High-Temperature Shock Tube and Modeling Studies on the

Article pubs.acs.org/JPCA

High-Temperature Shock Tube and Modeling Studies on the Reactions of Methanol with D‑Atoms and CH3‑Radicals S. L. Peukert and J. V. Michael* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: The shock tube technique has been used to study the hydrogen abstraction reactions D + CH3OH → CH2O + H + HD (A) and CH3 + CH3OH → CH2O + H + CH4 (B). For reaction A, the experiments span a T-range of 1016 K ≤ T ≤ 1325 K, at pressures 0.25 bar ≤ P ≤ 0.46 bar. The experiments on reaction B, CH3 + CH3OH, cover a T-range of 1138 K ≤ T ≤ 1270 K, at pressures around 0.40 bar. Reflected shock tube experiments, monitoring the depletion of D-atoms by applying Datom atomic resonance absorption spectrometry (ARAS), were performed on reaction A using gas mixtures of C2D5I and CH3OH in Kr bath gas. C2D5I was used as precursor for D-atoms. For reaction B, reflected shock tube experiments monitoring H-atom formation with H-ARAS, were carried out using gas mixtures of diacetyl ((CH3CO)2) and CH3OH in Kr bath gas. (CH3CO)2 was used as the source of CH3-radicals. Detailed reaction models were assembled to fit the D-atom and H-atom time profiles in order to obtain experimental rate constants for reactions A and B. Total rate constants from the present experiments on D + CH3OH and CH3 + CH3OH can be represented by the Arrhenius equations kA(T) = 1.51 × 10−10 exp(−3843 K/T) cm3 molecules−1 s−1 (1016 K ≤ T ≤ 1325 K) and kB(T) = 9.62 × 10−12 exp(−7477 K/T) cm3 molecules−1 s−1 (1138 K ≤ T ≤ 1270 K). The experimentally obtained rate constants were compared with available rate data from the literature. The results from quantum chemical studies on reaction A were found to be in good agreement with the present results. The present work represents the first direct experimental study on these bimolecular reactions at combustion temperatures and is important to the high-temperature oxidation of CH3OH.

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Truhlar13 have reported theoretical results for H-abstraction from CH3OH by H-atoms and CH3 radicals, yielding estimates of thermal rate constants, branching ratios, and kinetic isotope effects. In both studies rate constants were calculated by applying canonical variational transition state theory (CVT). To date, there are no direct experimental determinations of rate constants for the reaction of H-/D-atoms and CH3-radicals with CH3OH. Since these reactions will be important fuel destruction processes in methanol combustion, we have been motivated to carry out the present shock tube and modeling study on these reactions:

INTRODUCTION Since methanol (CH3OH) is an important alternative fuel in combustion, its pyrolysis has been extensively investigated.1−6 The most recent elementary kinetic investigations on CH3OH decomposition were conducted by Srinivasan et al., using a long absorption path multipass optical system for OH-radical detection coupled to a shock tube,7 and by Lu and co-workers,8 measuring the formation of H-atoms behind reflected shock waves with atomic resonance absorption spectrometry (ARAS). In contrast, the present work deals with bimolecular reactions of CH3OH. By measuring the depletion of OH-radicals and combining their own data with the results from previous evaluations on OH + CH3OH → H2CO + H2O + H, Srinivasan et al.7 derived a modified Arrhenius expression for this Habstraction reaction over a T-range 210−1710 K. Using conventional transition state theory (CTST), Jodkowski et al.9 derived theoretical rate constants for the hydrogen abstractions CH3/OH/H + CH3OH → CH4/H2O/ H2 + products. Optimized structures and rovibrational properties were obtained from ab initio calculations at the MP2/6-311G** level of theory. Carvalho et al.10 have also performed a quantum chemical study for the H + CH3OH abstraction reaction. Structures, barrier heights, and rate constants were calculated with DFT, MP2, and CCSD(T) methods coupled with variational transition state theory (VST). Theoretical rate constants for the two H-abstraction channels in the H + CH3OH reaction have also been reported by Moses et al.11 Most recently, Meana-Pañeda et al.12 and Alecu and © 2013 American Chemical Society

D + CH3OH → HD + CH 2O + H

(1)

CH3 + CH3OH → CH4 + CH 2O + H

(2)

For both reactions, 1 and 2, there are two possible channels: The H-abstraction can take place from the methyl- or from the hydroxyl-site. In each case, both channels result in the formation of H and CH2O. Use of H in 1 instead of D gives no net loss of H since H is a product. However, using D as a surrogate, rate constants can be determined by measuring the depletion of D-atoms with the sensitive ARAS method. Total rate constants can then be derived for reaction 1. On the other hand, measuring the formation of H-atoms, resulting from Received: June 14, 2013 Revised: August 7, 2013 Published: August 22, 2013 10186

dx.doi.org/10.1021/jp4059005 | J. Phys. Chem. A 2013, 117, 10186−10195

The Journal of Physical Chemistry A

Article

Figure 1. Measured photomultiplier signal for a D + CH3OH experiment at T5 = 1116 K, P5 = 0.37 bar, [CH3OH]0 = 7.38 × 1014 cm−3, and [C2D5I]0 = 3.89 × 1012 cm−3. (a) Photomultiplier signal that is not corrected for the fraction of nonresonant radiation. (b) Photomultiplier signal corrected for the fraction of nonresonant radiation. For this experiment, in the reflected shock wave regime, I0 is 22.51 mV.

LC334A oscilloscope. For H-atom detection, the microwave driven resonance lamp was operated at 35 W and 1.5 Torr of research-grade He (99.9999%) (effective Doppler temperature: 470 K).23 Due to lamp gas hydrogeneous impurities in research grade He (even cooled with liquid N2), LyαH radiation is emitted from the lamp along with a low percent of radiation that is extraneous (nonresonant). In order to measure the fraction of nonresonant radiation present in the lamp, an H2 discharge flow system, an atom filter, is used to create large [H] (∼1 × 1014 atoms cm−3) between the lamp and shock tube window19,23−25 thereby removing all of the LyαH in the emission lamp. The path length of the atomic filter section is 3 cm. It can be shown using line absorption theory19,23,26 that 3 cm of [H] = 1 × 1014 atoms cm−3 at room temperature will remove 99.6% of LyαH. The fraction of nonresonant emission is ∼10 - 15%. This fraction is subtracted from the measured photomultiplier-signal, meaning that 85−90% of the measured signal-intensity is LyαH radiation. D-Atom ARAS Detection. D-atom ARAS detection was used to follow [D]t, the absolute D-atom concentration as a function of time, quantitatively. The experimental setup is the same as described for H-atom ARAS. As previously mentioned, due to lamp gas hydrogeneous impurities in research grade He, LyαH radiation is emitted from the lamp along with a low percent of nonresonant radiation. In order to determine this nonresonant fraction of the emitted light, the atom filter is used again. Since there is a wavelength difference between LyαH and LyαD of 0.033 nm, D can be detected in the presence of H by performing experiments with the H2 discharge system turned on. As with H-ARAS, the fraction of nonresonant light is ∼15%, meaning that 85% of the measured photomultiplier-signal in the D-ARAS experiments is LyαD radiation. The D-ARAS experiments require metering very small amounts of D2 into the resonance lamp such that the lamp intensity is similar to that for the H lamp. This ensures that the D-atom concentration will be very low in the lamp, and that the lamp will be effectively unreversed, i.e., a completely defined LadenburgReiche Gaussian line shape.19,26 In this case, D-atoms in the presence of H-atoms can be directly detected by carrying out the experiment with the H2 discharge flow system turned on (i.e., removing LyαH) during the D-atom experiment. Gases. High purity He (99.995%), used as the driver gas, was from AGA Gases. Research grade Kr (99.999%), the diluent gas in reactant mixtures, was from Praxair, Inc. The ∼10 ppm impurities (N2 < 5 ppm, O2 < 2 ppm, Ar < 1 ppm, CO2 < 0.5 ppm, H2 < 1 ppm, H2O < 3 ppm, Xe < 2 ppm, and THC < 0.2 ppm) are all either inert or in sufficiently low concentration so as to not perturb H- or D-atom profiles. H2 (research grade)

