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The oxidation of widely-used herbicide metazachlor (MTZ) by hydroxyl radical (HO

International audience; The microhydration of iodous acid has been theoretically studied using the ωB97XD/aug‐cc‐pVTZ level of theory. Two hydration processes have been examined considering the addition of either successive water molecules or water clusters with a number of water molecules from 1 to 4. The singlet potential energy surface exploration revealed: four monohydrates, seven dihydrates, nine trihydrates, and ten tetrahydrates. The thermodynamic properties of the hydration reactions have been calculated at different levels of theory including DFT and coupled-cluster calculations DK-CCSD(T) with the ANO‐RCC‐VQZP basis sets. Standard reaction enthalpy and standard Gibbs free reaction energy were computed. The temperature dependence of ΔrG°(T) was evaluated for all studied aggregates over the temperature range 200 - 400 K. The results showed that the hydrated forms of HOIO can exist in tropospheric conditions. Atmospheric implications in terms of assessing the molecular concentrations of HOIO and water aggregates are discussed.

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Theoretical calculations are carried out on benzylperoxy radical four-center isomerization reaction. Geometry optimizations and vibrational frequency calculations are performed using three methods (B3LYP, MPW1K, and MP2) and seven basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ). Single-point energy calculations are performed with the highly-correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations (CCSD(T)) using the 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p), 6-311+G(3df,2p), and cc-pVTZ basis sets, and with the CASPT2/ANO-L-VDZP level of theory. Canonical transition state theory with a Wigner tunneling correction is used to calculate the high-pressure limit rate constant. The rate constants at 773 K calculated with the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ and CASPT2/ANO-L-VDZP//B3LYP/aug-cc-pVDZ levels of theory are in very good agreement with the literature value from Ellis et al. These levels of theory are then used to compute the temperature dependence of rate constant and leads to the following three-parameter Arrhenius expressions over the range 600–2000 K: k (s−1) = 1.34 × 1010 T0.79 exp(−133.1 kJ mol−1/RT) and k (s−1) = 1.85 × 1010 T0.78 exp(−133.9 kJ mol−1/RT) at the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ and CASPT2/ANO-L-VDZP//B3LYP/aug-cc-pVDZ levels of theory, respectively. These relations can be used in oxidation thermokinetic models involving toluene and alkylbenzenes.

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of cyclohexane at both low and high temperatures. Rules for reaction rate constants are developed for the low-temperature combustion of cyclohexane. These rules can be used for in chemical kinetic mechanisms for other cycloalkanes. Because cyclohexane produces only one type of cyclohexyl radical, much of the low-temperature chemistry of cyclohexane is described in terms of one potential energy diagram showing the reaction of cyclohexyl radical with O2 through five-, six-, and seven-membered-ring transition states. The direct elimination of cyclohexene and HO2 from RO2 is included in the treatment using a modified rate constant of Cavallotti et al. (Proc. Combust. Inst. 2007, 31, 201). Published and unpublished data from the Lille rapid compression machine, as well as jet-stirred reactor data, are used to validate the mechanism. The effect of heat loss is included in the simulations, an improvement on previous studies on cyclohexane. Calculations indicated that the production of 1,2-epoxycyclohexane observed in the experiments cannot be simulated according to the current understanding of low-temperature chemistry. Possible "alternative" H-atom isomerizations leading to different products from the parent O2QOOH radical were included in the low-temperature chemical kinetic mechanism and were found to play a significant role.

This study describes a new technique of reduction of detailed mechanisms for autoignition. It is based on two analysis methods. An analysis of reaction rates is coupled to an analysis of reaction sensitivity for the detection of redundant reactions. Thresholds associated with the two analyses have a great influence on the size and efficiency of the reduced mechanism. Rules of selection of the thresholds are defined. The reduction technique has been successfully applied to detailed autoignition mechanisms of two reference hydrocarbons: n-heptane and isooctane. The efficiency of the technique and the ability of the reduced mechanisms to reproduce well the results generated by the full mechanism are discussed. A speedup of calculations by a factor of 5.9 for n-heptane mechanism and by a factor of 16.7 for isooctane mechanism is obtained without losing accuracy of the prediction of autoignition delay times and concentrations of intermediate species. © 2007 Wiley Periodicals, Inc. 39: 181–196, 2007

