO + H, or may have been contaminated by secondary reactions.
A comprehensive, semidetailed kinetic scheme describing hydrocarbon oxidation is applied to the simulation of premixed, rich, sooting, ethylene laminar flames. The main goal of this work is to investigate the soot precursor and aromatic pathways under different operative conditions in terms of temperatures and feed composition. The modeling computations are in good agreement with the experimental data and are also comparable with predictions of different kinetic schemes present in the literature. The recombination reactions of resonantly stabilized radicals (such as propargyl) and C2H2 addition on linear dehydrogenated molecules (C4Hx) are taken into account to explain formation of the first aromatic ring. Two major channels of benzene and polycyclic aromatic hydrocarbon (PAH) formation are observed in the conditions under analysis. The former, which is not included in previous literature schemes, is faster and occurs first (where the conversion is still low). It is governed by ethylene and vinyl radical, which, through butadiene and butenyl radicals, explain the formation of cyclopentadiene and through further successive additions give rise to benzene and styrene. This mechanism should be the starting point for the initial formation of heavy highly hydrogenated compounds. Acetylene and resonantly stabilized radicals are mainly responsible for the successive aromatic growth. The study of such pathways is also important for the analysis of low NOx burners and new process alternatives, such as recirculating flue gases, where pollutant emission reductions are pursued by the use of low-temperature flames. The comparisons with experimental data for pure ethylene pyrolysis at lower temperatures (1100 K) confirms the validity of the assumed mechanism.
Detailed chemical kinetic modeling has been performed to investigate aromatic and polyaromatic hydrocarbon formation pathways in rich, sooting, methane and ethane premixed flames. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.5 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph / mass spectrometer technique. Measurements were made in the flame and post-flame zone for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-aromatic fused rings. The modeling results show the key reaction sequences leading to aromatic and polycyclic aromatic hydrocarbon formation primarily involve the combination of resonantly stabilized radicals. In particular, propargyl and I-methylallenyl combination reactions lead to benzene and methyl substituted benzene formation, while polycyclic aromatics are formed from cyclopentadienyl and fused rings that have a shared C3, side structure. Naphthalene production through the reaction step of cyclopentadienyl self-combination, and phenanthrene formation from indenyl and cyclopentadienyl combination were shown to be important in the flame modeling study. The removal of phenyl by O2leading to cyclopentadienyl formation isexpccted to playa pivotal role in the PAH or soot precursor growth process under fuel-rich oxidation conditions.
Using the BAC-MP4 potential surface parameters of Melius and Binkley, we have predicted the thermal rate coefficients for the two reactions: O + HCN ..-->.. NCO + H (a) and O + HCN ..-->.. NH + CO (b). Several levels of approximation are used in the theoretical treatment: a, canonical theory; b, canonical theory with Wigner tunneling correction; c, microcanonical theory (energy conserving); d, microcanonical/J-conservative theory (conserves both energy and angular momentum); e, microcanonical/J-conservative theory with one-dimensional tunneling. At high temperature the available experimental results are predicted accurately by even the crudest theoretical treatment (canonical theory). At lower temperature the theoretical predictions using the basic BAC-MP4 parameters are too low. However, adjustments to the BAC-MP4 energy barriers within their stated error limits lead to satisfactory agreement with experiment over the entire temperature range where experimental results are available (500 to 2500 K). The most important results of the investigation concern the dependence of the predictions on the level of approximation. Each successive refinement in the theory produces larger values of k/sub b/. The details of the theoretical treatment and comparisons with experiment are described in detail.
Two soluble models of chemical reaction kinetics are treated. It is found that the equilibrium approximation overestimates the true rate constants by a factor that is typically about 2 and may often be much larger. The mathematical mechanism by which a high activation energy is reflected in a long chemical relaxation time is studied and is found to be the same as the mechanism by which, in quantum mechanics, tunneling through a high potential barrier between two wells in resonance leads to a slight separation between the two lowest energy levels.
The idea that the chemical principle of detailed balance is a consequence of microscopic reversibility is examined in terms of the stochastic theory of chemical reaction rates, particularly as developed by Snider. It is emphasized that detailed balance is a purely macroscopic requirement for all cases except those in which the chemical reaction mechanism contains a closed loop, or where forward and reverse rate constants for an elementary reaction step are measured in different experimental situations, each designed to isolate one unidirectional elementary reaction from its counterpart. Snider's theory is extended to a simpe triangular (closed‐loop) isomerization mechanism, and it is shown that detailed balance is not obtained as a result; this is in contrast to local‐equilibrium theories, for which the relationship between microscopic reversibility and detailed balance is well established. An attempt to overcome this difficulty by reformulating Snider's theory, in terms of the theory of lumping error, was only partly successful. The limitations of the present approach are discussed, and consequences for universally valid interpretations of phenomenological chemical kinetics are suggested.