reaction 2, makes it possible to derive total rate constants for the H-abstraction process 2. Deuterated iodo-ethane, C2D5I, was used as precursor compound for D-atoms, whereas diacetyl (butane-2,3-dione), (CH3CO)2, was used as a precursor compound for CH3-radicals. With respect to the experiments on CH3 + CH3OH, the present study is a new implementation of the H-ARAS technique for measuring rate constants of a “slow” reaction (k ∼ 10−14 cm3 molecules−1 s−1).

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EXPERIMENTAL SECTION The present experiments, in Kr diluent, were performed with the reflected shock tube technique using H- and D-atom ARAS detection. The methods and the apparatus currently being used have been previously described.14,15 The shock-tube was constructed entirely from a 7-m (9.74 cm i.d.) 304 stainless steel tube with the cylindrical section being separated from the He driver chamber by a 4 mil unscored 1100-H18 aluminum diaphragm. The tube was routinely pumped between experiments to less than 1.3 × 10−11 bar by an Edwards Vacuum Products Model CR100P packaged pumping system. Shock-wave velocities were measured with eight equally spaced pressure transducers (PCB Piezotronics, Inc., Model 113A21) mounted along the downstream part of the test section and recorded with the LeCroy model LC334A oscilloscope. Since there is no appreciable attenuation of the shock wave in this shock tube, the velocity of the incident shock wave is calculated as the average over 7 time intervals, with standard deviations of about ±0.3 to 0.7%. In order to calculate temperature and density in the reflected shock-wave regime, the preshock-conditions (T and p) as well as the speed of the incident shock wave are required. This procedure has been given previously, and corrections for boundary layer perturbations have been applied.16−18 The oscilloscope was triggered by a pulse derived from the last velocity gauge signal. The photometer systems were radially located at 6 cm from the end plate. H-Atom ARAS Detection. H-atom ARAS detection was used to quantitatively follow [H]t, the absolute H-atom concentration as a function of time. The optical components (windows and lenses) were crystalline MgF2, and the resonance lamp beam intensity (filtered through 4 cm of air (21% O2) at 1 atm to isolate the Lyman-αH (LyαH) wavelength at 121.6 nm) was measured by a Hamamatsu R8487 solar blind photomultiplier tube, as described previously.19−22 The atmospheric O2 filter serves as a monochromator since there is a narrow region of high transmittance in the O2 absorption spectrum at 121.6 nm. Signals were recorded with the LeCroy model 10187



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Figure 2. (a) [D]t profile for an experiment at T5 = 1116 K and P5 = 0.37 bar. (b) Local D-atom sensitivity analysis for this experiment using the reaction model in Table 1 and the modeled rate constant for k1 (k1 = k1a + k1b). The normalized D-atom-sensitivity is defined as S = (dXD/dki) × (ki/ XD,local).

The solid curve in the figure is the simulated profile using the reaction mechanism provided in Table 1. The reaction model includes the major thermal decomposition channels of CH3OH, bimolecular reactions of H/D with CH3OH, decomposition of the D-atom precursor, C2D5I (characterized in an earlier study), as well as secondary chemistry (e.g., the CH3 + CH3 self-reactions, CH3 + OH/O2/ D/O etc.). Figure 2b shows a sensitivity analysis for the D-atom profile shown in 2a. There are only three reactions showing significant D-atom sensitivity: The two initial C2D5I decomposition reactions, and the overall abstraction reaction 1, D + CH3OH → CH2O + HD + H. Best fits of simulated profiles to the experimental data were obtained by adjusting the rate constant, k1, for each experiment. In the fitting procedure, the total rate constants for C2D5I decomposition were varied by up to ∼ ± 30%, and the initial concentration of C2D5I, by ±5%. In the temperature range of the present experiments, the branching ratio, between the dominant C−I bond dissociation and total dissociation including the elimination of DI, is ∼0.62.27 The experimental conditions and the best fit rate constants k1 are summarized in Table 2. To the best of our knowledge, these are the first experimentally determined hightemperature total rate constants for D + CH3OH. There are prior quantum chemical studies,9−12 as well as rate constant recommendations and estimations28,29 on k1. The experimental results from the present work and the theoretical9−12 and estimated rate constants,28,29 including the rate constant recommendation from Tsang, are shown in Figure 3. The values reported by Vandooren and Van Tiggelen29 show the greatest disagreement with the present experimentally measured rate constants. Vandooren and Van Tiggelen investigated methanol oxidation in lean premixed methanol flames using molecular beam sampling coupled with mass spectrometric analysis. Due to the complexity of the chemistry in these flames, it is not possible to isolate single elementary reactions, and therefore rate constants had to be estimated. However, with the estimation from ref 29 and the results from methanol oxidation studies at lower temperatures, a rate constant recommendation for reaction 1 was determined by Tsang.28 The overall agreement is satisfactory, since the rate constant values from ref 28 deviate on average by about a factor of 1.8 from the present experimental results. Considering the results from the available quantum chemical studies, the theoretical rate constants obtained from a variety of ab initio TST calculations agree well with the present experimental rate constants; i.e., within ±35%. Figure 3

was obtained from Airgas, and D2 was from AGA. D2 was 99.5% isotopically pure. The following compounds were used in this study: iodoethane-d5 (C2D5I) with a purity ∼99.7 D-atom % (CDN Isotopes), CH3OH (Purity: ≥ 99.0%, Sigma Aldrich), and diacetyl ((CH3CO)2, 97% Purity; Sigma-Aldrich). All compounds were further purified by bulb-to-bulb distillation, retaining only middle thirds for mixture preparation. Gas mixtures of CH 3 OH/C 2 D 5 I/Kr and CH 3 OH/ (CH3CO)2/Kr were accurately prepared from pressure measurements using a Baratron capacitance manometer in an all glass high-purity vacuum line.

