Rate coefficients and equilibrium constant for the CH2CHO + O2 reaction system

The kinetics of the CH2CHO + O2 reaction was experimentally studied in two quasi-static reactors and a discharge flow-reactor at temperatures ranging from 298 to 660 K and pressures between 1 mbar and 46 bar with helium as the bath gas. The CH2CHO radicals were produced by the laser-flash photolysis of ethyl vinyl ether at 193 nm and by the reaction F + CH3CHO, respectively. Laser-induced fluorescence excited at 337 or 347.4 nm was used to monitor the CH2CHO concentration. The reaction proceeded via reversible complex formation with subsequent isomerization and fast decomposition: CH2CHO + O2 <= => O2CH2CHO --> HO2CH2CO --> products. The rate coefficients for the first and second steps were determined (k1, k-1, k2) and analyzed by a master equation with specific rate coefficients from the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Molecular and transition-state parameters were obtained from quantum chemical calculations. A third-law analysis led to the following thermodynamic parameters for the first step: Delta(R)S degrees 300K(1) = -144 J K(-1) mol(-1) (1 bar) and Delta(R)H degrees 300K(1) = (-101 +/- 4) kJ mol(-1). From the falloff analysis, the following temperature dependencies for the low- and high-pressure limiting rate coefficients were obtained: k1(0) = 5.14 x 10(-14) exp(210 K/T) cm(-3) s(-1); k1(infinity) = 1.7 x 10(-12) exp(-520 K/T) cm(-3) s(-1); and k2(infinity) = 1.3 x 10(12) exp[-(82 +/- 4) kJ mol(-1)/RT] s(-1). Readily applicable analytical representations for the pressure and temperature dependence of k1 were derived to be used in kinetic modeling.     

Investigation of the radical product channel of the CH3COO2 + HO2 reaction in the gas phase

The reaction of CH(3)C(O)O(2) with HO(2) has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/CH(3)CHO/CH(3)OH/air mixtures. The branching ratio for the reaction channel forming CH(3)C(O)O, OH and O(2) (reaction ) has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on benzene concentration was found to be consistent with its formation from the OH-initiated oxidation of benzene, thereby confirming the presence of OH radicals in the system. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH(3)OH](0)/[CH(3)CHO](0), is consistent with OH formation resulting mainly from the reaction of CH(3)C(O)O(2) with HO(2) in the early stages of the experiments, such that the limiting yield of phenol at high benzene concentrations is well-correlated with that of CH(3)C(O)OOH, a well-established product of the CH(3)C(O)O(2) + HO(2) reaction (via channel (3a)). However, a delayed source of phenol was also identified, which is attributed mainly to an analogous OH-forming channel of the reaction of HO(2) with HOCH(2)O(2) (reaction ), formed from the reaction of HO(2) with product HCHO. This was investigated in additional series of experiments in which Cl(2)/CH(3)OH/benzene/air and Cl(2)/HCHO/benzene/air mixtures were photolysed. The various reaction systems were fully characterised by simulations using a detailed chemical mechanism. This allowed the following branching ratios to be determined: CH(3)C(O)O(2) + HO(2)--> CH(3)C(O)OOH + O(2), k(3a)/k(3) = 0.38 +/- 0.13; --> CH(3)C(O)OH + O(3), k(3b)/k(3) = 0.12 +/- 0.04; --> CH(3)C(O)O + OH + O(2), k(3c)/k(3) = 0.43 +/- 0.10: HOCH(2)O(2) + HO(2)--> HCOOH + H(2)O + O(2), k(17b)/k(17) = 0.30 +/- 0.06; --> HOCH(2)O + OH + O(2), k(17c)/k(17) = 0.20 +/- 0.05. The results therefore provide strong evidence for significant participation of the radical-forming channels of these reactions, with the branching ratio for the title reaction being in good agreement with the value reported in one previous study. As part of this work, the kinetics of the reaction of Cl atoms with phenol (reaction (14)) have also been investigated. The rate coefficient was determined relative to the rate coefficient for the reaction of Cl with CH(3)OH, during the photolysis of mixtures of Cl(2), phenol and CH(3)OH, in either N(2) or air at 296 K and 760 Torr. A value of k(14) = (1.92 +/- 0.17) x 10(-10) cm(3) molecule(-1) s(-1) was determined from the experiments in N(2), in agreement with the literature. In air, the apparent rate coefficient was about a factor of two lower, which is interpreted in terms of regeneration of phenol from the product phenoxy radical, C(6)H(5)O, possibly via its reaction with HO(2).     

