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The kinetics of peroxy radical (RO

Dimethyl ether (DME) oxidation is a model chemical system with a small number of prototypical reaction intermediates that also has practical importance for low-carbon transportation. Although it has been studied experimentally and theoretically, ambiguity remains in the relative importance of competing DME oxidation pathways in the low-temperature autoignition regime. To focus on the primary reactions in DME autoignition, we measured the time-resolved concentration of five intermediates, CH

The development of high-fidelity mechanisms for chemically reactive systems is a challenging process that requires the compilation of rate descriptions for a large and somewhat ill-defined set of reactions. The present unified combination of modeling, experiment, and theory provides a paradigm for improving such mechanism development efforts. Here we combine broadband rotational spectroscopy with detailed chemical modeling based on rate constants obtained from automated ab initio transition state theory-based master equation calculations and high-level thermochemical parametrizations. Broadband rotational spectroscopy offers quantitative and isomer-specific detection by which branching ratios of polar reaction products may be obtained. Using this technique, we observe and characterize products arising from H atom substitution reactions in the flash pyrolysis of acetone (CH3C(O)CH3) at a nominal temperature of 1800 K. The major product observed is ketene (CH2CO). Minor products identified include acetaldehyde (CH3CHO), propyne (CH3CCH), propene (CH2CHCH3), and water (HDO). Literature mechanisms for the pyrolysis of acetone do not adequately describe the minor products. The inclusion of a variety of substitution reactions, with rate constants and thermochemistry obtained from automated ab initio kinetics predictions and Active Thermochemical Tables analyses, demonstrates an important role for such processes. The pathway to acetaldehyde is shown to be a direct result of substitution of acetone's methyl group by a free H atom, while propene formation arises from OH substitution in the enol form of acetone by a free H atom.

Cycloalkanes and alkenes are important components of real fuels but there is little kinetic and mechanistic data on the dissociation of most large cyclic and olefinic molecules at elevated temperatures. We present here the first experimental and theoretical investigation of dissociation of cycloheptane and the initial product from ring opening, 1-heptene. Experiments were performed in a diaphragmless shock tube using laser schlieren densitometry. Pyrolysis of cycloheptane (0.5–4% in Kr) was studied over 1450–2000 K and 30–120 Torr. Experiments with 1-heptene (1–4% in Kr) covered 1200–1650 K and 30–120 Torr. A newly developed chemical kinetic mechanism for pyrolysis of cycloheptane and 1-heptene is presented herein. Simulations are in very good agreement with the experimental measurements. Rate coefficients for the initial ring-opening process in cycloheptane, k1, and dissociation of 1-heptene, k2, were determined from the experiments. Both k1 and k2 are in falloff, and the pressure and temperature dependencies were well reproduced by theoretical calculations allowing extrapolation to conditions beyond the scope of this work. These calculations yielded the following expressions for k1 and k2 with the uncertainties estimated as ±40% and ±50% respectively: k 1 , ∞ = 5.94 × 10 17 exp ( − 44 , 521 T ) s − 1 and k 2 , ∞ = 8.86 × 10 16 exp ( − 35 , 887 T ) s − 1 . The results of this study indicate that cycloheptane dissociates similarly to cyclopentane and cyclohexane, i.e. ring-opening via C C scission to a diradical that rapidly isomerizes to a conjugate 1-alkene. The secondary chemistry is dominated by the dissociation products of the 1-alkenes i.e. allyl and n-alkyl radicals. Furthermore, rates of dissociation of the cycloalkanes are size dependent and kcyclopentane

Recent theoretical studies have shown that termolecular chemistry can be facilitated through reactions of flame radicals (H, O, and OH) or O2 with highly-energized collision complexes (either radical or stable species) formed in exothermic reactions. In this work, radical-radical recombination reaction induced termolecular chemistry and its impact on combustion modeling was studied. Two recombination reactions, H + CH3 + M → CH4 + M and H + OH + M → H2O + M, were analyzed using ab-initio master equation analyses guided by quasiclassical trajectory results. The dynamics results and the master equation calculations indicate that CH4⁎ and H2O⁎ (formed in the two radical-radical reactions outlined above) react rapidly with flame radicals and O2 at rates that are competitive with collisional cooling. The addition of these processes into conventional combustion modeling requires two modifications: the inclusion of the new nonthermal termolecular reaction rates and the simultaneous reduction of the competing recombination reaction rates. The former is described with newly derived Arrhenius expressions based on quasiclassical trajectories, and the latter is achieved by perturbing the recombination reaction rate during the simulation. Kinetic modeling was used to gauge the impact of including this nonthermal chemistry for H2/CH4-air laminar flames speeds. Inclusion of this nonthermal chemistry has a noticeable impact on simulated flame speeds. The procedure developed here can be utilized to properly quantify the effects of such nonthermal reactions in macroscopic kinetic models.

The chemistry of low-temperature ignition in propane/air mixtures is analyzed using a recently developed pathway representation of the chemical kinetics. The “Sum Over Histories Representation” allows time-dependent kinetic observables to be computed using an expansion over global chemical pathways that follow chemical moieties as they move through a complex reaction network. This methodology assigns probabilities to complete chemical pathways through which specific intermediate or product species are generated. The growth of the radical pool during the ignition process is analyzed by enumerating chemical pathways that constitute catalytic cycles, in particular the catalyzed production of the highly reactive OH-radical. In addition to the well-known reaction route followed in low-temperature ignition of hydrocarbons which involves the QOOH and keto-hydroperoxide species, we have explicitly identified several other cycles that are responsible for most of the remaining OH-production.