We have studied the oxidation of allene in a rich ( φ = 1.5), low-pressure H2/O2/Ar flame to which one percent altene was added. Temperatures were measured by thermocouple and by OH laser-induced fluorescence. Stable species profiles were determined from mass spectrometer measurements using a quartz microprobe, and OH concentrations were determined using LIF. The experiments were analyzed with the aid of a chemical kinetic model. Significant quantities of methane and acetylene are formed early in the flame as a consequence of recombination of propargyl with hydrogen atoms,H + C2H3 (+ M) ↔ C3H4PH + C3H4P ↔CH3 + C2H2H+CH3( + M) ↔ CH4( + M),where C3H4P is propyne. However, the main oxidation path appears to beC2H3 + H↔ C2H2 + H2C3H2 + O2 ↔ productsThe underlying flame chemistry is discussed in depth, including the thermochemistry and molecular structure of various C3,H2 isomers.
We present a method for estimating the core-correlation contribution to the binding energy of molecules without electronic structure calculations. The method is parametrized for molecules containing H, Li, Be, B, C, N, O, F, Al, Si, P, S, and Cl. This method can be used for the prediction and estimation and thermochemical properties of large molecules.
The recombination of propargyl radicals is believed to be the single most important elementary step leading to cyclization, PAH growth, and ultimately soot in flames burning aliphatic fuels. The time-dependent, seven-well master equation were analyzed to determine rate coefficients and product distributions of the C 3H 3 + C 3H 3 reaction as a function of temperature and pressure. BAC-MP4 potential-energy surface information was used to describe the internal rearrangements of the C 6H 6 adduct. For computational reasons, the temperature and pressure ranges studied were 1000-2000 K and 1-10 atm. Benzene was not a very important product (maximum yield was only about 1%). At lower temperatures, the dominant products were phenyl + H, 2, ethynyl-1, 3 butadiene, and fulvene, with the former dominating at low pressure and the latter at high pressure. However, at 1000 K other C 6H 6 species begin to appear, most notably dimethylene-cyclobutene in the 1 atm < p = 10 atm range. Original is an abstract.
Using the BAC-MP4 potential surface parameters of Melius and Binkley, we have predicted the thermal rate coefficients for the two reactions: O + HCN ..-->.. NCO + H (a) and O + HCN ..-->.. NH + CO (b). Several levels of approximation are used in the theoretical treatment: a, canonical theory; b, canonical theory with Wigner tunneling correction; c, microcanonical theory (energy conserving); d, microcanonical/J-conservative theory (conserves both energy and angular momentum); e, microcanonical/J-conservative theory with one-dimensional tunneling. At high temperature the available experimental results are predicted accurately by even the crudest theoretical treatment (canonical theory). At lower temperature the theoretical predictions using the basic BAC-MP4 parameters are too low. However, adjustments to the BAC-MP4 energy barriers within their stated error limits lead to satisfactory agreement with experiment over the entire temperature range where experimental results are available (500 to 2500 K). The most important results of the investigation concern the dependence of the predictions on the level of approximation. Each successive refinement in the theory produces larger values of k/sub b/. The details of the theoretical treatment and comparisons with experiment are described in detail.
The purpose of the work described in this paper is to investigate the utility of ab initio molecular orbital calculations for the prediction of rate constants and activation parameters of reactions occurring in hydrocarbon combustion. The reaction of vinyl radical with oxygen has been chosen because there exist reliable experimental data against which the calculations can be calibrated. The results suggest that good agreement (within a factor of 2) between observed and calculated rate constants can be achieved but only ifa mechanism different from the one previously assumed is employed. The new mechanism involves cyclization of the first-formed vinylperoxy radical to a three-membered-ring dioxiranylmethyl radical rather than the four-membered-ring dioxetanyl radical that was assumed in earlier mechanisms. The agreement of the computed rate constants with existing experimental data, as well as the identification of the new mechanism, would appear to imply that ab initio calculations of the type described can have a useful role in the analysis of combustion processes. Predictions of results expected in shock-tube studies of the reaction are presented. It is shown that shock-tube experiments should provide definitive distinction between the old and new mechanisms for the reaction.