Mechanism of the OH-initiated oxidation of hydroxyacetone over the temperature range 236-298 K

The mechanism of the gas-phase reaction of OH radicals with hydroxyacetone (CH3C(O)CH2OH) was studied at 200 Torr over the temperature range 236-298 K in a turbulent flow reactor coupled to a chemical ionization mass-spectrometer. The product yields and kinetics were measured in the presence of O2 to simulate the atmospheric conditions. The major stable product at all temperatures is methylglyoxal. However, its yield decreases from 82% at 298 K to 49% at 236 K. Conversely, the yields of formic and acetic acids increase from about 8% to about 20%. Other observed products were formaldehyde, CO2 and peroxy radicals HO2 and CH3C(O)O2. A partial re-formation of OH radicals (by approximately 10% at 298 K) was found in the OH + hydroxyacetone + O2 chemical system along with a noticeable inverse secondary kinetic isotope effect (k(OH)/k(OD) = 0.78 +/- 0.10 at 298 K). The observed product yields are explained by the increasing role of the complex formed between the primary radical CH3C(O)CHOH and O2 at low temperature. The rate constant of the reaction CH3C(O)CHOH + O2 --> CH3C(O)CHO + HO2 at 298 K, (3.0 +/- 0.6) x 10(-12) cm3 molecule(-1) s(-1), was estimated by computer simulation of the concentration-time profiles of the CH3C(O)CHO product. The detailed mechanism of the OH-initiated oxidation of hydroxyacetone can help to better describe the atmospheric oxidation of isoprene, in particular, in the upper troposphere.     

A reinvestigation of the kinetics and the mechanism of the CH3C(O)O2 + HO2 reaction using both experimental and theoretical approaches

The kinetics and the mechanism of the reaction CH(3)C(O)O(2)+ HO(2) were reinvestigated at room temperature using two complementary approaches: one experimental, using flash photolysis/UV absorption technique and one theoretical, with quantum chemistry calculations performed using the density functional theory (DFT) method with the three-parameter hybrid functional B3LYP associated with the 6-31G(d,p) basis set. According to a recent paper reported by Hasson et al., [J. Phys. Chem., 2004, 108, 5979-5989] this reaction may proceed by three different channels: CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OOH + O(2) (1a); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OH + O(3) (1b); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)O + OH + O(2) (1c). In experiments, CH(3)C(O)O(2) and HO(2) radicals were generated using Cl-initiated oxidation of acetaldehyde and methanol, respectively, in the presence of oxygen. The addition of amounts of benzene in the system, forming hydroxycyclohexadienyl radicals in the presence of OH, allowed us to answer that channel (1c) is <10%. The rate constant k(1) of reaction (1) has been finally measured at (1.50 +/- 0.08) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, after having considered the combination of all the possible values for the branching ratios k(1a)/k(1,)k(1b)/k(1,)k(1c)/k(1) and has been compared to previous measurements. The branching ratio k(1b)/k(1), determined by measuring ozone in situ, was found to be equal to (20 +/- 1)%, a value consistent with the previous values reported in the literature. DFT calculations show that channel (1c) is also of minor importance: it was deduced unambiguously that the formation of CH(3)C(O)OOH + O(2) (X (3)Sigma(-)(g)) is the dominant product channel, followed by the second channel (1b) leading to CH(3)C(O)OH and singlet O(3) and, much less importantly, channel (1c) which corresponds to OH formation. These conclusions give a reliable explanation of the experimental observations of this work. In conclusion, the present study demonstrates that the CH(3)C(O)O(2)+ HO(2) is still predominantly a radical chain termination reaction in the tropospheric ozone chain formation processes.     

Room temperature and shock tube study of the reaction HCO+O2 using the photolysis of glyoxal as an efficient HCO source

The rate of the reaction 1, HCO+O2-->HO2+CO, has been determined (i) at room temperature using a slow flow reactor setup (20 mbarH2+HCO+CO, into additional HCO radicals. The rate constants of reaction 4 were determined from unperturbed photolysis experiments to be k4(295 K)=(3.6+/-0.3)x10(10) cm3 mol-1 s-1 and k4(769-1107 K)=5.4x10(13)exp(-18 kJ mol-1/RT) cm3 mol-1 s-1(Delta log k4=+/-0.12).     