OH + molecules are an important class of reactions in combustion and atmospheric chemistry. Consequently, numerous studies have measured rate constants for these processes over an extended temperature range. A large majority of these experimental studies have utilized the decay of [OH] profiles (monitored either by absorption or laser-induced fluorescence) to obtain total rate constants. However, there are limited direct measurements of channel specific rate constants in this important class of reactions, particularly at combustion relevant temperatures. In the present experiments, we have directly measured site-specific rate constants for abstraction of the secondary C H bond in OH + C3H8 at high temperatures. Atomic resonance absorption spectrometry (ARAS) was used to monitor the formation of H-atoms from shock-heated mixtures of tert-butylhydroperoxide and C3H8 at high temperatures. Simulations for the experimental H-atom profiles are sensitive only to abstraction of the secondary C H bond leading to unambiguous measurements of the rate constants for this reaction. Over the T-range, 921 K k = ( 3.935 ± 1.387 ) × 10 − 11 exp ( − 1681 ± 362 K / T ) c m 3 molecul e − 1 s − 1 Simulations of the lower temperature data (T

The high temperature gas phase chemistry of the four butyl radical isomers (n-butyl, sec-butyl, iso-butyl, and tert-butyl) was investigated in a combined experimental and theoretical study. Organic nitrites were used as convenient and clean sources of each of the butyl radical isomers. Rate coefficients for dissociation of each nitrite were obtained experimentally and are at, or close to, the high pressure limit. Low pressure experiments were performed in a diaphragmless shock tube with laser schlieren densitometry at post-shock pressures of 65, 130, and 260 Torr and post-shock temperatures of 700–1000 K. Additional experiments were conducted with iso-butyl radicals at 805 K and 8.7 bar to elucidate changes in mechanism at higher pressures. These experiments were performed in a miniature shock tube with synchrotron-based photoionization mass spectrometry. The mass spectra confirmed that scission of the O–NO bond is the primary channel by which the precursors dissociate, but they also provided evidence of a minor channel (

A new version of global sensitivity analysis is developed in this paper. This new version coupled with tools from statistics, machine learning, and optimization can devise small sample sizes that allow for the accurate ordering of sensitivity coefficients for the first 10-30 most sensitive chemical reactions in complex chemical-kinetic mechanisms, and is particularly useful for studying the chemistry in realistic devices. A key part of the paper is calibration of these small samples. Because these small sample sizes are developed for use in realistic combustion devices, the calibration is done over the ranges of conditions in such devices, with a test case being the operating conditions of a compression ignition engine studied earlier. Compression-ignition engines operate under low-temperature combustion conditions with quite complicated chemistry making this calibration difficult, leading to the possibility of false positives and false negatives in the ordering of the reactions. So an important aspect of the paper is showing how to handle the trade-off between false positives and false negatives using ideas from the multiobjective optimization literature. The combination of the new global sensitivity method and the calibration are sample sizes a factor of approximately 10 times smaller than were available with our previous algorithm.

The formation and destruction pathways of the formyl radical (HCO) occupy a pivotal role in the conversion of fuel molecules (and their intermediates) to eventual products CO and CO2, and therefore, HCO has been a prescient indicator for heat release in combustion. In this work, we have characterized the impact of including non-equilibrium effects for HCO, i.e. “prompt” dissociation of HCO to H + CO, in simulations of laminar flame speeds for archetypal hydrocarbon and oxygenated molecules relevant to combustion. Prompt dissociation probabilities for HCO were systematically applied to all elementary reactions that included this radical (as either a product or reactant) in literature combustion kinetics models. Simulations with the prompt HCO dissociation corrected models predicted a 7–13% increase in laminar flame speeds at 1 atm for the fuels characterized here (CH4, n-C7H16, CH3OH, CH3OCH3) relative to the predictions using the original models. It is evident that simulations of other fuel-air flames at 1 atm will be similarly impacted, suggesting the indispensability of incorporating these non-equilibrium effects for predictive flame modeling. Simulations of higher pressure (10 atm) heptane-air flames predicted a more modest effect ( O2 Ar mixtures were also impacted to a noticeable extent. Lastly, it is also worth noting that prompt dissociations are a ubiquitous feature of all weakly-bound radicals; the kinetics of many of which (C2H3, C2H5, CH3O, CH2OH, etc.) are central to our current understanding of combustion chemistry. Theory/modeling studies are in progress to address the relevance of prompt dissociations in these weakly-bound radicals to combustion modeling.

Quasiclassical trajectories are used to compute nonthermal rate constants, k

Laminar natural gas flames are investigated at engine-relevant thermochemical conditions where the ignition delay time τ is short due to very high ambient temperatures and pressures. At these conditions, it is not possible to measure or calculate well-defined values for the laminar flame speed sl, laminar flame thickness δl, and laminar flame time scale τ l = δ l / s l due to the explosive thermochemical state. Here, the corresponding reference values, sR, δR, and τ R = δ R / s R , that account for the effects of autoignition, are numerically estimated to investigate the enhancement of flame propagation, and the competition with autoignition that arises under nominally autoignitive conditions (characterised here by the number τ/τR). Large values of τ/τR indicate that autoignition is unimportant, values near or below unity indicate that flame propagation is not possible, and intermediate values indicate that a combination of both flame propagation and autoignition may be important, depending upon factors such as device geometry, turbulence, stratification, et cetera. The reference quantities are presented for a wide range of temperatures, equivalence ratios, pressures, and hydrogen concentrations, which includes conditions relevant to stationary gas turbine reheat burners and boosted spark ignition engines. It is demonstrated that the transition from flame propagation to autoignition is only dependent on residence time, when the results are non-dimensionalised by the reference values. The temporal evolution of the reference values are also reported for a modelled boosted SI engine. It is shown that the nominally autoignitive conditions enhance flame propagation, which may be an ameliorating factor for the onset of engine knock. The calculations are performed using a recently-developed, detailed 177 species mechanism for C0–C3 chemistry that is derived from theoretical chemistry and is suitable for a wide range of thermochemical conditions as it is not tuned or optimised for a particular operating condition.

Over the last 20 to 25 years theoretical chemistry (particularly theoretical chemical kinetics) has played an increasingly important role in developing chemical kinetics models for combustion. Theoretical methods of obtaining rate parameters are now competitive in accuracy with experiment, particularly for small molecules. Moreover, theoretical methods can deal with conditions that experiments frequently cannot. In addition to increased accuracy, theory has rejuvenated methods and discovered phenomena that were completely unappreciated, or at least underappreciated, in the 20th century. Our primary interest here is in molecular-level issues, i.e. in calculating rate and transport parameters. However, dealing with kinetics models that involve thousands of reactions and hundreds of species is important for practical applications and is relatively new to the 21st century. Theory, in a general sense, and theoretical methods development have a role to play here too. We discuss in this review all these topics in some detail with an emphasis on issues and methods that have emerged in the last 20 years or so. Even so, our review is selective, rather than comprehensive, out of necessity.