International audience; The chemistry of oxidation and autoignition of 1-, 2-, and 3-hexene has been studied after rapid compression between 630 and 850 K for stoichiometric mixtures with “air.” The phenomenology of autoignition has been recognized, and intermediate products formed before autoignition have been identified and analyzed. They mainly comprise of hexadienes, O-heterocycles, and aldehydes. There are many common products, because some of the intermediate alkenyl or alkenylperoxy radicals are delocalized. Saturated O-heterocycles are specific products formed by addition of HO2 to the double bond. Unsaturated O-heterocycles are products typical of the long alkenyl chain. Saturated and unsaturated lower aldehydes are the products of OH addition to the double bond of hexenes and hexadienes. The relative abundance of the intermediates enables a better insight into the competition between the reactivity of the double bond and the reactivity of the alkenyl chain. According to the position of the double bond, the behavior of 3-hexene is dominated by the properties of the double bond whereas the behavior of 1-hexene is dominated by the properties of the alkenyl chain. The reactivity of the alkenyl chain is related to the type and number of C–H bonds, the ability of stabilized radicals to react, and the cyclic strain of the transition state of isomerization reactions. Therefore, 1-hexene reacts much more with the typical features of alkanes like a two-stage ignition with a cool flame and a negative temperature coefficient. 3-Hexene does not have typical features and 2-hexene has an intermediate behavior.

Detailed chemical kinetic reaction mechanisms are developed for combustion of all nine isomers of heptane (C 7 H 16 ), and these mechanisms are tested by simulating autoignition of each isomer under rapid compression machine conditions. The reaction mechanisms focus on the manner in which the molecular structure of each isomer determines the rates and product distributions of possible classes of reactions. The reaction pathways emphasize the importance of alkylperoxy radical isomerizations and addition reactions of molecular oxygen to alkyl and hydroperoxyalkyl radicals. A new reaction group has been added to past models, in which hydroperoxyalkyl radicals that originated with abstraction of an H atom from a tertiary site in the parent heptane molecule are assigned new reaction sequences involving additional internal H atom abstractions not previously allowed. This process accelerates autoignition in fuels with tertiary C−H bonds in the parent fuel. In addition, the rates of hydroperoxyalkylperoxy radical isomerization reactions have all been reduced so that they are now equal to rates of analogous alkylperoxy radical isomerizations, significantly improving agreement between computed and experimental ignition delay times in the rapid compression machine. Computed ignition delay times agree well with experimental results in the few cases where experiments have been carried out for specific heptane isomers, and predictive model calculations are reported for the remaining isomers. The computed results fall into three general groups: the first consists of the most reactive isomers, including n -heptane, 2-methyl hexane, and 3-methyl hexane. The second group consists of the least reactive isomers, including 2,2-dimethyl pentane, 3,3-dimethyl pentane, 2,3-dimethyl pentane, 2,4-dimethyl pentane, and 2,2,3-trimethyl butane. The remaining isomer, 3-ethyl pentane, was observed computationally to have an intermediate level of reactivity. These observations are generally consistent with knocking tendencies of these isomers, as measured by octane ratings, although the correlations are only approximate.