Mutual sensitization of the oxidation of nitric oxide and a natural gas blend in a JSR at elevated pressure: experimental and detailed kinetic modeling study

The mutual sensitization of the oxidation of NO and a natural gas blend (methane-ethane 10:1) was studied experimentally in a fused silica jet-stirred reactor operating at 10 atm, over the temperature range 800-1160 K, from fuel-lean to fuel-rich conditions. Sonic quartz probe sampling followed by on-line FTIR analyses and off-line GC-TCD/FID analyses were used to measure the concentration profiles of the reactants, the stable intermediates, and the final products. A detailed chemical kinetic modeling of the present experiments was performed yielding an overall good agreement between the present data and this modeling. According to the proposed kinetic scheme, the mutual sensitization of the oxidation of this natural gas blend and NO proceeds through the NO to NO2 conversion by HO2, CH3O2, and C2H5O2. The detailed kinetic modeling showed that the conversion of NO to NO2 by CH3O2 and C2H5O2 is more important at low temperatures (ca. 820 K) than at higher temperatures where the reaction of NO with HO2 controls the NO to NO2 conversion. The production of OH resulting from the oxidation of NO by HO2, and the production of alkoxy radicals via RO2 + NO reactions promotes the oxidation of the fuel. A simplified reaction scheme was delineated: NO + HO2 --> NO2 + OH followed by OH + CH4 --> CH3 + H2O and OH + C2H6 --> C2H5 + H2O. At low-temperature, the reaction also proceeds via CH3 + O2 (+ M) --> CH3O2 (+ M); CH3O2 + NO --> CH3O + NO2 and C2H5 + O2 --> C2H5O2; C2H5O2 + NO --> C2H5O + NO2. At higher temperature, methoxy radicals are produced via the following mechanism: CH3 + NO2 --> CH3O + NO. The further reactions CH3O --> CH2O + H; CH2O + OH --> HCO + H2O; HCO + O2 --> HO2 + CO; and H + O2 + M --> HO2 + M complete the sequence. The proposed model indicates that the well-recognized difference of reactivity between methane and a natural gas blend is significantly reduced by addition of NO. The kinetic analyses indicate that in the NO-seeded conditions, the main production of OH proceeds via the same route, NO + HO2 --> NO2 + OH. Therefore, a significant reduction of the impact of the fuel composition on the kinetics of oxidation occurs.     

Thermal stability of carbonyl radicals. Part II. Reactions of methylglyoxyl and methylglyoxylperoxy radicals at 1 bar in the temperature range 275-311 K

Reactions of methylglyoxyl and methylglyoxylperoxy radicals were investigated at a total pressure of 1 bar in oxygen. Methylglyoxyl radicals were generated by stationary photolysis of Br2-CH3C(O)C(O)H-NO2-O2-N2 mixtures at wavelengths > or =480 nm and of Cl2-CH3C(O)C(O)H-NO2-O2-N2 mixtures in the wavelength range 315-460 nm. In the bromine system, rate constant ratios for the reactions CH3C(O)CO --> CH3CO + CO (kdis) and CH3C(O)CO + O2 --> CH3C(O)C(O)O2 (kO2) were measured as a function of temperature in the range 275-311 K. Assuming the constant value kO2 = 5.1 x 10(-12) cm3 molecule(-1) s(-1) for our reaction conditions, kdis = 1.2 x 10(10.0+/-0.7) x exp(-11.7 +/- 3.8 kJ mol(-1)/RT) s(-1) (2sigma errors) was obtained for ptot = 1 bar (M = O2), in good agreement with the kinetic parameters calculated by Méreau et al. [R. Méreau, M.-T. Rayez, J.-C. Rayez, F. Caralp and R. Lesclaux, Phys. Chem. Chem. Phys., 2001, 3, 4712]. CH3C(O)C(O)O2 radicals oxidise NO2, forming NO3, CH3CO and CO2. This experimental result is supported by DFT and ab initio calculations. Possible mechanisms for the observed formation of several % of ketene and bromoacetyl peroxynitrate are discussed. Use of Cl rather than Br atoms to abstract the aldehydic H atom from methylglyoxal leads to chemically activated CH3C(O)CO radicals, thus substantially increasing the fraction of CH3C(O)CO radicals that decompose rather than add O2.     

Kinetics and mechanism of the C6H5 + CH3CHO reaction: experimental measurement and theoretical prediction of the reactivity toward four molecular sites.