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RESULTS AND DISCUSSION D + CH3OH. Figure 1 shows a measured photomultipliersignal, obtained from the absorption of LyαD radiation from the D-atom lamp, in an experiment on D + CH3OH at T = 1116 K with [CH3OH]0 = 7.38 × 1014 cm−3 and [C2D5I]0 = 3.89 × 1012 cm−3. The signal in panel ‘a’ is not corrected for the fraction of nonresonant radiation. Since CH3OH absorbs some of the resonance radiation at λ ∼ 121.6 nm in the initial, incident, and reflected shock wave regimes, one can observe two jumps in the signal-intensity. The stepwise changes in signal-intensity are caused by the arrival of the incident and reflected shock waves at the photometer position. The signal before the arrival of the incident shock wave is ∼32.40 mV and is indicated by the top solid line in panel ‘a’. The level of signal resulting from the arrival of the incident shock wave is shown as the dotted line in Figure 1a. The signal intensity of the first step is 31.15 mV. We determine the nonresonant light-fraction of the LyαHlamp to be 5.20 mV. In separate experiments we have shown that D-atoms do not absorb the nonresonant radiation. Hence the nonresonant radiation has to be subtracted from all signal values over the whole time range of the experiment. Subtraction of 5.20 mV results in the corrected photomultiplier signal shown in Figure 1b. Since almost no CH3OH is destroyed in the experiment, the I0 was taken to be the signal at long times (t ≥ 1.5 ms): I0 will be S∞ - background. This gives I0 = 22.51 mV for this particular experiment. The solid line at the right side of Figure 1b indicates the signal corresponding to I0. The absolute value of absorbance is then ABS = ln(I0/It), where It = St − background. (ABS)t is then transformed to [D]t with line absorption calculations using the known oscillator strength for the LyαD transition. Figure 2a shows a D-atom profile derived from the experiment presented in Figure 1. 10188



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Table 1. Reaction Model Used to Simulate D and CH3 + CH3OH Experiments C2D5I → C2D4 + D + I C2D5I → C2D4 + DI H + C2D4 → C2D3H + D D + C2H4 → C2H3D + H CH3 + D → CH2D + H H + DI → DH + I (CH3CO)2 (+M) → CH3CO + CH3CO (+M)

CH3CO → CH3 + CO (CH3CO)2 + H → CH3CO + CH2CO + H2 (CH3CO)2 + CH3 → CH3CO + CH2CO + CH4 CH3OH (+M) → CH3 + OH (+M)

CH3OH (+M) → CH2(S)+H2O (+M) CH3OH (+M) → 2H + CH2O (+M) CH3OH + D → CH2O + HD + H CH3OH + CH3 → CH2O + CH4 + H CH3OH + OH → CH2O + H2O + H C2H6 (+M) = CH3 + CH3 (+M) CH3 + CH3 → C2H4 + 2H CH3 + H (+M) = CH4 (+M)

CH2O + Kr → HCO + H +Kr CH2O + Kr → H2 + CO + Kr HCO + Kr → H + CO + Kr CH2O + OH → H2O + HCO CH2O + O → OH + HCO H2O + Kr → OH + H + Kr H + O2 → OH + O OH + O → H + O2 H2 + O = OH + H CH3 + O2 → H2CO + O + H CH3 + O2 → H2CO + OH CH3 + OH = CH2(S) + H2O CH2(S) + Kr = CH2 + Kr CH2 + OH = H2CO + H CH2 + CH2 = C2H2 + 2H CH2 + CH3 = C2H4 + H CH2 + H = CH + H2 CH3 + O = H2 + CO + H C2H6 + OH → C2H4 + H + H2O C2H6 + H → C2H4 + H + H2 a

k = 2.49 × 1010 exp(−17729 K/T) × (0.3037 + (2.744 × 10−4)T) k = 2.49 × 1010 exp(−17729 K/T) × (0.6963 − (2.744 × 10−4)T) k = 3.48 × 10−10 exp(−2784 K/T) k = 2.56 × 10−10 exp(−2797 K/T) k = 2.21 × 10−10 k = 9.30 × 10−11 exp(−59 K/T) k∞ = 1.30 × 1028 T −3.40 exp(−37663 K/T) s−1 k0 = 1.32 × 1047 T −9.17 exp(−26083 K/T) cm3molecule−1s−1 α = 0.52/T*** = 3074/T* = 496/T** = 4100 k = 3.16 × 1016 T−2.09 exp(−7649 K/T) k = 1.52 × 10−16 T2.00 exp(−2516 K/T) k = 4.68 × 10−24 T4.00 exp(−3976 K/T) k∞ = 3.45 × 10−6 T −0.61 exp(−46571 K/T) s−1 k0 = 2.44 × 1024 T −8.44 exp(−50555 K/T) cm3 molecule−1 s−1 α = 1.402/T*** = 1157/T* = 5.34 × 1024/T** = 1.00 × 106 k∞ = 5.18 × 10−6 T −1.02 exp(−46154 K/T) k0 = 1.01 × 1023 T −8.23 exp(−48951 K/T) α = 1.846/T*** = 1714.03/T* = −7.90 × 1021/T** = 1.00 × 106 k∞ = 1.31 × 10−26 T 5.08 exp(−42510 K/T) k0 = 5.51 × 1018 T −7.24 exp(− 52954 K/T) α = 2.72 × 10−7/T*** = 3980.00/T* = 4155.00/T** = 37714 k = 1.51 × 10−10 exp(−3843 K/T) k = 9.62 × 10−12 exp(−7477 K/T) k = 2.96 × 10−16 T1.44 exp(−57 K/T) k∞ = 1.33 × 105 T −3.52 exp(−47983 K/T) k0 = 4.65 × 1048 T −15.10 exp(− 54222 K/T) α = 0.21/T*** = 1.00 × 10−30/T* = 1.00 × 1030 k = 5.25 × 10−11 exp(−7384 K/T) k∞ = 1.16 × 10−10 T0.19 exp(−T/25200 K) cm3 molecule−1 s−1 k0 = [M] 10−26.19 exp[(−T/21.22 K)0.5] cm6 molecule−2 s−1 Fc = 0.262 + [(T − 2950 K)/6100 K]2 F(x) = 1 − (1 − Fc) exp(−[log(1.5x)/N]2/N*)a k = 1.02 × 10−8 exp(−38706 K/T) k = 4.66 × 10−9 exp(−32110 K/T) k = 6.00 × 10−11 exp(−7722 K/T) k = 5.69 × 10−15 T1.18 exp(− 225 K/T) k = 6.92 × 10−13 T0.57 exp(− 1390 K/T) k = 2.43 × 10−10 exp(−47117 K/T) k = 2.79 × 10−10 exp(−8120 K/T) k = 2.05 × 10−11 k = 3.10 × 10−10 exp(−13620 K/T) k = 4.47 × 10−12 exp(−12573 K/T) k = 1.06 × 10−12 exp(−6802 K/T) k = 1.15 × 10−9 × T −0.49 k = 4.00 × 10−14 T 0.93 k = 1.11 × 10−10 T 0.017 exp(−9 K/T) k = 2.39 × 10−10 T 0.025 exp(−17 K/T) k = 1.89 × 10−10 T −0.13 exp(−8 K/T) k = 1.99 × 10−10 k = 1.40 × 10−10 k = 2.67 × 10−18 T2.22 exp(−373 K/T) k = 9.90 × 10−10 exp(−6475 K/T)