The oxidation and auto-ignition of cyclohexane, cyclohexene, and cyclohexa-1,3-diene have been studied by rapid compression between 600 K to 900 K and 0.7 MPa to 1.4 MPa to identify the low-temperature pathways leading to benzene from cyclohexane. Auto-ignition delay times were measured and concentration-time profiles of the C 6 intermediate products of oxidation were measured during the auto-ignition delays. Cyclohexane showed two-stage ignition at low temperatures, but single-stage ignition at higher temperatures, and a well-marked negative-temperature coefficient. By contrast there was neither a cool flame, nor a negative-temperature coefficient for cyclohexa-1,3-diene. Cyclohexene behaved in an intermediate way without a cool flame, but with a narrow, not very marked negative-temperature coefficient. The identified C 6 products belong to three families: the bicyclic epoxides and cyclic ketones, the unsaturated aliphatic aldehydes, and the conjugated alkenes, which are always the major products. The formation of C 6 products from cyclohexane is explained by the classical scheme for low-temperature oxidation, taking into account addition of O 2 to cyclohexyl radicals and the various isomerizations of the resulting peroxy radicals. Most of the C 6 products from cyclohexene are predicted by the same scheme, beginning with the formation of the allylic cyclohexenyl radical. However, addition of HO 2 to the double bond has to be included to predict the formation of 1,2-epoxycyclohexane. For cyclohexa-1,3-diene, the classical scheme is not valid: the C 6 oxygenated products are only formed by addition of HO 2 to the double bond. For all three hydrocarbons, the pathways to benzene are those leading to conjugated alkenes, and they are always more efficient than those producing oxygenated products, either by adding HO 2 to double bonds, or by addition of O 2 to the initial cyclic radical.

Experiments in a rapid compression machine were used to examine the influences of variations in fuel molecular structure on the autoignition of isomers of pentane. Autoignition of stoichiometric mixtures of the three isomers of pentane were studied at compressed gas initial temperatures between 640 K and 900 K and at precompression pressures of 300 and 400 torr. Numerical simulations of the same experiments were carried out using a detailed chemical kinetic reaction mechanism. The results are interpreted in terms of a low-temperature oxidation mechanism involving addition of molecular oxygen to alkyl and hydroperoxyalkyl radicals. Results indicate that in most cases, the reactive gases experience a two-stage autoignition. The first stage follows a low-temperature alkylperoxy radical isomerization pathway that is effectively quenched when the temperature reaches a level where dissociation reactions of alkylperoxy and hydroperoxyalkylperoxy radicals are more rapid than the reverse addition steps. The second stage is controlled by the onset of dissociation of hydrogen peroxide. At the highest compression temperatures achieved, little or no first-stage ignition is observed. Particular attention is given to the influence of heat transfer and the importance of regions of variable temperature within the compressed gas volume. Implications of this work on practical ignition problems are discussed.

The pre-autoignition chemistry of n-pentane and 1-pentene was studied by rapid compression in the low temperature range (600–900 K). The pressure traces, light emissions, intensities of cool flames, autoignition delays, and hydrocarbon conversions before final ignition indicate that there are similarities of behavior, but a lower reactivity of 1-pentene over the whole temperature range. Chemical analysis of the stable intermediate species after the cool flame, but before final ignition, shows marked differences in selectivities for O-heterocycles and aldehydes. Relatively high amounts of propyloxirane and butanal in the oxidation of 1-pentene suggest additions of oxidizing radicals to the double bond. The classical low temperature peroxidation scheme of alkanes can be applied, not only to n-pentane, but also to 1-pentene, if the higher reactivity of the allylic hydrogens and direct addition of OH and HO2 radicals are taken into account. Some peroxy radicals are common to both fuels and are responsible for their similar features of pre-autoignition chemistry. However, oxidation of 1-pentene is still deeply marked by the presence of an olefinic bond.

Autoignitions of n-pentane and 1-pentene are studied by rapid compression between 600 and 900 K at high pressure. Both hydrocarbons show a two-stage ignition and a negative temperature coefficient region (NTC). However, 1-pentene is less reactive. Ignition temperature limit is 50 K higher; cool flames and NTC are weaker and confined to a narrower temperature range. Chemical analyses are performed on the reacting mixture for fuel consumption and cyclic ethers. n-Pentane and 1-pentene give very different distribution patterns for cyclic ethers. 2-Methyltetrahydrofuran dominates the n-pentane pattern, whereas propyloxirane is by far the major cyclic ether formed by 1-pentene. Detailed mechanisms based on a common skeleton scheme are developed and used to simulate the experiments. They are validated for ignition delay times, cool flame intensities, and cyclic ether distributions. Good results are obtained for 1-pentene only if (1) direct addition channels of OH and HO2 to the double bond are included and (2) if a higher rate constant for the decomposition of the hydroperoxyalkyl radicals into cyclic ethers is used when this radical is formed by direct HO2 addition instead of isomerization of alkylperoxy radicals. The sensitivity analysis of the low-temperature scheme for 1-pentene points out that the total ignition delay time is dependent upon the competition between the decomposition channels of hydroperoxyalkyl radical into the branching sequence and into alkenes. The cool flame delay time is less sensitive but depends mainly upon the decomposition rate of unsaturated ketohydroperoxides.