Analysis of large-scale, realistic models incorporating detailed chemistry can be challenging because each simulation is computationally expensive, and a complete analysis may require many simulations. This paper addresses one such problem of this type, chemical-reaction selection in engine simulations. In this computationally challenging case, it is demonstrated how the important concept of sparsity can facilitate chemical-reaction selection, which is the process of finding the most important chemical reactions for modeling a chemical process. It is difficult to perform accurate reaction selection for engine simulations using realistic models of the chemistry, as each simulation takes processor weeks to complete. We developed a procedure to efficiently accomplish this selection process with a relatively small number of simulations using a form of global sensitivity analysis based on sparse regression. The chemical-reaction selection leads to an analysis of the ignition chemistry as it evolves within the compression-ignition engine simulations and allows for the spatial development of the selected chemical reactions to be studied in detail.

The reactions of D/H with n-C 4 H 10 and i-C 4 H 10 have been studied with both shock-tube experiments and ab initio transition state theoretical calculations. D-atom profiles were measured behind reflected shock waves using D-atom atomic resonance absorption spectrometry (ARAS) in mixtures with C 2 D 5 I (D-atom precursor, 200 ppm), over the T-range 1063–1327 K, at pressures ≅0.5 atm. D-atom depletion in the present experiments is sensitive only to the reactions, D + n - C 4 H 10 → products ( A ) D + i - C 4 H 10 → products . ( B ) Simulations of the measured D-atom profiles allow for determinations of total rate constants for the processes (A) and (B). The experimental rate constants are well represented by the Arrhenius equations, k A = 2.11 × 10 - 9 exp ( - 5661 K / T ) cm 3 molecules - 1 s - 1 ( 1074 – 1253 K ) k B = 2.57 × 10 - 9 exp ( - 5798 K / T ) cm 3 molecules - 1 s - 1 ( 1063 – 1327 K ) The title reactions have also been characterized using electronic structure theory at the CCSD(T)/cc-pV∞Z//M06-2X/cc-pvtz level of theory. Over the T-range of the present experiments, the ab initio based transition state theory (TST) kinetics predictions for the isotope effects, k D / k H , are near unity. The theoretical predictions are in good agreement with the experimental results and can be represented by the modified Arrhenius equations, k A,THEORY = 6.677 × 10 - 17 T 2.118 exp ( - 2700 K / T ) cm 3 molecules - 1 s - 1 ( 500 – 2000 K ) k B,THEORY = 5.627 × 10 - 20 T 2.934 exp ( - 1225 K / T ) cm 3 molecules - 1 s - 1 ( 500 – 2000 K ) To our knowledge, the present experiments are the first direct measurements for the title reactions and the rate constants from this combined experimental/theoretical effort are recommended for use in combustion modeling. Results from the present studies on n-C 4 H 10 and i-C 4 H 10 along with prior studies on C 2 H 6 and C 3 H 8 suggest the applicability of rate rules for H + Alkanes that are based on generic primary, secondary, and tertiary abstraction sites.

Although the reactions of fuel-radicals with other dominant flame radicals such as H and CH 3 are important reactions in low-pressure flames, they have not been well studied. These reactions may occur through either recombination to form stabilized molecular complexes or direct abstractions and chemically activated addition–eliminations to yield bimolecular products. Here, the role of such reactions in low-pressure flames of ethanol and methylformate is studied through a combination of theoretical characterizations of key reactions and detailed kinetic modeling. In particular, H and CH 3 + fuel-radical reactions have been characterized theoretically in this work and these are shown to make a pronounced impact on the formation of intermediates. Theoretical calculations for H + CH 3 CHOH and CH 3 + CH 3 CHOH predict that at low pressures recombinations are minor processes with well-skipping (addition–eliminations) dominating the reaction flux. Direct abstraction was also considered in H + CH 3 CHOH and theory suggests that abstraction at the CH 3 -site forming CH 2 CHOH is the only important channel. Notably, this result is counter to analogy based predictions that CH 3 CHO should be the dominant abstraction product. Low-pressure ethanol flame simulations indicate that addition–elimination reactions from H + CH 3 CHOH and CH 3 + CH 3 CHOH are a major source for C 2 H 4 and C 3 H 6 profiles, respectively. Similar results are observed in simulations of a low-pressure methylformate flame, where addition–elimination reactions of H + CH 2 OCHO and CH 3 + CH 2 OCHO have a significant impact on CH 3 OH and C 2 H 4 mole fraction profiles, respectively. The present results suggest that the well-skipping reactions of relatively stable fuel-radicals with ubiquitous flame radicals such as H, O, OH, and CH 3 should be considered extensively in combustion models.

Quasiclassical trajectories are used to compute nonthermal rate constants, k*, for abstraction reactions involving highly-excited methane CH4* and the radicals H, O, OH, and O2. Several temperatures and internal energies of methane, Evib, are considered, and significant nonthermal rate enhancements for large Evib are found. Specifically, when CH4* is internally excited close to its dissociation threshold (Evib ≈ D0 = 104 kcal/mol), its reactivity with H, O, and OH is shown to be collision-rate-limited and to approach that of comparably-sized radicals, such as CH3, with k* > 10−10 cm3 molecule−1 s−1. Rate constants this large are more typically associated with barrierless reactions, and at 1000 K, this represents a nonthermal rate enhancement, k*/k, of more than two orders of magnitude relative to thermal rate constants k. We show that large nonthermal rate constants persist even after significant internal cooling, with k*/k > 10 down to Evib ≈ D0/4. The competition between collisional cooling and nonthermal reactivity is studied using a simple model, and nonthermal reactions are shown to account for up to 35%–50% of the fate of the products of H + CH3 = CH4* under conditions of practical relevance to combustion. Finally, the accuracy of an effective temperature model for estimating k* from k is quantified.