ref 27 ref 27 refs 25, 33 refs 25, 33 ref 27 ref 27 ref 31

ref ref ref ref

31 31 31 34

ref 34 ref 34 this work this work ref 7 ref 35 ref 21 ref 36

ref 7 ref 7 ref 7 ref 7 ref 7 ref 37 ref 38 ref 38 ref 39 ref 40 ref 6 refs 41, 42 ref 7 ref 7 ref 7 ref 7 ref 43 ref 44 ref 45

x = k0/k∞; N = 0.75−1.27 × log(1.5x); N* = 2 for log(1.5x) > 0; N* = 2 × [1 − 0.15 log(1.5x)] for log(1.5x) < 0.

a suitable surrogate for H + CH3OH at combustion temperatures. Since H-abstraction can take place from either the CH3- or OH-sites, there are two abstraction channels

shows that the best agreement is with the results of the CCSD(T) and VST calculations from Carvalho et al.10 These workers also carried out B3LYP and MP2 calculations, but they are not shown in Figure 3 since these levels of calculation result in rate constants that are almost the same as those from Jodkowski et al.,9 Moses et al.,11 and Meana-Pañeda et al.12 In the Meana-Pañeda et al. study, overall rate constants for D + CH3OH are reported. According to their theory, the kinetic isotope effect in the temperature range of the present shock wave experiments is 5%, kD/kH ∼ 1.05. This confirms that the elementary kinetic investigation on D + CH3OH can be used as

D + CH3OH → HD + CH 2OH (ΔHr 0 = −8.69 kcal/mol)

(1a)

D + CH3OH → HD + CH3O (ΔHr 0 = −0.39 kcal/mol) 10189

(1b)



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constant, k1a + k1b, for reaction 1. Therefore, experimental branching ratios cannot be derived. However, the theoretical studies9−12 report branching ratios for channels 1c and 1d, and Figure 4 shows the theoretical branching ratios over the temperature range 1000−1400 K.

Table 2. Summary of Experimental Conditions for D + CH3OH Experiments P1 / Torra

Ms

ρ5 / (1018 cm−3)b −4

15.95 15.83 15.87 15.76 15.74 15.88 15.89 15.81 15.91 10.89 10.88 10.88 10.89 10.85 10.83 10.87 10.84 10.99

T5 / K

k1 / cm3 molecule−1 s−1c

−4

X(CH3OH) = 3.05 × 10 × 10 /X(C2D5I) = 1.61 × 10−6 16 Torr Experiments 2.043 2.432 1100 4.61 × 10−12 2.156 2.575 1208 5.31 × 10−12 2.193 2.633 1245 6.64 × 10−12 2.090 2.471 1144 6.48 × 10−12 2.059 2.423 1116 5.31 × 10−12 2.062 2.449 1117 4.32 × 10−12 1.952 2.283 1016 3.32 × 10−12 2.042 2.400 1102 4.48 × 10−12 2.025 2.391 1087 3.65 × 10−12 10 Torr Experiments 2.116 1.722 1164 4.90 × 10−12 2.203 1.808 1252 7.97 × 10−12 2.274 1.875 1325 8.47 × 10−12 2.180 1.781 1232 6.48 × 10−12 2.209 1.802 1262 6.48 × 10−12 2.061 1.650 1114 5.31 × 10−12 2.066 1.667 1116 5.31 × 10−12 2.097 1.695 1146 5.98 × 10−12 2.138 1.761 1186 6.14 × 10−12

Figure 4. Branching ratios based on theoretical rate constant predictions for H-abstraction channels 1c and 1d.9−12

The branching ratio is defined as BR1c = k1c/ktotal, with ktotal = k1c + k1d. These theoretical branching ratios agree well with each other, and the four branching ratios can be averaged and then described by a second-order polynomial equation, giving the T-dependent expression

a

1 Torr corresponds to 133.32 Pascal. bQuantities with the subscript 5 refer to the thermodynamic state of the gas in the reflected shock wave region. cThe average uncertainty of k1 is estimated to be ±21%. k1 = k1a + k1b (see text). There are prior quantum chemical studies,9−12 as well as rate constant recommendations and estimations28,29 on k1.

BR1c = 1.062 + (− 1.158 × 10−4T ) + 1.472 × 10−8T 2 (E1)

Over the T-range 1016−1325 K, the temperature dependence of the experimentally derived rate constants in Table 2 can be described by the Arrhenius expression: k1(T ) = 1.51 × 10−10 exp( − 3843 K/T ) cm 3 molecules−1 s−1

(E2)

The values from Table 2 are within ±21%, at the one standard deviation level, of those calculated from eq E2. The rate constants for k1a ∼ k1c and k1b ∼ k1d can then individually be determined by using eqs E1 and E2. The resulting values are fitted to a modified Arrhenius expression, k = A × Tn × exp(−Ea/T) k1a(T ) = 5.04 × 10−10T −0.169 exp( − 3923 K/T )

Figure 3. Experimentally obtained rate constants for D + CH3OH: ▲ Experiments at P5 ∼ 0.37 bar; ○ experiments at P5 ∼ 0.29 bar. Theoretical rate constants: (dash-dot-dot: − • • −) Vandooren and Van Tiggelen;29 (short dotted: ····) review: Tsang;28 (dotted: •••) Carvalho et al.;10 (solid curve: ) Moses et al.;11 (short dashed: ----) Meana-Pañeda et al.;12 (dashed: − − −) Jodkowski et al.9

cm 3 molecules−1 s−1 k1b(T ) = 1.24 × 10−9T −0.401 exp( − 6428 K/T )

(E4) cm 3 molecules−1 s−1 The site specific H-abstraction reactions described by E3 and E4 are plotted in Figure 5a,b, respectively. As shown in Figure 3, the best agreement between theoretical ktotal predictions and the present modeled rate constants for D + CH3OH, is with the results of Carvalho et al.10 Over the T-range 1000−2000 K, the ab initio TST calculations from Carvalho et al. can be described with modified Arrhenius-expressions

H + CH3OH → H 2 + CH 2OH (ΔHr 0 = −8.12 kcal/mol) H + CH3OH → H 2 + CH3O

(E3)

