Shock Tube/Laser Absorption Measurements of Cyclopentadiene Pyrolysis.

High-temperature cyclopentadiene pyrolysis was examined behind reflected shock waves in a heated shock tube using several laser absorption diagnostic schemes. A two-color, online-offline sensor near 3335 cm-1 was used to measure time histories of acetylene, while a three-color scheme of diagnostics at 10.532, 10.675, and 11.345 µm yielded measurements of cyclopentadiene and ethylene. Species time histories of cyclopentadiene decomposition and acetylene formation as well as ethylene yields are reported from 1319 to 1678 K at 1.2-1.5 atm. In addition, the overall decomposition rate of cyclopentadiene is reported, and comparisons are made to a number of kinetic models.

Collisional excitation kinetics for O(3s

Collisional excitation kinetics for atomic oxygen is studied behind reflected shock waves in 1%O_{2}/Ar mixtures over 10 000-11 000K using laser absorption spectroscopy of the O(3s^{5}S^{o}) to O(3p^{5}P_{3}) transition at 777 nm and the O(3p^{5}P_{3}) to O(3d ^{5}D_{2,3,4}^{o}) transitions at 926 nm. Four time histories are inferred simultaneously from the absorbance of the two transitions: the population density of level 4 of atomic oxygen, i.e., the O(3s ^{5}S^{o}) state, n_{4}; the population density of level 6 of atomic oxygen, i.e., the O(3p^{5}P_{3}) state, n_{6}; the electron number density, n_{e}; and the heavy-particle translational temperature, T_{tr}. Atomic oxygen in the levels 4 and 6 are not in equilibrium with the ground-state atomic oxygen as the measurements of n_{4} and n_{6} are generally 3-20 times smaller than the corresponding values under Boltzmann equilibrium at T_{tr}. However, these two states are close to partial equilibrium with each other within the test time, indicating strong heavy-particle cross coupling between levels 4 and 6 of atomic oxygen. A simplified two-temperature collisional-radiative (CR) model is developed to study the thermal and chemical nonequilibrium of atomic oxygen following shock heating. The four measured time histories are used to optimize the 12 collisional rate constants in the CR model using a stochastic gradient descent (SGD) algorithm. The time-history results, diagnostic methods, and collisional-radiative model presented in the current study are potentially useful in studies of high-enthalpy air, plasma processing, or other applications involving weakly ionized oxygen.

Determination of the JP10 + OH → Product Reaction Rate with Measured Fuel Concentrations in Shock Tube Experiments.

The overall reaction rate for JP10 + OH → products was measured directly via laser absorption of OH in shock tube experiments from 931 to 1308 K and 0.94 to 1.44 atm. The JP10 concentration of test gas mixtures was measured in the shock tube for several experiments using a 3.39 µm laser fuel diagnostic. The measured JP10 concentrations indicated fuel losses due to adsorption of 11-31% compared to values calculated manometrically from mixture preparation. OH was generated via rapid thermal decomposition of tert-butyl hydroperoxide behind reflected shock waves, and post-shock OH profiles were measured via laser absorption at 308.6 nm. The measured OH profiles were fit with a chemical kinetic model for JP10 chemistry to determine the overall JP10 + OH reaction rate. A recommendation is made for the JP10 + OH overall reaction rate over the temperature range explored in this study as k1 (931-1308 K) = 1.622 × 1014 exp(-1826/T [K]) ± 12%. To the authors' knowledge, these data are the first direct measurements of the overall reaction rate for JP10 + OH.

Shock Tube Measurement of the CH3 + C2H6 → CH4 + C2H5 Rate Constant.