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A set of ignition data of linear and branched chain alkanes (n-butane, n-pentane, neopentane, n-heptane, and isooctane) measured in an original rapid compression machine is provided. It allows a comparison of the ignition conditions of pressure, temperature and equivalence ratio for these hydrocarbons. Detailed mechanisms from different research groups based on a similar generic scheme of hydrocarbon oxidation are tested against the experimental ignition delays. The differences between experimental and modeling results are discussed.

New experimental data on autoignition delays and product distributions during two-stage autoignitions for the two primary reference fuels n -heptane and iso -octane (2,2,4-trimethylpentane) have been obtained by rapid compression in the low and intermediate range of temperature for enginelike conditions of stoichiometry and dilution. The lower reactivity of iso -octane has been compensated by a four times increase in pressure. A good correlation between our data and that published is obtained when the compressed charge density of the core gas is considered. Both fuels show many common features in this temperature range: a marked negative temperature coefficient region that shifts to higher temperatures as the pressure is increased and a similarity in the nature of the intermediate species. However, the importance of the cool flame zone is greater for n -heptane, and the negative temperature coefficient region extends toward higher temperatures. The evolution of the main intermediate products formed during the two-stage autoignition is presented and discussed according to a common generic mechanism that takes into account the various isomerizations of alkylperoxy radicals and scissions of the hydroperoxyalkyl radicals. Cyclic ethers are important intermediates. For both hydrocarbons, tetrahydrofurans are the major O heterocycles formed in cool flames, especially in the case of iso -octane. The observed high selectivity in the lower alkenes demonstrates the importance of β carbon-carbon scission of the hydroperoxyalkyl radicals that leads to terminal alkenes in the case of n -heptane and to methylpropene and substituted pentenes in the case of iso -octane. These channels have to be taken into account in the improvement of detailed mechanisms for good predictions of pollutants.

This paper presents computations based on a semidetailed kinetic scheme for reference fuel (n-heptane and 2,2,4-trimethyl-pentane) mixtures. Model results are compared with several experimental data previously appearing in the literature and with data presented here for the first time. New data, obtained in a pressurized flow reactor at 12.5 atm, report the heat release and gas composition of various dilute, stoichiometric mixtures of primary reference, fuels and oxygen under fixed reaction time. New data from rapid compression machine experiments characterize cool flame phenomena in the low temperature range and define total ignition-delay times for stoichiometric mixtures of primary reference fuels and air mixtures at high octane number. The new data confirm that negative temperature coefficient (NTC) and hot ignition characteristics are a nonlinear function of octane number (ON). A previous kinetic model developed and tested for the oxidation of the individual pure reference fuel is shown to reproduce accurately the autoignition and the various oxidation characteristics of reference fuel mixtures in the earlier experiments, as well as in shock tubes and in a motored engine experiment. Moreover, the model is shown to unify the intrinsic information of the different experimental sources and data and provides a skeletal mechanism from which to derive and validate simpler empirical kinetic models for inclusion in engine design codes. Results also suggest that new studies and experiments are required for a deeper understanding of oxidation processes to include the effects of other molecular structure classes, particularly fuel additives (e.g., ethers, alcohols) and aromatics.

The rate constants of the H-abstraction reactions from CH(3)Cl and CH(3)Br molecules by ClO and BrO radicals have been estimated over the temperature range of 300-2500 K using four different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using B3LYP and MP2 methods combined with the cc-pVTZ basis set. Single-point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using the cc-pVTZ and cc-pVQZ basis sets. Canonical transition-state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature. In order to choose the appropriate levels of theory with chlorine- and bromine-containing species, the reference reaction Cl ((2)P(3/2)) + CH(3)Cl → HCl + CH(2)Cl (R(ref)) was first theoretically studied because its kinetic parameters are well-established from numerous experiments, evaluation data, and theoretical studies. The kinetic parameters of the reaction R(ref) have been determined accurately using the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level of theory. This level of theory has been used for the rate constant estimation of the reactions ClO + CH(3)Cl (R(1)), ClO + CH(3)Br (R(2)), BrO + CH(3)Cl (R(3)), and BrO + CH(3)Br (R(4)). Six-parameter Arrhenius expressions have been obtained by fitting to the computed rate constants of these four reactions (including cis and trans pathways) over the temperature range of 300-2500 K.