The kinetics and mechanism of the reaction of C6H5 with CH3CHO have been investigated experimentally and theoretically. The total rate constant for the reaction has been measured by means of the cavity ring-down spectrometry (CRDS) in the temperature range 299-501 K at pressures covering 20-75 Torr. The overall bimolecular rate constant can be represented by the expression k = (2.8 +/- 0.2) x 10(11) exp[-(700 +/- 30)/T] cm3 mol-1 s-1, which is slightly faster than for the analogous C6H5 + CH2O reaction determined with the same method in the same temperature range. The reaction mechanism for the C6H5 + CH3CHO reaction was also explored with quantum-chemical calculations at various hybrid density functional theories (DFTs) and using ab initio high-level composite methods. The theories predict that the reaction may occur by two hydrogen-abstraction and two addition channels with the aldehydic hydrogen-abstraction reaction being dominant. The rate constant calculated by the transition state theory for the aldehydic hydrogen-abstraction reaction is in good agreement with the experimental result after a very small adjustment of the predicted reaction barrier (+0.3 kcal mol-1). Contributions from other product channels are negligible under our experimental conditions. For combustion applications, we have calculated the rate constants for key product channels in the temperature range of 298-2500 K under atmospheric-pressure conditions; they can be represented by the following expressions in units of cm 3mol-1 s-1: k1,cho = 8.8 x 10(3)T2.6 exp(-90/T), k2,ch3 = 6.0 x 10(1)T3.3 exp(-950/T), k3a(C6H5COCH3 + H) = 4.2 x 10(5)T0.6 exp(-410/T) and k3b(C6H5CHO + CH3) = 6.6 x 10(9)T-0.5 exp(-310/T).     

CH(A2Delta) Formation in hydrocarbon combustion: the temperature dependence of the rate constant of the reaction C2H + O2--> CH(A2Delta) + CO2

The temperature dependence of the rate constant of the chemiluminescence reaction C2H + O2 --> CH(A) + CO2, k1e, has been experimentally determined over the temperature range 316-837 K using pulsed laser photolysis techniques. The rate constant was found to have a pronounced positive temperature dependence given by k1e(T) = AT(4.4) exp(1150 +/- 150/T), where A = 1 x 10(-27) cm(3) s(-1). The preexponential factor for k1e, A, which is known only to within an order of magnitude, is based on a revised expression for the rate constant for the C2H + O(3P) --> CH(A) + CO reaction, k2b, of (1.0 +/- 0.5) x 10(-11) exp(-230 K/T) cm3 s(-1) [Devriendt, K.; Van Look, H.; Ceursters, B.; Peeters, J. Chem. Phys. Lett. 1996, 261, 450] and a k2b/k1e determination of this work of 1200 +/- 500 at 295 K. Using the temperature dependence of the rate constant k1e(T)/k1e(300 K), which is much more accurately and precisely determined than is A, we predict an increase in k(1e) of a factor 60 +/- 16 between 300 and 1500 K. The ratio of rate constants k2b/k1e is predicted to change from 1200 +/- 500 at 295 K to 40 +/- 25 at 1500 K. These results suggest that the reaction C2H + O2 --> CH(A) + CO2 contributes significantly to CH(A-->X) chemiluminescence in hot flames and especially under fuel-lean conditions where it probably dominates the reaction C2H + O(3P) --> CH(A) + CO.     

Reflected shock tube studies of high-temperature rate constants for OH + CH4 --> CH3 + H2O and CH3 + NO2 --> CH3O + NO

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm has been used to study the reactions OH + CH(4) --> CH(3) + H(2)O and CH(3) + NO(2) --> CH(3)O + NO. Over the temperature range 840-2025 K, the rate constants for the first reaction can be represented by the Arrhenius expression k = (9.52 +/- 1.62) x 10(-11) exp[(-4134 +/- 222 K)/T] cm(3) molecule(-1) s(-1). Since this reaction is important in both combustion and atmospheric chemistry, there have been many prior investigations with a variety of techniques. The present results extend the temperature range by 500 K and have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range 195-2025 K. A three-parameter expression describes the rate behavior over this temperature range, k = (1.66 x 10(-18))T(2.182) exp[(-1231 K)/T] cm(3) molecule(-1) s(-1). Previous theoretical studies are discussed, and the present evaluation is compared to earlier theoretical estimates. Since CH(3) radicals are a product of the reaction and could cause secondary perturbations in rate constant determinations, the second reaction was studied by OH radical production from the fast reactions CH(3)O --> CH(2)O + H and H + NO(2) --> OH + NO. The measured rate constant is 2.26 x 10(-11) cm(3) molecule(-1) s(-1) and is not dependent on temperature from 233 to 1700 K within experimental error.     