(1c)

(ΔHr 0 = 0.96 kcal/mol) (1d)

The reaction enthalpies are calculated with enthalpies of formation taken from the thermodynamic database of Goos, Burcat, and Ruscic.30 The present D-ARAS experiments do not permit differentiation between the two reaction channels, 1a and 1b. The measured D-atom depletion only depends on the total rate constant for D + CH3OH. No matter what branching ratio, BR1a = k1a/(k1a + k1b), is assumed, the modeling of the measured D-atom profiles results in the same total rate

k1a(T ) = 9.06 × 10−19T −2.496 exp( − 2496 K/T ) cm 3 molecules−1 s−1

(E5)

k1b(T ) = 5.02 × 10−18T −2.119 exp( −5052 K/T ) cm 3 molecules−1 s−1 10190

(E6)



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Figure 5. Comparison between rate constants of individual H-abstraction channels: (a) Channel 1c: [] This work, E3: Based on average branching ratio ⟨BR1c⟩ from theory applied to measured total rate constants for D + CH3OH assuming k1c ∼ k1a; [− −] Carvalho et al.,10 E5; (b) Channel 1d: [] This work, E4: Based on average branching ratio ⟨BR1d⟩ = 1 − ⟨BR1c⟩ applied to measured total rate constants for D + CH3OH assuming k1d ∼ k1b; [− −] Carvalho et al.,10 E6.

Figure 6. (a) Measured photomultiplier signal for a CH3 + CH3OH experiment at T5 = 1219 K, and P5 = 0.44 bar, [CH3OH]0 = 5.41 × 1014 cm−3 and [(CH3CO)2]0 = 1.30 × 1013 cm−3. For this experiment, the signal intensity in the incident shock wave regime is 15.12 mV, and in the reflected shock wave regime, I0 is 13.87 mV. (b) [H]t profile for this experiment. The solid curve [] refers to a simulation using the Table 1 mechanism.

analysis, pressure- and T-dependent rate constants were derived. As an alternative to observing the depletion of CH3-radicals caused by reaction 2, in this work the formation of [H]t was measured. Since the detection limit is ∼5 × 1010 atoms cm−3, with H-ARAS, experiments can be designed that inhibit complications arising from secondary reactions. In contrast, the detection limit of experiments using a CH3 multipass absorption technique is approximately 5 × 1013 radicals cm−3.32 Therefore detecting CH3-radical depletion in order to investigate reaction 2 would require gas mixtures having substantially higher reactant concentrations resulting in an increased influence of secondary chemistry. Methyl−methyl recombination then becomes the most sensitive reaction under the conditions of the present experiments. Therefore, measuring H-atom formation from reaction 2, using H-ARAS, makes it possible to minimize secondary chemistry and to chemically isolate k2. A photomultiplier-signal from the absorption of LyαH in a CH3 + CH3OH experiment at T = 1219 K is shown in Figure 6a. The initial reactant concentrations are [(CH3CO)2]0 = 1.30 × 1013 cm−3 and [CH3OH]0 = 5.41 × 1013 cm−3. Figure 6a is already corrected for nonresonant light. The signal shows jumps (as in Figure 2) that are caused by reactant absorption. The stepwise changes in signal-intensity again are caused by the arrival of the incident and reflected shock wave at the photometer position. The I0 was taken to be the signal intensity of the second step in the photomultiplier signal. The level of signal resulting from the arrival of the incident shock wave is shown as the dashed line in Figure 6a. The signal

Equations E5 and E6 are plotted in Figure 5a,b, showing that after extracting individual rate constants for 1c and 1d based on the average of theoretical branching ratios and the present experimental results on k1, the calculations from Carvalho et al. give the best agreement with the present results for the major H-abstraction channel 1c. The theoretical rate constants from Carvalho et al. for 1c, panel ‘a’, deviate by less than +15% and −10% from the experimentally based values. For reaction 1d the deviation is larger as seen in panel ‘b’. Over the T-range of the present shock tube experiments, the theoretical rate constants from Carvalho et al. are on average 25% larger than the present rate constants, k1b. According to theory, reaction 1d is the minor channel with an average branching ratio of ∼0.06 in the T-range of the present experiments. Therefore this deviation is of no practical relevance. Therefore, our conclusion is that in the T-range of 1000−1400 K, both sets of rate constants, eqs E3 and E4 as well as eqs E5 and E6, can be used in detailed chemical kinetic modeling of methanol combustion equally well. CH3 + CH3OH. For this reaction, diacetyl ((CH3CO)2) was used as a source for CH3-radicals. Dissociation results in the formation of two acetyl radicals (CH3CO), that then rapidly decompose to CH3 and CO: (CH3CO)2 → 2CH3CO

(3)

CH3CO → CH3 + CO

(4)

The thermal dissociation of diacetyl has been characterized by Yang et al.31 behind incident shock waves with laser schlieren densitometry. Using an ab initio/Master Equation 10191



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intensity of the first step is 15.12 mV. The solid line indicates the signal-intensity caused by reactant absorption in the reflected shock wave regime. This gives I0 = 13.87 mV for this particular experiment. With I0 known for each experiment, (ABS)t = ln(I0/It) for H was calculated and then transformed to [H]t with line absorption calculations with the known oscillator strength for the LyαH transition. Figure 6b shows the H-atom profile for this experiment at T = 1219 K. The solid curve represents the simulated H-profile using the Table 1 reaction mechanism. The reaction model includes the dissociation of the CH3 precursor, (CH3CO)2, the reaction between CH3-radicals and CH3OH. The remaining steps in the reaction model are identical with the mechanism used for modeling the D + CH3OH experiments (CH3OH decomposition channels and secondary chemistry). Best fit Hatom profiles were obtained by only adjusting the rate constant k2 for each experiment with all other rate constants taken as known. Figure 7 shows a brute force sensitivity check, varying

Table 3. Summary of Experimental Conditions for CH3 + CH3OH Experiments P1 / Torra

Ms

ρ5 / (1018 cm−3)b

T5 / K

k2 / cm3 molecule−1 s−1c

−4

15.82 15.90 15.97 15.87 15.89 15.87 15.84 15.78 15.80 15.97

X(CH3OH) = 2.06 × 10 /X(CH3CO)2 = 4.96 × 10−6 2.087 2.365 1138 1.24 × 10−14 2.191 2.643 1238 2.82 × 10−14 2.171 2.628 1219 1.99 × 10−14 2.222 2.680 1270 2.82 × 10−14 2.191 2.641 1238 1.66 × 10−14 2.088 2.485 1142 9.96 × 10−15 2.111 2.514 1165 1.33 × 10−14 2.109 2.509 1158 1.66 × 10−14 2.113 2.518 1162 1.74 × 10−14 2.108 2.538 1157 2.41 × 10−14

a

1 Torr corresponds to 133.32 Pascal. bQuantities with the subscript 5 refer to the thermodynamic state of the gas in the reflected shock wave region. cThe average uncertainty of k2 is estimated to be ±40%. k2 = k2a + k2b (see text).