o-Xylene could be a good candidate to represent the family of aromatic hydrocarbons in a surrogate fuel. This study uses computational chemistry to calculate standard enthalpies of formation at 298 K, Δ(f)H°(298 K), standard entropies at 298 K, S°(298 K), and standard heat capacities C(p)°(T) over the temperature range 300 K to 1500 K for ten target species present in the low-temperature oxidation mechanism of o-xylene: o-xylene (1), 2-methylbenzyl radical (2), 2-methylbenzylperoxy radical (3), 2-methylbenzyl hydroperoxide (4), 2-(hydroperoxymethyl)benzyl radical (5), 2-(hydroperoxymethyl)benzaldehyde (6), 1-ethyl-2-methylbenzene (7), 2,3-dimethylphenol (8), 2-hydroxybenzaldehyde (9), and 3-hydroxybenzaldehyde (10). Δ(f)H°(298 K) values are weighted averages across the values calculated using five isodesmic reactions and five composite calculation methods: CBS-QB3, G3B3, G3MP2, G3, and G4. The uncertainty in Δ(f)H°(298 K) is also evaluated. S°(298 K) and C(p)°(T) values are calculated at B3LYP/6-311G(d,p) level of theory from molecular properties and statistical thermodynamics through evaluation of translational, rotational, vibrational, and electronic partition functions. S°(298 K) and C(p)°(300 K) values are evaluated using the rigid-rotor-harmonic-oscillator model. C(p)°(T) values at T ≥ 400 K are calculated by treating separately internal rotation contributions and translational, external rotational, vibrational, and electronic contributions. The thermochemical properties of six target species are used to develop six new additivity groups taking into account the interaction between two substituents in ortho (ORT/CH2OOH/ME, ORT/ET/ME, ORT/CHO/OH, ORT/CHO/CH2OOH) or meta (MET/CHO/OH) positions, and the interaction between three substituents (ME/ME/OH123) located one beside the other (positions numbered 1, 2, 3) for two- or three-substituted benzenic species. Two other additivity groups are also developed using the thermochemical properties of benzenic species taken from the literature: the C/CB/H2/OO and the CB/CO groups. These groups extend the capacities of the group additivity method to deal with substituted benzenic species.

Theoretical calculations are performed to obtain thermochemical properties ( Δ f H 298 K ∘ , S 298 K ∘ , CP = f(T)) for the electronic ground state of the NCN (3Σ−) biradical. In order to validate our methodology, the thermochemical properties ( Δ f H 298 K ∘ , S 298 K ∘ , CP = f(T)) are determined also for seventeen species involved in the prompt-NO mechanism. The standard enthalpies of formation at 298 K are estimated using atomization reaction based on the CBS-QB3, CBS-APNO, G3B3, G3, and G4 calculation levels. In the case of the NCN biradical, an isogyric set of four reactions is also used to assess its standard enthalpy of formation at 298 K. Standard molar entropies at 298 K and heat capacities at constant pressure are estimated at the B3LYP/cc-pVQZ level of theory. Based on our results, we recommend the use of our calculated thermochemical properties for the modeling of the prompt-NO mechanism: Δ f H 298 K ∘ (NCN) = (448.7 ± 3.4) kJ mol−1, S 298 K ∘ = 225.8 J mol−1 K−1, CP (in J mol−1 K−1) = 41.9, 45.8, 49.0, 51.6, 55.2, 57.3, 59.9, 60.9, 61.7, 62.0, 62.1 at 300, 400, 500, 600, 800, 1000, 1500, 2000, 3000, 4000, 5000 K, respectively.