We study the role of individual reaction rates on engine performance, with an emphasis on the contribution of quantum tunneling. It is demonstrated that the effect of quantum tunneling corrections for the reaction HO2 + HO2 = H2O2 + O2 can have a noticeable impact on the performance of a high-fidelity model of a compression-ignition (e.g., diesel) engine, and that an accurate prediction of ignition delay time for the engine model requires an accurate estimation of the tunneling correction for this reaction. The three-dimensional model includes detailed descriptions of the chemistry of a surrogate for a biodiesel fuel, as well as all the features of the engine, such as the liquid fuel spray and turbulence. This study is part of a larger investigation of how the features of the dynamics and potential energy surfaces of key reactions, as well as their reaction rate uncertainties, affect engine performance, and results in these directions are also presented here.

The shock tube technique was used to study the thermal decomposition of ozone, O3, with a view to using this as a thermal precursor of O-atoms at high temperatures. The formation of O-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range, 819 K ≤ T ≤ 1166 K, at pressures 0.13 bar ≤ P ≤ 0.6 bar. Unimolecular rate theory provides an excellent representation of the falloff characteristics from the present and literature data on ozone decomposition at high temperatures. The present decomposition study on ozone permits its usage as a thermal source for O-atoms allowing measurements for, O + CH3OC(O)OCH3 → OH + CH3OC(O)OCH2 [A]. Reflected shock tube experiments monitoring the formation and decay of O-atoms were performed on reaction A using mixtures of O3 and CH3OC(O)OCH3, (DMC), in Kr bath gas over the T-range, 862 K ≤ T ≤ 1167 K, and pressure range, 0.15 bar ≤ P ≤ 0.33 bar. A detailed model was used to fit the O-atom temporal profile to obtain experimental rate constants for reaction A. Rate constants from the present experiments for O + DMC can be represented by the Arrhenius expression: kA(T) = 2.70 × 10(-11) exp(-2725 K/T) cm(3) molecule(-1) s(-1) (862-1167 K). Transition state theory calculations employing CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for reaction A are in good agreement with the experimental rate constants. The theoretical rate constants can be well represented (to within ±10%) over the 500-2000 K temperature range by: kA(T) = 1.87 × 10(-20)T(2.924) exp(-2338 K/T) cm(3) molecule(-1) s(-1). The present study represents the first experimental measurement and theoretical study on this bimolecular reaction which is of relevance to the high temperature oxidation of DMC.

The reactions of D/H with methyl formate (MF) and methyl acetate (MA) have been studied with both shock-tube experiments and ab initio transition state theoretical calculations. D-atom profiles were measured behind reflected shock waves using D-atom atomic resonance absorption spectrometry (ARAS) over the temperature range 1050–1270 K, at pressures ≅0.5 atm. The title reactions have been theoretically studied at the CCSD(T)/cc-pv∞z//MP2/aug-cc-pvtz and CCSD(T)/cc-pv∞z//B3LYP/6-311++G(d,p) levels of theory. Theoretical calculations suggest the dominance of the abstraction processes in comparison to addition processes in the 300–2000 K T-range. Over the T-range of the present experiments, the theoretically predicted isotope effects, kD/kH, are near unity. D-atom depletion in the present experiments is sensitive only to the reactions, (A) D + CH 3 OC(O)H → products (B) D + CH 3 OC(O)CH 3 → products Simulations of the measured D-atom profiles allow for determinations of total rate constants for the processes (A) , (B) . In combination with results obtained from recent H-ARAS experiments from our laboratory on MF decomposition, total experimental rate constants kA can be described by the Arrhenius equation, k A = ( 4.47 ± 1.54 ) × 10 - 10 exp ( - 5843 ± 416 K / T ) cm 3 molecule - 1 s - 1 ( 1050 – 1270 K ) For H/D + MF, total experimental rate constants, kA, and branching ratios agree well with theoretical predictions. For D/H + MA, total rate constants predicted by theory are in reasonable agreement with the experimental data. The theoretical predictions are preferred for use, with kB represented by the modified Arrhenius equation, k B = 3.078 × 10 - 19 T 2.78 exp ( - 3261 K / T ) cm 3 molecule - 1 s - 1 ( 500 – 2000 K ) To our knowledge, the present experiments are the first direct measurements for the title reactions and the rate constants from this combined experimental/theoretical effort are recommended for use in combustion modeling.

Methylbutanoate (MB), a C 4 methyl ester, represents the simplest surrogate that captures the chemical effects of the ester moiety in biodiesel and biodiesel surrogates. An updated chemical kinetic model has been developed to characterize the ignition and flame characteristics of MB. The mechanistic elements within this model that relate to the MB and smaller ester/oxygenate sub-mechanisms are drawn from the prototypical Fisher et al. model and from more recent theory and modeling efforts. The MB model development which is based on an iterative procedure involving global sensitivity analyses to identify elementary reactions that govern ignition and subsequent high level ab initio based theoretical updates to these reaction rates are presented. The MB model makes reasonable predictions of ignition delays and laminar flame speeds. The C 5 –C 7 submechanisms from the LLNL n -heptane (NH) model were merged with the present MB model to obtain a detailed chemical kinetics model for a surrogate blend representing biodiesel. The detailed MB-NH model (661 species) was reduced using graph based techniques. The robust reduction techniques employed result in a reduced model (145 species) that is in good agreement with the detailed model over a wide range of conditions. 3-D compression ignition (CI) engine simulations utilizing this reduced chemistry model for MB-NH blends as a surrogate for biodiesel show good agreement with the experimental data suggesting the utility of this model for predictions of combustion and emission characteristics of biodiesel in realistic CI engine simulations.