The rate constant for the CH3 + C2H6 → CH4 + C2H5 reaction was studied behind reflected shock waves at temperatures between 1369 and 1626 K and pressures from 8.6 to 47.4 atm in mixtures of methane, ethane, and argon. Ethylene time histories were measured using laser absorption of radiation from a carbon dioxide gas laser near 10.532 µm. The resulting rate constant data can be represented by the Arrhenius equation k (T) = 3.90 × 1013 exp(-16670 cal/mol/RT) cm3 mol-1 s-1. We believe this is the first study to extend experimental data for this rate constant to temperatures above 1400 K. The overall 2σ uncertainty of the current data is +18%/-21% resulting primarily from uncertainties associated with the influence of secondary reactions and the fitting of rapidly changing species time histories at the higher temperatures.

Shock Tube Measurement of the C2H4 + H ⇔ C2H3 + H2 Rate Constant.

The rate constant for the reaction C2H4 + H ⇔ C2H3 + H2 was studied behind reflected shock waves at temperatures between 1619 and 1948 K and pressures near 10 atm in a mixture of C2H4, CH4, H2, and argon. C2H4 time histories were measured using laser absorption of a CO2 gas laser near 10.53 µm. Experimental mixtures were designed to optimize sensitivity to the title reaction with only weak sensitivity to secondary reactions. Two mechanisms, FFCM1 and ARAMCO v2, are used for data analysis. The well-selected operating conditions and Monte Carlo sampling data analysis procedure resulted in mechanism-independent reaction rate constant measurements with a 2σ uncertainty of ±35%. The current data disagree with a broadly used theoretical calculation (Knyazev et al. (1996)), but they are in good consensus with one of the review studies (Baulch et al. (2005)), k = (3.9 × 1022) T3.62 exp(-5670/ T) cm3 molecule-1 s-1. To the best of our knowledge, this work provides the first high-temperature study of the C2H4 + H ⇔ C2H3 + H2 reaction rate constant with well-defined uncertainty.

Shock Tube and Laser Absorption Study of CH2O Oxidation via Simultaneous Measurements of OH and CO.

The oxidation of Ar-diluted stoichiometric CH2O-O2 mixtures was studied behind reflected shock waves over temperatures of 1332-1685 K, at pressures of about 1.5 atm and initial CH2O mole fractions of 500, 1500, and 5000 ppm. Quantitative and time-resolved concentration histories of OH and CO (at both v″ = 0 and v″ = 1) were measured by narrow-linewidth laser absorption at 306.7 and 4854 nm, respectively. A time delay was observed between the formation of v″ = 0 and v″ = 1 states of CO, suggesting that CO was kinetically generated primarily in the ground state and then collisionally relaxed toward vibrational equilibrium. The measured CO and OH time-histories were used to evaluate the performance of four detailed reaction mechanisms regarding the oxidation chemistry of CH2O. Further analyses of these time-history data have also led to improved determination for the rate constants of two key reactions, namely H + O2 = O + OH (R1) and OH + CO = CO2 + H (R2), as follows: k1 = 8.04 × 1013 exp(-7370 K/T) cm3 mol-1 s-1, k2 = 1.90 × 1012 exp(-2760 K/T) cm3 mol-1 s-1; both expressions are valid over 1428-1685 K and have 1σ uncertainties of approximately ±10%.

Combined Ab Initio, Kinetic Modeling, and Shock Tube Study of the Thermal Decomposition of Ethyl Formate.