The influence of N-methylaniline (NMA) on flame stability and structure of a two-stage butane-air flame is studied. The stability of the flame system is reduced as increasing amount of the inhibitor are added, particularly at lower pressures. At 1.8 bar and a low level of NMA (0.3% of butane) the structure of the two-stage butane-air flame, which was described in a previous communication with a particular emphasis on the H2O2/HO2 chemistry, is slightly modified. Among the 18 chemical species profiles studied, the hydrogen peroxide and peroxy radicals profiles are most sensitive to inhibition. The second stage ignition is particularly affected as it results from balanced concentration of the branching agent H2O2 controlled by the inhibitor. At higher concentration of NMA, the inhibition is more important, as revealed by the lifted positions of the flames.

Une machine a compression rapide originale a ete construite pour l’etude de l’autoinflammation des hydrocarbures et la validation des mecanismes reactionnels a haute pression dans la gamme de temperature 600-900 K environ. Cette machine est pilotee par microordinateur et est munie d’un systeme d’echantillonnage. Les problemes lies a la determination des temperatures et la vitesse de compression des gaz sont discutes. Divers exemples d’autoinflammation sont donnes notamment dans le cas d’autoinflammations "douces" et "severes".

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International audience

The rate constants of the 2-, 3-, and 4-methylbenzylperoxy isomerization reactions have been computed using the elaborated CASPT2 method. Geometry optimizations and vibrational frequency calculations are performed with two methods (B3LYP and MPW1K) combined with the cc-pVDZ and 6-31+G(d,p) basis sets, respectively. Single-point energy calculations are performed at the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory as recommended by Canneaux et al. (J. Phys. Chem. A 2008, 112, 6045). Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. They are given by the following relations for the 2-, 3-, and 4-methylbenzylperoxy (MBP) (1,3s) isomerizations and for the 2-methylbenzylperoxy (1,6p) isomerization, respectively: k(2-MBP(1,3s))(600-2000 K) (in s(-1)) = (3.33 x 10(10))T(0.79) exp((-142.6 in kJ mol(-1))/RT); k(3-MBP(1,3s))(600-2000 K) (in s(-1)) = (0.74 x 10(10))T(0.79) exp((-130.7 in kJ mol(-1))/RT); k(4-MBP(1,3s))(600-2000 K) (in s(-1)) = (1.12 x 10(10))T(0.79) exp((-133.6 in kJ mol(-1))/RT); k(2-MBP(1,6p))(600-2000 K) (in s(-1)) = (5.10 x 10(8))T(0.85) exp((-87.1 in kJ mol(-1))/RT). These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. In the case of the 2-methylbenzylperoxy radical, the (1,6p) H-atom transfer reaction is consistently the most important channel over the studied temperature range, and the (1,3s) H-atom transfer reaction is not energetically favored.

The autoignition of a series of C 4 to C 8 fatty acid methyl esters has been studied in a rapid compression machine in the low and intermediate temperature region (650–850 K) and at increasing pressures (4–20 bar). Methyl hexanoate was selected for a full investigation of the autoignition phenomenology, including the identification and determination of the intermediate products of low temperature oxidation. The oxidation scheme and overall reactivity of methyl hexanoate has been examined and compared to the reactivity of C 4 to C 7 n -alkanes in the same experimental conditions to evaluate the impact of the ester function on the reactivity of the n -alkyl chain. The low temperature reactivity leading to the first stage of autoignition is similar to n -heptane. However, the negative temperature coefficient region is located at lower temperature than in the case of the n -alkanes of corresponding reactivity. An evaluation of the distribution of esteralkyl radicals R and esteralkylperoxy radicals ROO gives an insight into the main reaction pathways.

The autoignition of butane has been studied by two techniques. At a relatively low pressure (1.8 bar), a two-stage flame has been fully described by following the stable-species and peroxy-radical evolution. At higher pressures, studies have been conducted in a rapid-compression machine in order to investigate the evolution of the autoignition delay times with temperature. These experimental results are compared with predictions obtained from a butane-oxidation model based on a complete mechanism of 133 species and 689 reversible reactions as proposed and tested at low pressure by Pitz, Wilk, Westbrook and Cernansky. In a reduced version (45 species, 272 reversible reactions) the model agrees with the measured major species produced in the second stage of a burner-stabilized two-stage flame. In its complete version, it also predicts the negative temperature coefficient observed at high pressure in the rapid-compression machine. However to account for the ignition delay, it was necessary to modify the rate constants associated with the low-temperature mechanism as suggested by Pitz, Leppard and Westbrook.