However, prior quantum chemical studies have been reported on this H-abstraction9,13 by CH3. The present experimental values for k2 as well as theoretical and estimated rate constants, and a rate constant evaluation from Tsang,28 are presented in Figure 9.

Figure 7. Comparison between measured and calculated [H]t-profiles. Brute force sensitivity analysis for reaction 2. Profiles were calculated with the reaction model presented in Table 1. []: Best fit simulation obtained by adjusting only k2; [••••] k2 × 1.30; [− − −]: k2 × 0.7.

k2 by ±30%. H-atom formation clearly depends on k2 as seen in Figure 8 where the local H-atom sensitivity analysis for the 1219 K profile is shown. Figure 9. ▲ Experimental rate constants (P5 ∼ 0.40 bar). (solid curve: ) 2-parameter Arrhenius fit: This work, E8. (short dotted: ••••) Spindler and Wagner;2 (dash-dot-dot: − • • −) Jodkowski et al.;9 (short dashed: − − − −) Alecu and Truhlar;13 (dash-dot: − • −) Tsang review.28

Jodkowski et al.9 carried out ab initio TST calculations on reaction 2. Whereas their MP2 calculations result in good agreement with the present data on D + CH3OH, their MP2/ cTST calculations on CH3 + CH3OH give values ∼4 − 5 times larger than the present experimental determination. Alecu and Truhlar13 used the M08-HX method for their electronic structure calculations. With the M08-HX based molecular properties, they performed kinetic calculations by applying the method of multistructural canonical variational-transition-state theory with multidimensional tunneling (MS-CVT/MT). Compared to the results from Jodkowski et al., the values of Alecu and Truhlar are much closer to the present k2-results. The theoretical rate constants from Alecu and Truhlar are only ∼80% larger than the experiments. Spindler and Wagner conducted an earlier less direct shock tube study on CH3OH dissociation by measuring OH- and CH3-radical formation behind reflected shock waves. They used UV−vis absorption spectrometric detection.2 Under their experimental conditions, the reaction CH3 + CH3OH exhibited

Figure 8. Local H-atom sensitivity analysis for the experiment at T5 = 1219 K using the Table 1 reaction model and the modeled rate constant for k2 (k2 = k2a + k2b). The normalized H-atom-sensitivity is defined as S = (dXH/dki) × (ki/XH,local).

Even though the CH3−CH3 self-reactions show some sensitivity, the H-abstraction reaction, 2, clearly exhibits the largest H-atom sensitivity over the whole time period of 2 ms. The experimental conditions and the best fit rate constants k2 are summarized in Table 3. To the best of our knowledge, these are the first directly determined high temperature experimental total rate constants for CH3 + CH3OH. 10192



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Figure 10. Comparison between rate constants of individual H-abstraction channels: (a) Channel 2a: [] This work, E9: Based on average branching ratio ⟨BR2a⟩ from theory applied to measured total rate constants for CH3 + CH3OH; [− • • − ] Jodkowski et al.;9 [− −] Alecu and Truhlar;13 [••••] Tsang.28 (b) Channel 2b: [] This work, E10: Based on average branching ratio ⟨BR2b⟩ = 1 − ⟨BR2a⟩ applied to measured total rate constants for CH3 + CH3OH; [− • • −] Jodkowski et al.;9 [− −] Alecu and Truhlar;13 [••••] Tsang.28

be determined by combining eqs E7 and E8. Over the T-range 1138−1270 K, 3-parameter fits were obtained for reactions 2a and 2b.

only minor sensitivity. In their reaction mechanism, they did have to supply an estimate for k2. As can be seen in Figure 9, this estimation gives values ∼10 times higher than the present experimental data. The rate constant recommendation from Tsang,28 however, is in excellent agreement with the present experimental values being on average ∼25% larger than the present experimental results. By analogy to D + CH3OH, there are again two abstraction channels for CH3 + CH3OH:

k 2a(T ) = 8.14 × 10−12T 0.013 exp(− 7589 K/T ) cm 3 molecules−1 s−1 k 2b(T ) = 3.84 × 10−11T −0.475 exp( − 7345 K/T ) cm 3 molecules−1 s−1

CH3 + CH3OH → CH4 + CH 2OH (ΔHr 0 = −8.92 kcal/mol)

CH3 + CH3OH → CH4 + CH3O (2b)

Reaction enthalpies are calculated from standard enthalpies of formation given in the thermodynamic database by Goos, Burcat, and Ruscic.30 As in the case of D + CH3OH, the present H-ARAS experiments cannot be used to extract branching ratios between these two channels. Irrespective of the branching ratio between 2a and 2b that is used in modeling, H-atom formation is sensitive only to the total rate constant, ktotal, where ktotal = k2a + k2b. The two theoretical studies9,13 on CH3 + CH3OH report the branching ratio BR2a = k2a/ktotal, and both studies nearly agree with each other. Over the T-range, 1000−1400 K, Jodkowski et al.’ s branching ratio, BR2a, ranged from 80% to 83%. Over the same T-range, Alecu and Truhlar obtained BR2a ranging from 85% to 88%. We have averaged these two sets, and the temperature dependence of this averaged BR2a can be described by a second-order polynomial:

■

CONCLUSION The shock tube technique was used to measure total rate constants for two H-abstraction reactions that are important in the high temperature oxidation of methanol, D + CH3OH → CH2O + H + HD (1) and CH3 + CH3OH → CH4 + CH2O + H (2). With D-ARAS, [D]t depletion was measured to study reaction 1, but reaction 2 was measured by observing H-atom formation. Using D- and H-atom ARAS, experiments can be carried out under chemically isolated conditions thereby minimizing the influence of secondary chemistry. The experimental rate constants for 1 and 2 are compared to ab initio based theoretical rate constants from already published literature sources. Theory and experiment are in good agreement. With respect to reaction 2, only two sets of ab initio based rate constants are available and among these two

−4

BR 2a = 0.671 + (2.163 × 10 T ) + ( −5.843 × 10−8T 2)

(E7)

Over the T-range 1138−1270 K, the temperature dependence of the experimental rate constants in Table 3 can be described by the following expression: k 2(T ) = 9.62 × 10−12 exp( −7477 K/T ) cm 3 molecules−1 s−1

(E10)

The comparison between rate constants for k2a and k2b are shown in Figure 10. The discrepancies between theoretical and experimental rate constants for 2a and 2b reflect the deviations in total rate constants, k2, shown in Figure 10. There is good agreement between the modeled total rate constants and the recommended values for k2 from Tsang,28 and this is reflected in the good agreement between the experimentally based values for k2a and the recommendation for k2a by Tsang. Over the T-range of the present experiments, ca. 1100−1300 K, the recommended values by Tsang are ∼17% larger than the experimentally based rate constants. Concerning the channel 2b, Tsang’s recommendation for k2b gives rate constants that are ∼60% larger than those from E10. Since reaction 2b is the minor abstraction channel, with an averaged theoretical branching ratio of 0.16, this discrepancy in k2b is not important in detailed chemical kinetic modeling. Therefore, we conclude that in the T-range of 1100−1300 K, eqs E9 and E10, or equally Tsang’ s recommendation for k2a and k2b can be used in chemical kinetic modeling of methanol combustion.