The potential energy surfaces (PESs) and reaction rate constants of the unimolecular decomposition of ethyl formate (EF) were investigated using high-precision theoretical methods at the CCSD(T)/CBS(T-Q)//M06-2X/6-311++G(d,p) level of theory. The calculated PESs of EF dissociation and molecular decomposition reactions indicate that the intramolecular H-shift to produce formic acid and ethylene is the dominant decomposition pathway. A detailed chemical kinetic mechanism for EF pyrolysis was constructed by incorporating the important reactions of EF and its radicals into an existing mechanism previously developed for small methyl esters. The updated mechanism was first used to reproduce CO, CO2, and H2O concentration time histories during EF pyrolysis in the shock tube reported by Ren et al. [ Ren , W. , Mitchell Spearrin , R. , Davidson , D. F. , and Hanson , R. K. J. Phys. Chem. A 2014 , 118 , 1785 - 1798 ]. The rate of production and sensitivity analyses show that the competing dehydration and decarboxylation channels of the intermediate formic acid control the final product yields of EF pyrolysis. The EF mechanism was further validated against the shock tube data of OH, CO, CO2, and H2O time histories measured during EF oxidation (equivalence ratio Φ = 1.0) at 1331-1615 K and 1.52-1.74 atm. This revised EF mechanism captured all of the species' time histories over the entire temperature range. Such modeling capability was due to the more accurate rate constants of EF reactions determined by high-precision theoretical calculations and a high-fidelity C0-C2 basis mechanism.

Kinetics of Excited Oxygen Formation in Shock-Heated O2-Ar Mixtures.

The formation of electronically excited atomic oxygen was studied behind reflected shock waves using cavity-enhanced absorption spectroscopy. Mixtures of 1% O2-Ar were shock-heated to 5400-7500 K, and two distributed-feedback diode lasers near 777.2 and 844.6 nm were used to measure time-resolved populations of atomic oxygen's 5S° and 3S° electronic states, respectively. Measurements were compared with simulated population time histories obtained using two different kinetic models that accounted for thermal nonequilibrium effects: (1) a multitemperature model and (2) a reduced collisional-radiative model. The former assumed a Boltzmann distribution of electronic energy, whereas the latter allowed for non-Boltzmann populations by treating the probed electronic states as pseudospecies and accounting for dominant electronic excitation/de-excitation processes. The effects of heavy-particle collisions were investigated and found to play a major role in the kinetics of O atom electronic excitation at the conditions studied. For the first time, rate constants (kM) for O atom electronic excitation from the ground state (3P) due to collisions with argon atoms were directly inferred using the reduced collisional-radiative model, kM(3P → 5S°) = 7.8 × 10-17T0.5 exp(-1.061 × 105K/T) ± 25% cm3 s-1 and kM(3P → 3S°) = 2.5 × 10-17T0.5 exp(-1.105 × 105K/T) ± 25% cm3 s-1.

Shock Tube Measurement for the Dissociation Rate Constant of Acetaldehyde Using Sensitive CO Diagnostics.

The rate constant of acetaldehyde thermal dissociation, CH3CHO = CH3 + HCO, was measured behind reflected shock waves at temperatures of 1273-1618 K and pressures near 1.6 and 0.34 atm. The current measurement utilized sensitive CO diagnostics to track the dissociation of CH3CHO via oxygen atom balance and inferred the title rate constant (k1) from CO time histories obtained in pyrolysis experiments of 1000 and 50 ppm of CH3CHO/Ar mixtures. By using dilute test mixtures, the current study successfully suppressed the interferences from secondary reactions and directly determined the title rate constant as k1(1.6 atm) = 1.1 × 10(14) exp(-36 700 K/T) s(-1) over 1273-1618 K and k1(0.34 atm) = 5.5 × 10(12) exp(-32 900 K/T) s(-1) over 1377-1571 K, with 2σ uncertainties of approximately ±30% for both expressions. Example simulations of existing reaction mechanisms updated with the current values of k1 demonstrated substantial improvements with regards to the acetaldehyde pyrolysis chemistry.

Improved Shock Tube Measurement of the CH4 + Ar = CH3 + H + Ar Rate Constant using UV Cavity-Enhanced Absorption Spectroscopy of CH3.