The shock tube technique was used to study the high temperature thermal decomposition of methylformate (MF) and methylacetate (MA). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1194–1371 K at pressures ∼0.5 atm. The H-atom profiles were simulated using a detailed chemical kinetic mechanism for MF and MA thermal decomposition. The simulations were used to derive rate constants for sensitive decomposition and H-abstraction reactions in MF and MA. In methylformate, the most sensitive reactions that determine H-atom profiles are: equation(A) CH3OC(O)H→HCO2+CH3CH3OC(O)H→HCO2+CH3 equation(B) CH3OC(O)H+H→CH3OCO+H2CH3OC(O)H+H→CH3OCO+H2 where H is formed from HCO2 → H + CO2. In methylacetate the most sensitive reactions affecting H-atom formation are: equation(C) CH3OC(O)CH3→CH3+OC(O)CH3CH3OC(O)CH3→CH3+OC(O)CH3 equation(D) CH3OC(O)CH3+H→CH2OC(O)CH3+H2CH3OC(O)CH3+H→CH2OC(O)CH3+H2 Minor sensitivity was observed for the energetically higher lying bond fission, equation(E) CH3OC(O)CH3→CH3+CH3OCOCH3OC(O)CH3→CH3+CH3OCO and H-atom abstraction from MA by CH3 through, equation(F) CH3OC(O)CH3+CH3→CH2OC(O)CH3+CH4CH3OC(O)CH3+CH3→CH2OC(O)CH3+CH4 equation(G) CH3OC(O)CH3+CH3→CH3OC(O)CH2+CH4CH3OC(O)CH3+CH3→CH3OC(O)CH2+CH4 Unlike MF, where H-atoms are formed instantaneously at high-temperatures from (A), in MA, H-atoms form from the CH3 radicals (through CH3 + CH3 → C2H4 + 2H) generated primarily through the C–O bond fission channel (C) with minor contributions from (E). A master equation analysis was performed using CCSD(T)/cc-pv∞z//B3LYP/6-311++G(d,p) energetics and molecular properties for all thermal decomposition processes in MF and MA. The theoretical predictions were found to be in good agreement with the present experimentally derived rate constants for the bond fissions. TST calculations employing CCSD(T)/cc-pv∞z//MP2/aug-cc-pvtz energies and molecular properties for reactions (B) and (D) (the only sensitive abstraction processes in MF and MA) are in good agreement with the experimental rate constants. The theoretically derived rate constants for these processes can be represented by modified Arrhenius expressions for the bond fissions at 0.5 atm over the T-range 1000–2000 K and for the bimolecular abstractions over the 500–2000 K regime. kA(T)=9.79×1068T-15.95exp(-57,434K/T)s-1kB(T)=5.67×10-19T2.50exp(-3188K/T)cm3molecule-1s-1kC(T)=1.42×1084T-19.60exp(-63,608K/T)s-1kD(T)=1.18×10-18T2.58exp(-3714K/T)cm3molecule-1s-1kE(T)=1.90×1082T-19.30exp(-64,724K/T)s-1 Our theoretical predictions for MA + CH3 give over the T-range 500–2000 K, kF(T)=2.12×10-25T3.93exp(-4440K/T)cm3molecule-1s-1kG(T)=3.40×10-25T3.88exp(-4149K/T)cm3molecule-1s-1 To our knowledge this is the first study providing experimentally derived rate constant values for the primary bond fission and abstraction reactions in MF and MA.

The reactions of H/D with C2H6 and C3H8 have been studied with both shock-tube experiments and ab initio transition-state theory calculations. Rate constants for the reactions of D with C2H6 and C3H8 have been measured in reflected shock wave experiments over the temperature range, 1128–1299 K, at pressures 0.3–1 atm. D atoms are detected using atomic resonance absorption spectrometry. The measured D-atom profiles are sensitive to only the thermal dissociation of the D-atom source molecule, C2D5I, and to the title reactions. Since the dissociation has been previously studied in this laboratory, modeling the temporal evolution of the D-atom profiles allows determinations of the rate constants for the title reactions. Over the T range, 1128–1228 K, rate constants from the present experiments for D + C2H6 can be represented by the Arrhenius expression The experimental total rate constants for D + C3H8 over the T range, 1128–1299 K, can be represented by the Arrhenius expression The title reactions have been studied at the CCSD(T)/aug-cc-pv∞z level of theory using saddle point geometries at B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory. The reaction endothermicities are in good agreement with current Active Thermochemical Tables values. Theoretical rate constants were estimated using transition state theory. The theoretical rate constants are in good agreement with the present experiments and lower temperature literature data. Over the T range of the present experiments, the theoretically predicted isotope effects are close to unity. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 194–205, 2012

Weakly bound free radicals have low-dissociation thresholds such that at high temperatures, time scales for dissociation and collisional relaxation become comparable, leading to significant dissociation during the vibrational-rotational relaxation process. Here we characterize this "prompt" dissociation of formyl (HCO), an important combustion radical, using direct dynamics calculations for OH + CH2O and H + CH2O (key HCO-forming reactions). For all other HCO-forming reactions, presumption of a thermal incipient HCO distribution was used to derive prompt dissociation fractions. Inclusion of these theoretically derived HCO prompt dissociation fractions into combustion kinetics models provides an additional source for H-atoms that feeds chain-branching reactions. Simulations using these updated combustion models are therefore shown to enhance flame propagation in 1,3,5-trioxane and acetylene. The present results suggest that HCO prompt dissociation should be included when simulating flames of hydrocarbons and oxygenated molecules and that prompt dissociations of other weakly bound radicals may also impact combustion simulations.

The thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure both the total decomposition rate and the branching to radical products versus molecular products, with the molecular products arising predominantly through roaming according to the theoretical analysis. The experimental observations also provide a measure of the rate coefficient for H + CH3OCH3. An evaluation of the available experimental results for H + CH3OCH3 can be expressed by a three parameter Arrhenius expression as

The thermal decomposition of toluene-d5 (C6D5CH3) has been studied at high temperatures with the reflected shock-tube technique using H- and D-atom ARAS. The experiments were performed at high-T (1469–1859 K) at nominal pressures ≈0.25–1.50 atm. The present study utilizing the ultra-sensitive H-atom ARAS technique has provided a direct measurement of the branching ratio in the two-channel high-temperature thermal decomposition of toluene-d5 giving (1) C6D5CH2 + H and (2) C6D5 + CH3. Fall-off is observed in both decomposition channels at high-T (>1700 K), but the lower-T rate constants can be well represented by simple Arrhenius expressions in units of s−1 as, k1=6.91×1013exp(-40180/T)(1469–1714K)k2=5.99×1015exp(-50060/T)(1469–1714K) The experimental results were also used to obtain rate constants for total toluene-d5 decomposition. Arrhenius analysis gives in units of s−1, kTotal=2.21×1014exp(-41760/T)(1469–1714K) The isotope effect is minimal and the present results therefore represent direct measurements of branching ratios and rate constants for toluene decomposition. The branching ratios to benzyl + H vary from ≈0.9 at lower-T ( 1700 K). The excellent agreement between the present experiments and the theoretical predictions by Klippenstein et al. lead us to conclude that the high-T thermal decomposition of toluene is now well-characterized.

The thermal decompositions of benzyl (C6H5CH2) and benzyl,α,ά-d2 (C6H5CD2) have been studied at high temperatures with the reflected shock tube technique using H- and D-atom Atomic Resonance Absorption Spectroscopy (ARAS). The experiments were performed at high-T (1437–1801 K) at nominal pressures ≈ 0.3–1.3 atm. The present experiments utilizing the ultra-sensitive H- and D-atom ARAS technique reveal that there are three unique channels that contribute to benzyl decomposition: H-atom removal from the ring, H/D-atom removal from the side-chain, and a non-atom producing process (postulated to be cyclopentadienyl + acetylene). The rate constants for the three channels can be represented in s−1 by, k H,ring = 1.14 × 10 12 exp ( - 32180 K / T ) ( 1475 – 1801 K ) k H / D,side-chain = 4.50 × 10 13 exp ( - 40650 K / T ) ( 1487 – 1789 K ) k non-atom = 1.55 × 109 exp ( - 23470 K / T ) ( 1437 – 1627 K ) The experimental data was also used to obtain rate constants for total benzyl decomposition and an Arrhenius fit gives, k total = 1.05 × 10 12 exp ( - 31450 K / T ) ( 1437 – 1801 K ) The present study gives reliable determinations of rate constants for the three channels that, when coupled with the postulated mechanistic interpretations for these processes, should be useful in future theoretical and experimental characterizations of benzyl decomposition.

The thermal decomposition of ethanol and its reactions with OH and D have been studied with both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for ethanol have been measured at high T in reflected shock waves using OH optical absorption and high-sensitivity H-atom ARAS detection. The three dissociation processes that are dominant at high T are C2H5OH--> C2H4+H2O (A) -->CH3+CH2OH (B) -->C2H5+OH (C).The rate coefficient for reaction C was measured directly with high sensitivity at 308 nm using a multipass optical White cell. Meanwhile, H-atom ARAS measurements yield the overall rate coefficient and that for the sum of reactions B and C , since H-atoms are instantaneously formed from the decompositions of CH(2)OH and C(2)H(5) into CH(2)O + H and C(2)H(4) + H, respectively. By difference, rate constants for reaction 1 could be obtained. One potential complication is the scavenging of OH by unreacted ethanol in the OH experiments, and therefore, rate constants for OH+C2H5OH-->products (D)were measured using tert-butyl hydroperoxide (tBH) as the thermal source for OH. The present experiments can be represented by the Arrhenius expression k=(2.5+/-0.43) x 10(-11) exp(-911+/-191 K/T) cm3 molecule(-1) s(-1) over the T range 857-1297 K. For completeness, we have also measured the rate coefficient for the reaction of D atoms with ethanol D+C2H5OH-->products (E) whose H analogue is another key reaction in the combustion of ethanol. Over the T range 1054-1359 K, the rate constants from the present experiments can be represented by the Arrhenius expression, k=(3.98+/-0.76) x10(-10) exp(-4494+/-235 K/T) cm3 molecule(-1) s(-1). The high-pressure rate coefficients for reactions B and C were studied with variable reaction coordinate transition state theory employing directly determined CASPT2/cc-pvdz interaction energies. Reactions A , D , and E were studied with conventional transition state theory employing QCISD(T)/CBS energies. For the saddle point in reaction A , additional high-level corrections are evaluated. The predicted reaction exo- and endothermicities are in good agreement with the current Active Thermochemical Tables values. The transition state theory predictions for the microcanonical rate coefficients in ethanol decomposition are incorporated in master equation calculations to yield predictions for the temperature and pressure dependences of reactions A - C . With modest adjustments (

Primary and secondary reactions involved in the thermal decomposition of NH2OH are studied with a combination of shock tube experiments and transition state theory based theoretical kinetics. This coupled theory and experiment study demonstrates the utility of NH2OH as a high temperature source of OH radicals. The reflected shock technique is employed in the determination of OH radical time profiles via multipass electronic absorption spectrometry. O-atoms are searched for with atomic resonance absorption spectrometry. The experiments provide a direct measurement of the rate coefficient, k1, for the thermal decomposition of NH2OH. Secondary rate measurements are obtained for the NH2 + OH (5a) and NH2OH + OH (6a) abstraction reactions. The experimental data are obtained for temperatures in the range from 1355 to 1889 K and are well represented by the respective rate expressions: log[k/(cm3 molecule(-1) s(-1))] = (-10.12 +/- 0.20) + (-6793 +/- 317 K/T) (k1); log[k/(cm3 molecule(-1) s(-1))] = (-10.00 +/- 0.06) + (-879 +/- 101 K/T) (k5a); log[k/(cm3 molecule(-1) s(-1))] = (-9.75 +/- 0.08) + (-1248 +/- 123 K/T) (k6a). Theoretical predictions are made for these rate coefficients as well for the reactions of NH2OH + NH2, NH2OH + NH, NH + OH, NH2 + NH2, NH2 + NH, and NH + NH, each of which could be of secondary importance in NH2OH thermal decomposition. The theoretical analyses employ a combination of ab initio transition state theory and master equation simulations. Comparisons between theory and experiment are made where possible. Modest adjustments of predicted barrier heights (i.e., by 2 kcal/mol or less) generally yield good agreement between theory and experiment. The rate coefficients obtained here should be of utility in modeling NOx in various combustion environments.