Ab initio energies and product branching ratios for the O+C3H6 reaction

Intermediate and transition-state energies have been calculated for the O+C3H6 (propene) reaction using the compound ab initio CBS-QB3 and G3 methods in combination with density functional theory. The lowest-lying triplet and singlet potential energy surfaces of the O-C3H6 system were investigated. RRKM statistical theory was used to predict product branching fractions over the 300-3000 K temperature and 0.001-760 Torr pressure ranges. The oxygen atom adds to the C3H6 terminal olefinic carbon in the primary step to generate a nascent triplet biradical, CH3CHCH2O. On the triplet surface, unimolecular dissociation of CH3CHCH2O to yield H+CH3CHCHO is favored over the entire temperature range, although the competing H2CO+CH3CH product channel becomes significant at high temperature. Rearrangement of triplet CH3CHCH2O to CH3CH2CHO (propanal) via a 1,2 H-atom shift has a barrier of 122.3 kJ mol(-1), largely blocking this reaction channel and any subsequent dissociation products. Intersystem crossing of triplet CH3CHCH2O to the singlet surface, however, leads to facile rearrangement to singlet CH3CH2CHO, which dissociates via numerous product channels. Pressure was found to have little influence over the branching ratios under most conditions, suggesting that the vibrational self-relaxation rates for p相似文献   

Reactions of SO3 with the O/H radical pool under combustion conditions

The reactions of SO3 with H, O, and OH radicals have been investigated by ab initio calculations. For the SO3 + H reaction (1), the lowest energy pathway involves initial formation of HSO3 and rearrangement to HOSO2, followed by dissociation to OH + SO2. The reaction is fast, with k(1) = 8.4 x 10(9)T(1.22) exp(-13.9 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (700-2000 K). The SO3 + O --> SO2 + O2 reaction (2) may proceed on both the triplet and singlet surfaces, but due to a high barrier the reaction is predicted to be slow. The rate constant can be described as k(2) = 2.8 x 10(4)T(2.57) exp(-122.3 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) for T > 1000 K. The SO3 + OH reaction to form SO2 + HO2 (3) proceeds by direct abstraction but is comparatively slow, with k(3) = 4.8 x 10(4)T(2.46) exp(-114.1 kJ mol(-) 1/RT) cm(3) mol(-1) s(-1) (800-2000 K). The revised rate constants and detailed reaction mechanism are consistent with experimental data from batch reactors, flow reactors, and laminar flames on oxidation of SO2 to SO3. The SO3 + O reaction is found to be insignificant during most conditions of interest; even in lean flames, SO3 + H is the major consumption reaction for SO3.     

Shock-tube study of the thermal decomposition of CH3CHO and CH3CHO + H reaction

The thermal decomposition of acetaldehyde, CH3CHO + M --> CH3 + HCO + M (eq 1), and the reaction CH3CHO + H --> products (eq 6) have been studied behind reflected shock waves with argon as the bath gas and using H-atom resonance absorption spectrometry as the detection technique. To suppress consecutive bimolecular reactions, the initial concentrations were kept low (approximately 10(13) cm(-3)). Reaction was investigated at temperatures ranging from 1250 to 1650 K at pressures between 1 and 5 bar. The rate coefficients were determined from the initial slope of the hydrogen profile via k1 = [CH3CHO]0(-1) x d[H]/dt, and the temperature dependences observed can be expressed by the following Arrhenius equations: k1(T, 1.4 bar) = 2.9 x 10(14) exp(-38 120 K/T) s(-1), k1(T, 2.9 bar) = 2.8 x 10(14) exp(-37 170 K/T) s(-1), and k1(T, 4.5 bar) = 1.1 x 10(14) exp(-35 150 K/T) s(-1). Reaction was studied with C2H5I as the H-atom precursor under pseudo-first-order conditions with respect to CH3CHO in the temperature range 1040-1240 K at a pressure of 1.4 bar. For the temperature dependence of the rate coefficient the following Arrhenius equation was obtained: k6(T) = 2.6 x 10(-10) exp(-3470 K/T) cm(3) s(-1). Combining our results with low-temperature data published by other authors, we recommend the following expression for the temperature range 300-2000 K: k6(T) = 6.6 x 10(-18) (T/K) (2.15) exp(-800 K/T) cm(3) s(-1). The uncertainties of the rate coefficients k1 and k6 were estimated to be +/-30%.     