We report an improved measurement for the rate constant of methane dissociation in argon (CH4 + Ar = CH3 + H + Ar) behind reflected shock waves. The experiment was conducted using a sub-parts per million sensitivity CH3 diagnostic recently developed in our laboratory based on ultraviolet cavity-enhanced absorption spectroscopy. The high sensitivity of this diagnostic allowed for measurements of quantitatively resolved CH3 time histories during the initial stage of CH4 pyrolysis, where the reaction system is clean and free from influences of secondary reactions and temperature change. This high sensitivity also allowed extension of our measurement range to much lower temperatures (<1500 K). The current-reflected shock measurements were performed at temperatures between 1487 and 1866 K and pressures near 1.7 atm, resulting in the following Arrhenius rate constant expression for the title reaction: k(1.7 atm) = 3.7 × 10(16) exp(-42 200 K/T) cm(3)/mol·s, with a 2σ uncertainty factor of 1.25. The current data are in good consensus with various theoretical and review studies, but at the low temperature end they suggest a slightly higher (up to 35%) rate constant compared to these previous results. A re-evaluation of previous and current experimental data in the falloff region was also performed, yielding updated expressions for both the low-pressure limit and the high-pressure limit rate constants and improved agreement with all existing data.

Cavity-enhanced absorption spectroscopy with a ps-pulsed UV laser for sensitive, high-speed measurements in a shock tube.

We report the first application of cavity-enhanced absorption spectroscopy (CEAS) with a ps-pulsed UV laser for sensitive and rapid gaseous species time-history measurements in a transient environment (in this study, a shock tube). The broadband nature of the ps pulses enabled instantaneous coupling of the laser beam into roughly a thousand cavity modes, which grants excellent immunity to laser-cavity coupling noise in environments with heavy vibrations, even with an on-axis alignment. In this proof-of-concept experiment, we demonstrated an absorption gain of 49, which improved the minimum detectable absorbance by ~20 compared to the conventional single-pass strategy at similar experimental conditions. For absorption measurements behind reflected shock waves, an effective time-resolution of ~2 µs was achieved, which enabled time-resolved observations of transient phenomena, such as the vibrational relaxation of O(2) demonstrated here. The substantial improvement in detection sensitivity, together with microsecond measurement resolution implies excellent potential for studies of transient physical and chemical processes in nonequilibrium situations, particularly via measurements of weak absorptions of trace species in dilute reactive systems.

Shock-tube measurements of excited oxygen atoms using cavity-enhanced absorption spectroscopy.

We report the use of cavity-enhanced absorption spectroscopy (CEAS) using two distributed feedback diode lasers near 777.2 and 844.6 nm for sensitive, time-resolved, in situ measurements of excited-state populations of atomic oxygen in a shock tube. Here, a 1% O2/Ar mixture was shock-heated to 5400-8000 K behind reflected shock waves. The combined use of a low-finesse cavity, fast wavelength scanning of the lasers, and an off-axis alignment enabled measurements with 10 µs time response and low cavity noise. The CEAS absorption gain factors of 104 and 142 for the P35←S520 (777.2 nm) and P0,1,23←S310 (844.6 nm) atomic oxygen transitions, respectively, significantly improved the detection sensitivity over conventional single-pass measurements. This work demonstrates the potential of using CEAS to improve shock-tube studies of nonequilibrium electronic-excitation processes at high temperatures.

Shock Tube Measurement of the High-Temperature Rate Constant for OH + CH3 → Products.

The reaction between hydroxyl (OH) and methyl radicals (CH3) is critical to hydrocarbon oxidation. Motivated by the sparseness of its high-temperature rate constant data and the large uncertainties in the existing literature values, the current study has remeasured the overall rate constant of the OH + CH3 reaction and extended the measurement temperature range to 1214-1933 K, using simultaneous laser absorption diagnostics for OH and CH3 radicals behind incident and reflected shock waves. tert-Butyl hydroperoxide and azomethane were used as pyrolytic sources for the OH and CH3 radicals, respectively. The current study bridged the temperature ranges of existing experimental data, and good agreement is seen between the current measurement and some previous experimental and theoretical high-temperature studies. A recommendation for the rate constant expression of the title reaction, based on the weighted average of the high-temperature data from selected studies, is given by k1 = 4.19 × 10(1)(T/K)(3.15) exp(5270 K/T) cm(3) mol(-1) s(-1) ±30%, which is valid over 1000-2500 K.