High temperature experiments were performed with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The present experiments span a wide T-range, 801-1347 K, and represent the first direct measurements of the title rate constants at T>500 K for cyclopentane and cyclohexane and the only high temperature measurements for the corresponding methyl derivatives. The present work utilized 48 optical passes corresponding to a total path length 4.2 m. As a result of this increased path length, the high [OH] detection sensitivity permitted unambiguous analyses for measuring the title rate constants. The experimental rate constants in units, cm3 molecule-1 s-1, can be expressed in Arrhenius form as k{sub OH+Cyclopentane} = (1.90 {+-} 0.30) x 10{sup -10} exp(-1705 {+-} 156 K/T) (813-1341 K), k{sub OH+Cyclohexane} = (1.86 {+-} 0.24) x 10{sup -10} exp(-1513 {+-} 123 K/T) (801-1347 K), k{sub OH+Methylcyclopentane} = (2.02 {+-} 0.19) x 10{sup -10} exp(-1799 {+-} 96 K/T) (859-1344 K), k{sub OH+Methylcyclohexane} = (2.55 {+-} 0.30) x 10{sup -10} exp(-1824 {+-} 114 K/T) (836-1273 K). These results and lower-T experimental data were used to obtain three parameter evaluations of the experimental rate constants for the title reactions over an even wider T-range.more » These experimental three parameter fits to the rate constants in units, cm{sup 3} molecule{sup -1} s{sup -1}, are k{sub OH+Cyclopentane} = 1.390 x 10{sup -16}T{sup 1.779} exp(97 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (209-1341 K), k{sub OH+Cyclohexane} = 3.169 x 10{sup -16} T{sup 1.679} exp(119 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (225-1347 K), k{sub OH+Methylcyclopentane} = 6.903 x 10{sup -18}T{sup 2.148} exp(536 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (296-1344 K), k{sub OH+Methylcyclohexane} = 2.341 x 10{sup -18}T{sup 2.325} exp(602 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (296-1273 K). High level electronic structure methods were used to characterize the first three reactions in order to provide reliable extrapolations of the rate constants from 250-2000 K. The results of the theoretical predictions for OH + cyclohexane and OH + methylcyclopentane were sufficient to make a theoretical prediction for OH + methylcyclohexane. The present recommended rate expressions for OH with cyclohexane, and methylcyclohexane, give rate constants that are 15-25% higher (over the T-range 800-1300 K) than the rate constants utilized in recent modeling efforts aimed at addressing the oxidation of cyclohexane and methylcyclohexane. The current measurements reduce the uncertainties in rate constants for the primary cycloalkane consumption channel in a high temperature oxidation environment.« less

Rate constants for H-atom abstractions by OH radicals from a series of alkanes (propane, n-butane, i-butane and neo-pentane) have been measured at high temperatures with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The experiments represent the first direct measurements of these rate constants at T > 1000 K and span a wide T-range, 797–1259 K. The present work utilized 80 optical passes corresponding to a total path length of ∼7 m. As a result of this increased path length, the high [OH] detection sensitivity permitted pseudo-first-order analysis for unambiguously measuring the total rate constants. The experimental rate constants can be represented in Arrhenius form as, kC3H8+OH=6.671×10-11exp(-1543K/T)cm3molecule-1s-1(797–1248K)kn-C4H10+OH=9.674×10-11exp(-1569K/T)cm3molecule-1s-1(800–1236K)ki-C4H10+OH=9.114×10-11exp(-1654K/T)cm3molecule-1s-1(846–1221K)kneo-C5H12+OH=1.060×10-10exp(-1947K/T)cm3molecule-1s-1(841–1259K) The present results have been combined with prior lower-T measurements to generate three-parameter rate expressions that adequately represent the available direct measurements (within 25%) over a wide temperature regime (250–1250 K). High-level ab-initio electronic structure theory computations of the molecular properties of reactants, products and transition states have been used to estimate theoretical rate constants with conventional transition state theory (CTST). The theoretical rate constants are excellent representations of the available experimental data (deviations less than 25%) and thereby offer a reliable method for extrapolation to higher-T as well as for extracting branching ratios for primary, secondary and tertiary abstractions.

Pyrolysis experiments on n -heptane, 1-heptene and 1,6-heptadiene have been performed using the UIC High-Pressure Shock Tube (HPST) at pressures relevant to diesel combustion systems. The experimental pressures for these experiments ranged from 25 to 50 atm and temperatures varied from 1000 to 1350 K with reaction times ranging from 1 to 3 ms. Dilute reagent mixtures ∼100 ppm were prepared in bulk argon and shock heated to study the stable intermediates. The experimental data has been used to develop and validate an updated kinetic model for the pyrolysis of saturated and unsaturated C 7 hydrocarbons. The experimental results and their implication on increased NO emissions from biodiesel blends will also be discussed.

The water-gas shift reaction influences the chemistry between the postcombustion gases of a rocket and the rocket's graphite nozzle. The rocket's operating pressures (70-600 atm) exceed those for existing water-gas shift reaction data, and further study of the chemistry under similar conditions is essential for optimum rocket design. To investigate chemical kinetic effects at the pertinent pressure and temperature regime, experiments were performed using the University of Illinois at Chicago high-pressure shock-tube facility with experimental temperatures ranging from 1200-2100 K and pressures ranging from 194-490 atm with reaction times averaging 1.17 ms. Initial mole fractions of H 2 O and CO were varied from 115-983 ppm (0.0002-0.003 mol/L). The experimental data have been compared with predictions from a comprehensive model for synthesis gas (CO/H 2 /CO 2 ) combustion and a reduced four-step model with the chemistry relevant to the water-gas shift reaction. The rate coefficients at our elevated pressure and temperature conditions were found to be pressure independent when compared with prior lower temperature 1 atm measurements.