Direct dynamics study on the hydrogen abstraction reaction CH2O + HO2 --> CHO + H2O2

We present a direct ab initio dynamics study on the hydrogen abstraction reaction CH2O + HO2 --> CHO + H2O2, which is predicted to have four possible reaction channels caused by different attacking orientations of HO2 radical to CH2O. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of the four reaction channels are calculated at the B3LYP/cc-pVTZ level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of some single-point multilevel energy calculations (HL). The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method. The calculated results show that, in the whole temperature range, the more favorable reaction channels are Channels 1 and 3. The total ICVT/SCT rate constants of the four channels at the HL//B3LYP/cc-pVTZ level of theory are in good agreement with the available experiment data over the measured temperature ranges, and the corresponding three-parameter expression is k(ICVT/SCT) = 3.13 x 10(-20) T(2.70) exp(-11.52/RT) cm3 mole(-1) s(-1) in the temperature range of 250-3000 K. Additionally, the flexibility of the dihedral angle of H2O2 is also discussed to explain the different experimental values.     

Branching ratios for the reaction of selected carbonyl-containing peroxy radicals with hydroperoxy radicals

An important chemical sink for organic peroxy radicals (RO(2)) in the troposphere is reaction with hydroperoxy radicals (HO(2)). Although this reaction is typically assumed to form hydroperoxides as the major products (R1a), acetyl peroxy radicals and acetonyl peroxy radicals have been shown to undergo other reactions (R1b) and (R1c) with substantial branching ratios: RO(2) + HO(2) → ROOH + O(2) (R1a), RO(2) + HO(2) → ROH + O(3) (R1b), RO(2) + HO(2) → RO + OH + O(2) (R1c). Theoretical work suggests that reactions (R1b) and (R1c) may be a general feature of acyl peroxy and α-carbonyl peroxy radicals. In this work, branching ratios for R1a-R1c were derived for six carbonyl-containing peroxy radicals: C(2)H(5)C(O)O(2), C(3)H(7)C(O)O(2), CH(3)C(O)CH(2)O(2), CH(3)C(O)CH(O(2))CH(3), CH(2)ClCH(O(2))C(O)CH(3), and CH(2)ClC(CH(3))(O(2))CHO. Branching ratios for reactions of Cl-atoms with butanal, butanone, methacrolein, and methyl vinyl ketone were also measured as a part of this work. Product yields were determined using a combination of long path Fourier transform infrared spectroscopy, high performance liquid chromatography with fluorescence detection, gas chromatography with flame ionization detection, and gas chromatography-mass spectrometry. The following branching ratios were determined: C(2)H(5)C(O)O(2), Y(R1a) = 0.35 ± 0.1, Y(R1b) = 0.25 ± 0.1, and Y(R1c) = 0.4 ± 0.1; C(3)H(7)C(O)O(2), Y(R1a) = 0.24 ± 0.15, Y(R1b) = 0.29 ± 0.1, and Y(R1c) = 0.47 ± 0.15; CH(3)C(O)CH(2)O(2), Y(R1a) = 0.75 ± 0.13, Y(R1b) = 0, and Y(R1c) = 0.25 ± 0.13; CH(3)C(O)CH(O(2))CH(3), Y(R1a) = 0.42 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.58 ± 0.1; CH(2)ClC(CH(3))(O(2))CHO, Y(R1a) = 0.2 ± 0.2, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2; and CH(2)ClCH(O(2))C(O)CH(3), Y(R1a) = 0.2 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2. The results give insights into possible mechanisms for cycling of OH radicals in the atmosphere.     

Theoretical Study of Reaction Mechanism of 1-Propenyl Radical with NO

The reaction system of 1-propenyl radical with NO is an ideal model for studying the intermolecular and intramolecular reactions of complex organic free radicals containing C=C double bonds. On the basis of the full optimization of all species with the Gaussian 98 package at the B3LYP/6-311++G** level, the reaction mechanism was elucidated extensively using the vibrational mode analysis. There are seven reaction pathways and five sets of small molecule end products: CH2O+CH3CN, CH2CHCN+H2O, CH3CHO+HCN, CH3CHO+HNC, and CH3CCH+HNO. The channel of C3H5￠+NO→ IM1→TS1→IM2→TS2→IM3→TS3→CH3CHO+HCN is thermodynamically most favorable.     