Shock-Tube Measurement of Acetone Dissociation Using Cavity-Enhanced Absorption Spectroscopy of CO.

A direct measurement for the rate constant of the acetone dissociation reaction (CH3COCH3 = CH3CO + CH3) was conducted behind reflected shock wave, utilizing a sub-ppm sensitivity CO diagnostic achieved by cavity-enhanced absorption spectroscopy (CEAS). The current experiment eliminated the influence from secondary reactions and temperature change by investigating the clean pyrolysis of <20 ppm acetone in argon. For the first time, the acetone dissociation rate constant (k1) was directly measured over 5.5 orders of magnitude with a high degree of accuracy: k1 (1004-1494 K, 1.6 atm) = 4.39 × 10(55) T(-11.394) exp(-52 140K/T) ± 24% s(-1). This result was seen to agree with most previous studies and has bridged the gap between their temperature and pressure conditions. The current work also served as an example demonstration of the potential of using the CEAS technique in shock-tube kinetics studies.

Reaction rate constant of CH2O + H = HCO + H2 revisited: a combined study of direct shock tube measurement and transition state theory calculation.

The rate constant of the H-abstraction reaction of formaldehyde (CH2O) by hydrogen atoms (H), CH2O + H = H2 + HCO, has been studied behind reflected shock waves with use of a sensitive mid-IR laser absorption diagnostic for CO, over temperatures of 1304-2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxane as the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintain relatively constant H atom concentrations, the current study significantly boosted the measurement sensitivity of the target reaction and suppressed the influence of interfering reactions. The measured CH2O + H rate constant can be expressed in modified Arrhenius from as kCH2O+H(1304-2006 K, 1 atm) = 1.97 × 10(11)(T/K)(1.06) exp(-3818 K/T) cm(3) mol(-1)s(-1), with uncertainty limits estimated to be +18%/-26%. A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12 level of theory, is in good agreement with the shock tube measurement and extended the temperature range of the current study to 200-3000 K, over which a modified Arrhenius fit of the rate constant can be expressed as kCH2O+H(200-3000 K) = 5.86 × 10(3)(T/K)(3.13) exp(-762 K/T) cm(3) mol(-1)s(-1).

Experimental and modeling study of the thermal decomposition of C3-C5 ethyl esters behind reflected shock waves.

The thermal decomposition of three ethyl esters, ethyl formate (C3H6O2), ethyl acetate (C4H8O2), and ethyl propanoate (C5H10O2), was studied behind reflected shock waves using laser absorption to measure concentration time-histories of H2O, CO2, and CO. Experimental conditions covered temperatures of 1301-1636 K, pressures of 1.48-1.72 atm, and reactant concentrations of 2000 ppm in argon. Recently developed mid-infrared laser diagnostics for H2O (2.5 µm), CO2 (4.3 µm), and CO (4.6 µm) provide orders-of-magnitude greater detectivity compared to previous near-infrared absorption sensors. The experimental results have highlighted significant differences among these three ethyl esters: negligible CO2 production during ethyl formate pyrolysis, quite slow CO formation rate during ethyl acetate pyrolysis, and nearly equal formation rate of H2O, CO2, and CO during ethyl propanoate pyrolysis. Detailed kinetic modeling was performed to understand the destruction pathways of these three ethyl esters with different alkyl chain lengths. Rate of production and sensitivity analyses were also carried out to interpret the experimental results and to identify the key reactions affecting experimental results.

High-temperature measurements of the reactions of OH with ethylamine and dimethylamine.