A new kinetic mechanism has been developed for the formation of benzene and high-molecular-mass aromatic compounds in rich flames of aliphatic hydrocarbons. The kinetic scheme emphasizes both the role of resonantly stabilized radicals in the growth of aromatics and the standard acetylene addition mechanism. The model has been used to simulate premixed flames of acetylene and ethylene where the concentrations of radicals and high-molecular-mass compounds are known. The kinetic scheme accurately reproduces the concentrations and trends of radicals and stable species including benzene and total aromatic compounds. Formation of benzene is controlled by propargyl radical combination. The model reproduces well the profiles of benzene for the different hydrocarbons and in the different operating conditions. Key reactions in the formation of high-molecular-mass aromatics are the combinations of resonantly stabilized radicals, including cyclopentadienyl self-combination, propargyl addition to benzyl radicals, and the sequential addition of propargyl radicals to aromatic rings. The predicted amounts of total aromatic compounds increase at the flame front and remain constant in the postoxidation zone of the flames, attaining the final concentrations of soot, in slightly-sooting conditions. As a consequence, the carbonaceous species which contribute to soot formation are already present at the flame front as high-molecular-mass structures. Soot is formed through dehydrogenation and aromatization of the high-molecular-mass compounds, rather than by surface growth.
We propose a multi-coefficient modification (MCG3) of the Gaussian-3 (G3) electronic structure method that is suitable for calculating continuous potential energy surfaces. We tested it for atomization energies and found that it improves the accuracy by 8% as compared to G3 and reduces the cost of single-point energy calculations by 50%. The method should be useful for calculating bond energies, potential energy surfaces, and thermochemical data of molecules.
We have formulated a chemical kinetic model to predict the growth of higher hydrocarbons in a lightly sooting C2H2/O2/Ar flame. The predictions of the model compare favorably with the experimental results of Bastin et al. (Twenty-Second Combustion Symposium). Analysis of the mechanism shows that reactions of 1CH2 play a central role in forming the C3 and C4 hydrocarbons that ultimately lead to ring formation. Several possibilities are considered for the cyclization reaction. The most likely candidates involve reaction complexes in which the hydrogen atoms are not optimally placed. This point is discussed in some detail. We argue that the “first ring” is most likely formed by reaction of two propargyl radicals, C3H3 + C3H3 ⇐ C6H5 + H or C3H3 + C3H3 ⇐ C6H6.
The generation by combustion processes of airborne species of current health concern such as polycyclic aromatic hydrocarbons (PAH) and soot particles necessitates a detailed understanding of chemical reaction pathways responsible for their formation. The present review discusses a general scheme of PAH formation and sequential growth of PAH by reactions with stable and radical species, including single-ring aromatics, other PAH and acetylene, followed by the nucleation or inception of small soot particles, soot growth by coagulation and mass addition from gas phase species, and carbonization of the particulate material. Experimental and theoretical tools which have allowed the achievement of deeper insight into the corresponding chemical processes are presented. The significant roles of propargyl (C3H3) and cyclopentadienyl (C5H5) radicals in the formation of first aromatic rings in combustion of aliphatic fuels are discussed. Detailed kinetic modeling of well-defined combustion systems, such as premixed flames, for which sufficient experimental data for a quantitative understanding are available, is of increasing importance. Reliable thermodynamic and kinetic property data are also required for meaningful conclusions, and computational techniques for their determination are presented. Routes of ongoing and future research leading to more detailed experimental data as well as computational approaches for the exploration of elementary reaction steps and the description of systems of increasing complexity are discussed.
A general procedure is introduced for calculation of the electron correlationenergy, starting from a single Hartree–Fock determinant. The normal equations of (linear) configuration interaction theory are modified by introducing new terms which are quadratic in the configuration coefficients and which ensure size consistency in the resulting total energy. When used in the truncated configuration space of single and double substitutions, the method, termed QCISD, leads to a tractable set of quadratic equations. The relation of this method to coupled‐cluster (CCSD) theory is discussed. A simplified method of adding corrections for triple substitutions is outlined, leading to a method termed QCISD(T). Both of these new procedures are tested (and compared with other procedures) by application to some small systems for which full configuration interaction results are available.
Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
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