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.

International audience; New kinetic mechanisms for the oxidation of 1-pentene and 1-hexene at low temperature have been developed that require important improvements of the kinetic rules used by the EXGAS system for the automatic generation of mechanisms. This paper details the changes or additions necessary for the definition of the specific generic reactions involving alkenes and their free radicals, as well as the correlations to estimate the related rate constants. Tests have been performed to verify that these improvements still allow good simulations in the case of propene. New mechanisms for the oxidation of 1-pentene and 1-hexene at low temperature have been thus generated and validated using experimental data obtained in a rapid compression machine between 600 and 900 K. The mechanism for the oxidation of 1-pentene has also been tested in a plug flow reactor between 654 and 716 K. Results reveal acceptable agreement between simulated and experimental data for autoignition delays and for the distribution of products. The analysis of mechanisms demonstrates the importance of new reaction pathways specific to long chain alkenes. This study confirms the significant role played in autoignition delays by the reaction of addition of hydroxyl radicals to the double bond and by the specific reactivity of the allylic radical. The important role played by the reactions of allylic and alkenyl radicals with O2 to produce dienes is also emphasized and has allowed us to refine the kinetic value for these generic reactions.

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International audience

The ignition delay times of CH4/C2H6/C3H8 mixtures representative of an average natural gas composition have been measured in a rapid compression machine (RCM) at the University of Lille. The pressure at the end of compression (EOC) varied from 13 to 21 bar and the core gas temperature ranged from 850 to 925 K. Zero-dimensional modelling starting from the EOC was used to reproduce the experimental ignition delay times taking into account heat losses during the preignition phase. The experimental database served as basis for the development of a reaction mechanism suitable for HCCI like autoignition simulations on a stationary co-generation engine with a prechamber, which is under development at the laboratory. Different mechanisms for natural gas oxidation and combustion have been tested and their low temperature simulation ability investigated, showing difficulties to properly reproduce the low temperature ignition delay times. Starting from the GRI3.0 mechanism with an additional submodule improving the low temperature chemistry representation, a sensitivity analysis was performed to determine the most influential reactions. The rate constants of these reactions were then optimised within their range of uncertainty using a multi- objective strategy. The resulting optimised mechanism led to a strong improvement of the agreement between simulation and experimental RCM data. The optimised mechanism was also tested on experimental shock tube data between 900 and 1250 K and gave satisfying results within the temperature range where the optimisation was performed on. Therefore, the applied optimisation technique showed its efficiency.

The low-temperature oxidation of n -butylbenzene, an intermediate structure between alkanes and short-chain alkylaromatics, was studied between 640 and 840 K by rapid compression and by modeling. Delay times of one- and two-stage autoignitions were measured, and intermediate species after the cool flame were analyzed. First, a detailed mechanism for n -butane was developed with existing material. Then, an n -butylbenzene mechanism was built by taking into account the change of reactivity due to the introduction of the aromatic nucleus. Both mechanisms have been validated by simulations of the delays and the product concentrations. Finally, the n -butylbenzene mechanism was used to analyze the main low-temperature reaction pathways. The comparative calculation of the concentrations of alkyl, alkylperoxy, and hydroperoxyalkyl radicals in the cool flame of n -butane and n -butylbenzene illustrates the effects of the aromatic nucleus on the first steps of oxidation. A study of the competitive channels to the main molecular intermediate species shows that the internal transfer of a benzylic hydrogen to the peroxy sites is a major event in the development of reactions leading to branching and ignition. This can explain a previous observation that alkylaromatics with two oitho -alkyl groups or a long single lateral chain have the possibility of an internal transfer of a benzylic hydrogen and manifest a greater low-temperature reactivity than aromatics that have neither oitho -alkyl groups nor a long lateral chain.