(2a)

(ΔHr 0 = 0.17 kcal/mol)

(E9)

(E8)

The values from Table 3 are within ±40%, at the one standard deviation level, of those calculated from eq E8. Assuming that the theoretical branching ratios are correct, the rate constants for the two abstraction channels, 2a and 2b, can 10193



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(13) Alecu, I. M.; Truhlar, D. G. Computational Study of the Reactions of Methanol with the Hydroperoxyl and Methyl Radicals. 2. Accurate Thermal Rate Constants. J. Phys. Chem. A 2011, 115, 14599− 14611. (14) Michael, J. V. Measurement of Thermal Rate Constants by Flash or Laser Photolysis in Shock- Tubes - Oxidations of H2 and D2. Prog. Energy Combust. Sci. 1992, 18, 327−347. (15) Michael, J. V. The Measurement of Thermal Bimolecular Rate Constants by the Flash Photolysis-Shock Tube (FP-ST) Technique: Comparison of Experiment to Theory. In Advances in Chemical Kinetics and Dynamics; Barker, J. R., Ed.; JAI: Greenwich, CT, 1992; Vol. I, pp 47−112, for original references. (16) Michael, J. V.; Sutherland, J. W. The Thermodynamic State of the Hot Gas behind Reflected Shock-Waves − Implication to Chemical Kinetics. Int. J. Chem. Kinet. 1986, 18, 409−436. (17) Michael, J. V. Rate Constants for the Reaction O + D2 → OD + D by the Flash Photolysis- Shock Tube Technique over the Temperature-Range 825−2487 K - The H2 to D2 Isotope Effect. J. Chem. Phys. 1989, 90, 189−198. (18) Michael, J. V.; Fisher, J. R. Corrections for Non-ideal Effects in Reflected Shock Waves at Low Mach Numbers. In Seventeenth International Symposium on Shock Waves and Shock Tubes; Kim, Y. W., Ed.; AIP Conference Proceedings 208; American Institute of Physics: New York, 1990; p 210. (19) Michael, J. V.; Lifshitz, A. Chemical and Combustion Kinetics. In Handbook of Shock Waves; Ben-Dor, G., Igra, O., Elperin, T., Lifshitz, A., Eds.; Academic Press: New York, 2001; Vol. 3, p 77. (20) Kumaran, S. S.; Carroll, J. J.; Michael, J. V. The Branching Ratio in the Thermal Decomposition of H2CO. Proc. Combust. Inst. 1998, 27, 125−133. (21) Lim, K. P.; Michael, J. V. The Thermal Reactions of CH3. Proc. Comb. Inst. 1994, 25, 713−719 and references therein.. (22) Kumaran, S. S.; Su, M.-C.; Lim, K. P.; Michael, J. V. The Thermal Decomposition of C2H5I. Proc. Comb. Inst. 1996, 26, 605− 611. (23) Maki, R. G.; Michael, J. V.; Sutherland, J. W. Lyman-Alpha Photometry - Curve of Growth Determination, Comparison to Theoretical Oscillator Strength, and Line Absorption Calculations at High-Temperature. J. Phys. Chem. 1985, 89, 4815−4821. (24) Mielke, S. L.; Peterson, K. A.; Schwenke, D. W.; Garrett, B. C.; Truhlar, D. G.; Michael, J. V.; Su, M.-C. Sutherland, J. W. H + H2 Thermal Reaction: A Convergence of Theory and Experiment. Phys. Rev. Lett. 2003, 91, 063201. (25) Michael, J. V.; Su, M.-C.; Sutherland, J. W.; Harding, L. B.; Wagner, A. F. Rate Constants for D + C2H4 → C2H3D + H at High Temperature: Implications to the High Pressure Rate Constant for H + C2H4 → C2H5. Proc. Combust. Inst. 2005, 30, 965−973. (26) Mitchell, A. C. G.; Zemansky, M. W. Resonance Radiation and Excited States; Cambridge University Press: Cambridge, England, 1934. (27) Su, M.-C.; Michael, J. V. C2D5I Dissociation and D + CH3 → CH2D + H at High Temperature: Implications to the High-Pressure Rate Constant for CH4 Dissociation. Proc. Combust. Inst. 2002, 29, 1219−1227. (28) Tsang, W. Chemical Kinetic Data Base for Combustion Chemistry. Part 2. ‘Methanol’. J. Phys. Chem. Ref. Data 1987, 16, 471− 508. (29) Vandooren, J.; Van Tiggelen, P. J. Experimental Investigation of Methanol Oxidation in Flames: Mechanisms and Rate Constants of Elementary Steps. Proc. Combust. Inst. 1981, 18, 473−483. (30) Goos, E.; Burcat, A.; Ruscic, B. Ideal Gas Thermochemical Database with updates from Active Thermochemical Tables, February 2013 ftp://ftp.technion.ac.il/pub/supported/aetdd/thermodynamics/ BURCAT.THR (31) Yang, X.; Jasper, A.; Kiefer, J. H.; Tranter, R. S. The Dissociation of Diacetyl: A Shock Tube and Theoretical Study. J. Phys. Chem. A 2009, 113, 8318−8326. (32) Sivaramakrishnan, R.; Su, M.-C.; Michael, J. V.; Klippenstein, S. J.; Harding, L. B.; Ruscic, B. Shock Tube and Theoretical Studies on

sets, the theoretical results from Truhlar and Alecu show a better agreement to the present experimental data. The present experiments cannot be used to extract branching ratios for the single H-abstraction channels. Therefore branching ratios from the theory studies were averaged and applied to the present experimentally determined rate constants in order to obtain Arrhenius expressions for the site-specific Habstraction reactions.

■

AUTHOR INFORMATION

Corresponding Author

*Mailing address: D-183, Bldg. 200 Argonne National Laboratory, Argonne, IL 60439, USA. Phone: (630) 2523171; Fax: (630) 252-9570; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract No. DEAC0206CH11357.