Deuterium kinetic isotope effects on the gas-phase reactions of C2H with H2(D2) and CH4(CD4)

Kinetics of the ethynyl (C(2)H) radical reactions with H(2), D(2), CH(4) and CD(4) was studied over the temperature range of 295-396 K by a pulsed laser photolysis/chemiluminescence technique. The C(2)H radicals were generated by ArF excimer-laser photolysis of C(2)H(2) or CF(3)C(2)H and were monitored by the chemiluminescence of CH(A(2)Δ) produced by their reaction with O(2) or O((3)P). The measured absolute rate constants for H(2) and CH(4) agreed well with the available literature data. The primary kinetic isotope effects (KIEs) were determined to be k(H(2))/k(D(2)) = 2.48 ± 0.14 and k(CH(4))/k(CD(4)) = 2.45 ± 0.16 at room temperature. Both of the KIEs increased as the temperature was lowered. The KIEs were analyzed by using the variational transition state theory with semiclassical small-curvature tunneling corrections. With anharmonic corrections on the loose transitional vibrational modes of the transition states, the theoretical predictions satisfactorily reproduced the experimental KIEs for both C(2)H + H(2)(D(2)) and C(2)H + CH(4)(CD(4)) reactions.     

A kinetic and product study of the Cl + HO2 reaction

Absolute rate data and product branching ratios for the reactions Cl + HO2 --> HCl + O2 (k1a) and Cl + HO2 --> OH + ClO (k1b) have been measured from 226 to 336 K at a total pressure of 1 Torr of helium using the discharge flow resonance fluorescence technique coupled with infrared diode laser spectroscopy. For kinetic measurements, pseudo-first-order conditions were used with both reagents in excess in separate experiments. HO2 was produced by two methods: through the termolecular reaction of H atoms with O2 and also by the reaction of F atoms with H2O2. Cl atoms were produced by a microwave discharge of Cl2 in He. HO2 radicals were converted to OH radicals prior to detection by resonance fluorescence at 308 nm. Cl atoms were detected directly at 138 nm also by resonance fluorescence. Measurement of the consumption of HO2 in excess Cl yielded k1a and measurement of the consumption of Cl in excess HO2 yielded the total rate coefficient, k1. Values of k1a and k1 derived from kinetic experiments expressed in Arrhenius form are (1.6 +/- 0.2) x 10(-11) exp[(249 +/- 34)/T] and (2.8 +/- 0.1) x 10(-11) exp[(123 +/- 15)/T] cm3 molecule(-1) s(-1), respectively. As the expression for k1 is only weakly temperature dependent, we report a temperature-independent value of k1 = (4.5 +/- 0.4) x 10(-11) cm3 molecule(-1) s(-1). Additionally, an Arrhenius expression for k1b can also be derived: k1b = (7.7 +/- 0.8) x 10(-11) exp[-(708 +/- 29)/T] cm3 molecule(-1) s(-1). These expressions for k1a and k1b are valid for 226 K < or = T < or = 336 and 256 K < or = T < or = 296 K, respectively. The cited errors are at the level of a single standard deviation. For the product measurements, an excess of Cl was added to known concentrations of HO2 and the reaction was allowed to reach completion. HCl product concentrations were determined by IR absorption yielding the ratio k1a/k1 over the temperature range 236 K < or = T < or = 296 K. OH product concentrations were determined by resonance fluorescence giving rise to the ratio k1b/k1 over the temperature range 226 K < or = T < or = 336 K. Both of these ratios were subsequently converted to absolute numbers. Values of k1a and k1b from the product experiments expressed in Arrhenius form are (1.5 +/- 0.1) x 10(-11) exp[(222 +/- 17)/T] and (10.6 +/- 1.5) x 10(-11) exp[-(733 +/- 41)/T] cm3 molecule(-1) s(-1), respectively. These expressions for k1a and k1b are valid for 256 K < or = T < or = 296 and 226 K < or = T < or = 336 K, respectively. A combination of the kinetic and product data results in the following Arrhenius expressions for k1a and k1b of (1.4 +/- 0.3) x 10(-11) exp[(269 +/- 58)/T] and (12.7 +/- 4.1) x 10(-11) exp[-(801 +/- 94)/T] cm3 molecule(-1) s(-1), respectively. Numerical simulations were used to check for interferences from secondary chemistry in both the kinetic and product experiments and also to quantify the losses incurred during the conversion process HO2 --> OH for detection purposes.     