The overall rate constants of hydroxyl radicals (OH) with ethylamine (EA: CH3CH2NH2) and dimethylamine (DMA: CH3NHCH3) were investigated behind reflected shock waves using UV laser absorption of OH radicals near 306.7 nm. tert-Butyl hydroperoxide (TBHP) was used as the fast source of OH at elevated temperatures. Test gas mixtures of individual amines and TBHP, diluted in argon, were shock-heated to temperatures from 901 to 1368 K at pressures near 1.2 atm. The overall rate constants were determined by fitting the measured OH time-histories with the computed profiles using a detailed mechanism developed by Lucassen et al. (Combust. Flame 2012, 159, 2254-2279). Over the temperature range studied, the measured rate constants can be expressed as kEA+OH = 1.10 × 10(7)·T(1.93) exp(1450/T) cm(3) mol(-1) s(-1), and kDMA+OH = 2.26 × 10(4)·T(2.69) exp(1797/T) cm(3) mol(-1) s(-1). Detailed error analyses were performed to estimate the overall uncertainties of the measured reaction rate constants, and the estimated (2σ) uncertainties were found to be ±31% at 901 K and ±22% at 1368 K for kEA+OH, and ±29% at 925 K and ±21% at 1307 K for kDMA+OH. Variational transition state theory was used to compute the H-abstraction rates by OH for ethylamine and dimethylamine, with the potential energy surface, geometries, frequencies, and electronic energies calculated by Galano and Alvarez-Idaboy (J. Chem. Theory Comput. 2008, 4, 322-327) at CCSD(T)/6-311++G(2d,2p) level of theory. The calculated reaction rate constants are in good agreement with the experimental data.

High-temperature measurements of the reactions of OH with small methyl esters: methyl formate, methyl acetate, methyl propanoate, and methyl butanoate.

The overall rate constants for the reactions of hydroxyl radicals (OH) with four small methyl esters, namely methyl formate (CH(3)OCHO), methyl acetate (CH(3)OC(O)CH(3)), methyl propanoate (CH(3)OC(O)C(2)H(5)), and methyl butanoate (CH(3)OC(O)C(3)H(7)), were investigated behind reflected shock waves using UV laser absorption of OH radicals near 306.69 nm. Test gas mixtures of individual methyl esters and tert-butyl hydroperoxide (TBHP), a fast source of OH at elevated temperatures, diluted in argon were shock-heated to temperatures spanning from 876 to 1371 K at pressures near 1.5 atm. The overall rate constants were determined by matching the measured OH time-histories with the computed profiles from the comprehensive chemical kinetic mechanisms of Dooley et al. (2010) and Dooley et al. (2008), which were originally developed for the oxidation of methyl formate and methyl butanoate, respectively. These measured values can be expressed in Arrhenius form as k(CH(3)OCHO+OH) = 2.56 × 10(13) exp(-2026/T) cm(3) mol(-1) s(-1), k(CH(3)OC(O)CH(3)+OH) = 3.59 × 10(13) exp(-2438/T) cm(3) mol(-1) s(-1), k(CH(3)OC(O)C(2)H(5)+OH) = 6.65 × 10(13) exp(-2539/T) cm(3) mol(-1) s(-1), and k(CH(3)OC(O)C(3)H(7)+OH) = 1.13 × 10(14) exp(-2515/T) cm(3) mol(-1) s(-1) over the temperature ranges studied. Detailed error analyses were performed to estimate the overall uncertainties of these reactions, and the estimated (2σ) uncertainties were found to be ±29% at 913 K and ±18% at 1289 K for k(CH(3)OCHO+OH), ± 29% at 930 K and ±17% at 1299 K for k(CH(3)OC(O)CH(3)+OH), ± 25% at 909 K and ±17% at 1341 K for k(CH(3)OC(O)C2H(5)+OH), and ±24% at 925 K and ±16% at 1320 K for k(CH(3)OC(O)C(3)H(7)+OH). We believe these are the first direct high-temperature rate constant measurements for the reactions of OH with these small methyl esters. These measured rate constants were also compared with the estimated values employed in different comprehensive kinetic mechanisms. Additionally, the structure-activity relationship from Kwok and Atkinson (1995) was used to estimate these four rate constants, and the estimations from this group-additivity model are in good agreement with the measurements (within ~25%) at the present experimental conditions.