The high pressure oxidation of dilute CO mixtures doped with 150–200 ppm of H 2 has been studied behind reflected shock waves in the UIC high pressure single pulse shock tube. The experiments were performed over the temperature range from 1000 to 1500 K and pressures spanning 21–500 bars for stoichiometric ( Φ = 1) and fuel lean ( Φ = 0.5) oxidation. Stable species sampled from the shock tube were analyzed by standard GC, GC/MS techniques. The experimental data obtained in this work were simulated using a detailed model for H 2 /CO combustion that was validated against a variety of experimental observables/targets that span a wide range of conditions. These simulations have shown that within experimental error the model is able to capture the experimental trends for the lower pressure data sets (average nominal pressures of 24 and 43 bars). However the model under predicts the CO and O 2 decay and subsequent CO 2 formation for the higher pressure data sets (average nominal pressures of 256 and 450 bars). The current elevated pressure data sets span a previously unmapped regime and have served to probe HO 2 radical reactions which appear to be among the most sensitive reactions in the model under these conditions. With updated rate parameters for a key HO 2 radical reaction OH + HO 2 = H 2 O + O 2 , the model is able to reconcile the elevated pressure data sets thereby extending its capability to an extreme range of conditions.

The mechanism for the thermal decomposition of acetaldehyde has been revisited with an analysis of literature kinetics experiments using theoretical kinetics. The present modeling study was motivated by recent observations, with very sensitive diagnostics, of some unexpected products in high temperature microtubular reactor experiments on the thermal decomposition of CH3CHO and its deuterated analogs, CH3CDO, CD3CHO, and CD3CDO. The observations of these products prompted the authors of these studies to suggest that the enol tautomer, CH2CHOH (vinyl alcohol), is a primary intermediate in the thermal decomposition of acetaldehyde. The present modeling efforts on acetaldehyde decomposition incorporate a master equation reanalysis of the CH3CHO potential energy surface (PES). The lowest-energy process on this PES is an isomerization of CH3CHO to CH2CHOH. However, the subsequent product channels for CH2CHOH are substantially higher in energy, and the only unimolecular process that can be thermally accessed is a reisomerization to CH3CHO. The incorporation of these new theoretical kinetics predictions into models for selected literature experiments on CH3CHO thermal decomposition confirms our earlier experiment and theory-based conclusions that the dominant decomposition process in CH3CHO at high temperatures is C-C bond fission with a minor contribution (∼10-20%) from the roaming mechanism to form CH4 and CO. The present modeling efforts also incorporate a master-equation analysis of the H + CH2CHOH potential energy surface. This bimolecular reaction is the primary mechanism for removal of CH2CHOH, which can accumulate to minor amounts at high temperatures, T > 1000 K, in most lab-scale experiments that use large initial concentrations of CH3CHO. Our modeling efforts indicate that the observation of ketene, water, and acetylene in the recent microtubular experiments are primarily due to bimolecular reactions of CH3CHO and CH2CHOH with H-atoms and have no bearing on the unimolecular decomposition mechanism of CH3CHO. The present simulations also indicate that experiments using these microtubular reactors when interpreted with the aid of high-level theoretical calculations and kinetics modeling can offer insights into the chemistry of elusive intermediates in the high-temperature pyrolysis of organic molecules.

The pyrolysis of toluene, the simplest methyl-substituted aromatic molecule, has been studied behind reflected shock waves using a single pulse shock tube. Part 1 in this two-part series focused on the high-pressure experimental results and the high-pressure limiting rate coefficients for the primary steps in toluene decomposition. The present work focuses on the modeling of benzyl decomposition and the growth of key soot precursors (C2H2, C4H2, C8H6, and indene) from toluene pyrolysis with 81 among the 262 reactions in the detailed toluene model representing the chemistry that describes the formation and decomposition of these species. Feasible pathways for benzyl decomposition as well as phenylacetylene and indene formation have been tested. The simulations also show very good agreement with the single pulse shock tube profiles for the growth of key soot precursors such as C2H2, C4H2, C8H6, and indene.

The high-temperature, high-pressure pyrolysis of the prototype aromatic, benzene, has been studied behind reflected shock waves in the UIC High Pressure Single Pulse Shock Tube. Three sets of experiments were performed at nominal pressures of 30 and 50 bars in the high temperature regime from 1200–1800 K. Stable species sampled from the shock tube were analyzed offline using gas chromatographic techniques. The present data set was simulated using the three most recent models, two of the models developed and validated against high-temperature benzene pyrolysis shock-tube data for stable species profiles as well as H atom production rates and the third model, a “work-in-progress” model from our laboratory aimed at resolving the high-pressure combustion of primary aromatics such as benzene and toluene. The simulations reflect the complexities and uncertainties involved not only in describing the primary decay steps but also the subsequent high-temperature secondary chemistry for even the simplest ar...

An extensive experimental study of ethane oxidation and pyrolysis has been conducted in the high pressure shock tube at UIC covering reflected shock pressures from 5–1000 bar, reaction temperatures up to 1550 K and stoichiometric (Φ = 1), fuel rich (Φ = 5), and pyrolytic mixtures. The experimental data has been used to develop a single model that can simulate the whole dataset very well and is the first ethane model capable of simulating experimental results over such an extensive range of pressure, temperature, and stoichiometry. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 306–331, 2005

The high-pressure single pulse shock tube (HPST) at the University of Illinois at Chicago has been used to study the oxidation of toluene at reflected shock pressures from 22 to 550 bar and temperatures from 1210 to 1480 K. Experiments were performed for dilute stoichiometric, Φ = 1 , and rich, Φ = 5 , reagent mixtures. Stable species were analyzed using gas chromatography and gas chromatography/mass spectrometry. The resulting data set is the first that provides species concentrations over such extremes of pressure, temperature, and stoichiometry, and it serves as an excellent base for the validation and refinement of future detailed chemical kinetic models. Two literature models for the oxidation of toluene that have been validated against atmospheric pressure turbulent flow reactor and jet stirred reactor data were used to simulate the experimental data. Several modifications were made to one model to more accurately simulate the high-pressure/high-temperature experimental data. The modified model reproduces the Φ = 1 experimental data well and forms the first step in the development of a more comprehensive model for toluene combustion validated over wide ranges of temperature and pressure.