High-temperature shock tube measurements of methyl radical decomposition

We have studied the two-channel thermal decomposition of methyl radicals in argon, involving the reactions CH3 + Ar --> CH + H2 + Ar (1a) and CH3 + Ar --> CH2 + H + Ar (1b), in shock tube experiments over the 2253-3527 K temperature range, at pressures between 0.7 and 4.2 atm. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1311 nm. The collision-broadening coefficient for CH in argon, 2gamma(CH-Ar), was measured via repeated single-frequency experiments in the ethane pyrolysis system behind reflected shock waves. The measured 2gamma(CH-Ar) value and updated spectroscopic and molecular parameters were used to calculate the CH absorption coefficient at 431.1311 nm (23194.80 cm(-1)), which was then used to convert raw traces of fractional transmission to quantitative CH concentration time histories in the methyl decomposition experiments. The rate coefficient of reaction 1a was measured by monitoring CH radicals generated upon shock-heating highly dilute mixtures of ethane, C2H6, or methyl iodide, CH3I, in an argon bath. A detailed chemical kinetic mechanism was used to model the measured CH time histories. Within experimental uncertainty and scatter, no pressure dependence could be discerned in the rate coefficient of reaction 1a in the 0.7-4.2 atm pressure range. A least-squares, two-parameter fit of the current measurements, applicable between 2706 and 3527 K, gives k(1a) (cm(3) mol(-1) s(-1)) = 3.09 x 1015 exp[-40700/T (K)]. The rate coefficient of reaction 1b was determined by shock-heating dilute mixtures of C2H6 or CH3I and excess O2 in argon. During the course of reaction, OH radicals were monitored using the well-characterized R(1)(5) line of the OH A-X (0,0) band at 306.6871 nm (32606.52 cm(-1)). H atoms generated via reaction 1b rapidly react with O2, which is present in excess, forming OH. The OH traces are primarily sensitive to reaction 1b, reaction 9 (H + O2 --> OH + O) and reaction 10 (CH3 + O2 --> products), where the rate coefficients of reactions 9 and 10 are relatively well-established. No pressure dependence could be discerned for reaction 1b between 1.1 and 3.9 atm. A two-parameter, least-squares fit of the current data, valid over the 2253-2975 K temperature range, yields the rate expression k(1b) (cm(3) mol(-1) s(-1)) = 2.24 x 10(15) exp[-41600/T (K)]. Theoretical calculations carried out using a master equation/RRKM analysis fit the measurements reasonably well.     

Reflected shock tube studies of high-temperature rate constants for CH3 + O2, H2CO + O2, and OH + O2

The reflected shock tube technique with multipass absorption spectrometric detection of OH-radicals at 308 nm, corresponding to a total path length of approximately 2.8 m, has been used to study the reaction CH3 + O2 CH2O + OH. Experiments were performed between 1303 and 2272 K, using ppm quantities of CH3I (methyl source) and 5-10% O2, diluted with Kr as the bath gas at test pressures less than 1 atm. We have also reanalyzed our earlier ARAS measurements for the atomic channel (CH3 + O2 --> CH3O + O) and have compared both these results with other earlier studies to derive a rate expression of the Arrhenius form. The derived expressions, in units of cm3 molecule(-1) s(-1), are k = 3.11 x 10(-13) exp(-4953 K/T) over the T-range 1237-2430 K, for the OH-channel, and k = 1.253 x 10(-11) exp(-14241 K/T) over the T-range 1250-2430 K, for the O-atom channel. Since CH2O is a major product in both reactions, reliable rates for the reaction CH2O + O2 --> HCO + HO2 could be derived from [OH]t and [O]t experiments over the T-range 1587-2109 K. The combined linear least-squares fit result, k = 1.34 x 10(-8) exp(-26883 K/T) cm3 molecule(-1) s(-1), and a recent VTST calculation clearly overlap within the uncertainties in both studies. Finally, a high sensitivity for the reaction OH + O2 --> HO2 + O was noted at high temperature in the O-atom data set simulations. The values for this obtained by fitting the O-atom data sets at later times (approximately 1.2 ms) again follow the Arrhenius form, k = 2.56 x 10(-10) exp(-24145 K/T) cm3 molecule(-1) s(-1), over the T-range, 1950-2100 K.