High-temperature measurements of the reactions of OH with a series of ketones: acetone, 2-butanone, 3-pentanone, and 2-pentanone.

The overall rate constants for the reactions of hydroxyl radicals (OH) with a series of ketones, namely, acetone (CH(3)COCH(3)), 2-butanone (C(2)H(5)COCH(3)), 3-pentanone (C(2)H(5)COC(2)H(5)), and 2-pentanone (C(3)H(7)COCH(3)), were studied behind reflected shock waves over the temperature range of 870-1360 K at pressures of 1-2 atm. OH radicals were produced by rapid thermal decomposition of the OH precursor tert-butyl hydroperoxide (TBHP) and were monitored by the narrow line width ring dye laser absorption of the well-characterized R(1)(5) line in the OH A-X (0, 0) band near 306.69 nm. The overall rate constants were inferred by comparing the measured OH time histories with the simulated profiles from the detailed mechanisms of Pichon et al. (2009) and Serinyel et al. (2010). These measured values can be expressed in Arrhenius form as k(CH3COCH3+OH) = 3.30 × 10(13) exp(-2437/T) cm(3) mol(-1) s(-1), k(C2H5COCH3+OH )= 6.35 × 10(13) exp(-2270/T) cm(3) mol(-1) s(-1), k(C2H5COC2H5+OH) = 9.29 × 10(13) exp(-2361/T) cm(3) mol(-1) s(-1), and k(C3H7COCH3+OH) = 7.06 × 10(13) exp(-2020/T) cm(3) mol(-1) s(-1). The measured rate constant for the acetone + OH reaction from the current study is consistent with three previous experimental studies from Bott and Cohen (1991), Vasudevan et al. (2005), and Srinivasan et al. (2007), within ±20%. Here, we also present the first direct high-temperature rate constant measurements of 2-butanone + OH, 3-pentanone + OH, and 2-pentanone + OH reactions. The measured values for the 2-butanone + OH reaction are in close accord with the theoretical calculation from Zhou et al. (2011), and the measured values for the 3-pentanone + OH reaction are in excellent agreement with the estimates (by analogy with the H-atom abstraction rate constants from alkanes) from Serinyel et al. Finally, the structure-activity relationship from Kwok and Atkinson (1995) was used to estimate these four rate constants, and the estimated values from this group-additivity model show good agreement with the measurements (within ~25%) at the present experimental conditions.

Reactions of OH with butene isomers: measurements of the overall rates and a theoretical study.

Reactions of hydroxyl (OH) radicals with 1-butene (k(1)), trans-2-butene (k(2)), and cis-2-butene (k(3)) were studied behind reflected shock waves over the temperature range 880-1341 K and at pressures near 2.2 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH(3))(3)-CO-OH, and monitored by narrow-line width ring dye laser absorption of the well-characterized R(1)(5) line of the OH A-X (0, 0) band near 306.7 nm. OH time histories were modeled using a comprehensive C(5) oxidation mechanism, and rate constants for the reaction of OH with butene isomers were extracted by matching modeled and measured OH concentration time histories. We present the first high-temperature measurement of OH + cis-2-butene and extend the temperature range of the only previous high-temperature study for both 1-butene and trans-2-butene. With the potential energy surface calculated using CCSD(T)/6-311++G(d,p)//QCISD/6-31G(d), the rate constants and branching fractions for the H-abstraction channels of the reaction of OH with 1-butene were calculated in the temperature range 300-1500 K. Corrections for variational and tunneling effects as well as hindered-rotation treatments were included. The calculations are in good agreement with current and previous experimental data and with a recent theoretical study.