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REFERENCES

(1) Arnowitz, D.; Naegeli, D. W.; Glassman, I. Kinetics of the Pyrolysis of Methanol. J. Phys. Chem. 1977, 81, 2555−2559. (2) Spindler, K.; Wagner, H. Gg. Zum Thermischen Unimolekularen Zerfall von Methanol. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 2−13. (3) Cribb, P. H.; Dove, J. E.; Yamazaki, S. A Kinetic Study of the Pyrolysis of Methanol Using Shock Tube and Computer Simulation Techniques. Combust. Flame 1992, 88, 169−185. (4) Hidaka, Y.; Oki, T.; Kawano, H. Thermal Decomposition of Methanol in Shock Waves. J. Phys. Chem. 1989, 98, 7134−7139. (5) Dombrowsky, Ch.; Hoffmann, A.; Klatt, M.; Wagner, H. Gg. An Investigation of the Methanol Decomposition Behind Incident Shock Waves. Ber. Bunsen-Ges. 1991, 95, 1685−1687. (6) Krasnoperov, L. N.; Michael, J. V. High-Temperature Shock Tube Studies Using Multipass Absorption: Rate Constant Results for OH + CH3, OH + CH2, and the Dissociation of CH3OH. J. Phys. Chem. A 2004, 108, 8317−8323. (7) Srinivasan, N. K.; Su, M.-C.; Michael, J. V. Michael HighTemperature Rate Constants for CH3OH + Kr → Products, OH + CH3OH → Products, OH + (CH3)2CO → CH2COCH3 + H2O, and OH + CH3 → CH2 + H2O. J. Phys. Chem. A 2007, 111, 3951−3958. (8) Lu, K.-W.; Matsui, H.; Huang, C.-L.; Raghunath, P.; Wang, N.-S.; Lin, M. C. Shock Tube Study on the Thermal Decomposition of CH3OH. J. Phys. Chem. A 2010, 114, 5493−5502. (9) Jodkowski, J. T.; Rayez, M. T.; Rayez, J.-C.; Berces, T.; Dobe, S. Theoretical Study of the Kinetics of the Hydrogen Abstraction from Methanol. J. Phys. Chem. A 1999, 103, 3750−3765. (10) Carvalho, E. F. V.; Barauna, A. N.; Machado, F. B. C.; RobertoNeto, O. Theoretical Calculations of Energetics, Structures, and Rate Constants for the H + CH3OH Hydrogen Abstraction Reactions. Chem. Phys. Lett. 2008, 463, 33−37. (11) Moses, J. I.; Visscher, C.; Fortney, J. J.; Showman, A. P.; Lewis, N. K.; Griffith, C. A.; Klippenstein, S. J.; Shabram, M.; Friedson, A. J.; Marley, M. S.; et al. Disequilibrium Carbon, Oxygen, and Nitrogen Chemistry in the Atmospheres of HD 189733b and HD 209458b. Astrophys. J. 2011, 737 (15), 37. (12) Meana-Pañeda, R.; Truhlar, D. G.; Fernández-Ramos, A. HighLevel Direct-Dynamics Variational Transition State Theory Calculations Including Multidimensional Tunneling of the Thermal Rate Constants, Branching Ratios, and Kinetic Isotope Effects of the Hydrogen Abstraction Reactions from Methanol by Atomic Hydrogen. J. Chem. Phys. 2011, 134, 094302. 10194



Article

the Thermal Decomposition of Propane: Evidence for a Roaming Radical Channel. J. Phys. Chem. A 2011, 115, 3366−3379. (33) Sugawara, K.; Okazaki, K.; Sato, S. Temperature-Dependence of the Rate Constants of H and D-Atom Additions to C2H4, C2H3D, C2D4, C2H2, and C2D2. Bull. Chem. Soc. Jpn. 1981, 54, 2872−2877. (34) Jasper, A. W.; Klippenstein, S. J.; Harding, L. B.; Ruscic, B. Kinetics of the Reaction of Methyl Radical with Hydroxyl Radical and Methanol Decomposition. J. Phys. Chem. A 2007, 111, 3932−3950. (35) Kiefer, J. H.; Santhanam, S.; Srinivasan, N. K.; Tranter, R. S.; Klippenstein, S. J.; Oehlschlaeger, M. A. Dissociation, Relaxation and Incubation in the High-temperature Pyrolysis of Ethane, and a Successful RRKM Modeling. Proc. Combust. Inst. 2005, 30, 1129− 1135. (36) Troe, J.; Ushakov, V. G. J. Chem. Phys. The Dissociation/ Recombination Reaction CH4 (+M) → CH3 + H (+M): A Case Study for Unimolecular Rate Theory. J. Chem. Phys. 2012, 136, 214309. (37) Srinivasan, N. K.; Michael, J. V. The Thermal Decomposition of Water. Int. J. Chem. Kinet. 2006, 38, 211−219. (38) Pirraglia, A. N.; Michael, J. V.; Sutherland, J. W.; Klemm, R. B. A Flash-Photolysis Shock- Tube Kinetic-Study of the H Atom Reaction with O2: H + O2 ↔ OH + O (962 K ≤ T ≤ 1705 K) and H + O2 + Ar→HO2 + Ar (746 K ≤ T ≤ 987 K). J. Phys. Chem. 1989, 93, 282− 291. (39) Sutherland, J. W.; Michael, J. V.; Pirraglia, A. N.; Nesbitt, F. L.; Klemm, R. B. Rate Constant for the Reaction of O(3P) with H2 by the Flash Photolysis-Shock Tube and Flash Photolysis- Resonance Fluorescence Techniques; 504 K ≤ T ≤ 2495 K. Proc. Combust. Symp. 1986, 21, 929−941. (40) Srinivasan, N. K.; Su, M.-C.; Sutherland, J. W.; Michael, J. V. Reflected Shock Tube Studies of High-Temperature Rate Constants for CH3 + O2, H2CO + O2, and OH + O2. J. Phys. Chem. A 2005, 109, 7902−7914. (41) Langford, A. O.; Petek, H.; Moore, C. B. Collisional Removal of CH2(1A1) - Absolute Rate Constants for Atomic and Molecular Collisional Partners at 295 K. J. Chem. Phys. 1983, 78, 6650−6659. (42) Hancock, G.; Heal, M. R. Temperature Dependences of CH2 (∼a1A1) Removal Rates by Ar, NO, H2, and CH2CO in the Range 295−859 K. J. Phys. Chem. 1992, 96, 10316−10322. (43) Lim, K. P.; Michael, J. V. The Thermal-Decomposition of CH3Cl Using the Cl-Atom Absorption Method and the Bimolecular Rate-Constant for O + CH3 (1609−2002 K) with a Pyrolysis Photolysis-Shock Tube Technique. J. Chem. Phys. 1993, 98, 3919− 3928. (44) Krasnoperov, L. N.; Michael, J. V. Shock Tube Studies Using a Novel Multipass Absorption Cell: Rate Constant Results for OH + H2 and OH + C2H6. J. Phys. Chem. A 2004, 108, 5643−5648. (45) Sivaramakrishnan, R.; Michael, J. V.; Ruscic, B. HighTemperature Rate Constants for H/D + C2H6 and C3H8. Int. J. Chem. Kinet. 2012, 44, 194−205.

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