Shock Tube Investigation of CH₃ + CH₃OCH₃

The title reaction has been investigated in a diaphragmless shock tube by laser schlieren densitometry over the temperature range 1163–1629 K and pressures of 60, 120, and 240 Torr. Methyl radicals were produced by dissociation of 2,3-butanedione in the presence of an excess of dimethyl ether. Rate coefficients for CH₃ + CH₃OCH₃ were obtained from simulations of the experimental data yielding the following expression which is valid over the range 1100–1700 K: <i>k</i> = (10.19 ± 3.0)<i>T</i>^3.78 exp^(−4878/T) cm³ mol^–1s^–1. The experimental results are in good agreement with estimates by Curran and co-workers [Fischer, S. L.; Dryer, F. L.; Curran, H. J. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 713–740. Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 741–759] but about a factor of 2.6 lower than those of Zhao et al. [Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2008</b>, <i>40</i> (1), 1–18]

Shock Tube Investigation of CH₃ + CH₃OCH₃

The title reaction has been investigated in a diaphragmless shock tube by laser schlieren densitometry over the temperature range 1163–1629 K and pressures of 60, 120, and 240 Torr. Methyl radicals were produced by dissociation of 2,3-butanedione in the presence of an excess of dimethyl ether. Rate coefficients for CH₃ + CH₃OCH₃ were obtained from simulations of the experimental data yielding the following expression which is valid over the range 1100–1700 K: <i>k</i> = (10.19 ± 3.0)<i>T</i>^3.78 exp^(−4878/T) cm³ mol^–1s^–1. The experimental results are in good agreement with estimates by Curran and co-workers [Fischer, S. L.; Dryer, F. L.; Curran, H. J. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 713–740. Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 741–759] but about a factor of 2.6 lower than those of Zhao et al. [Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2008</b>, <i>40</i> (1), 1–18]

Shock Tube Investigation of CH₃ + CH₃OCH₃

The title reaction has been investigated in a diaphragmless shock tube by laser schlieren densitometry over the temperature range 1163–1629 K and pressures of 60, 120, and 240 Torr. Methyl radicals were produced by dissociation of 2,3-butanedione in the presence of an excess of dimethyl ether. Rate coefficients for CH₃ + CH₃OCH₃ were obtained from simulations of the experimental data yielding the following expression which is valid over the range 1100–1700 K: <i>k</i> = (10.19 ± 3.0)<i>T</i>^3.78 exp^(−4878/T) cm³ mol^–1s^–1. The experimental results are in good agreement with estimates by Curran and co-workers [Fischer, S. L.; Dryer, F. L.; Curran, H. J. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 713–740. Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2000</b>, <i>32</i> (12), 741–759] but about a factor of 2.6 lower than those of Zhao et al. [Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. <i>Int. J. Chem. Kinet.</i> <b>2008</b>, <i>40</i> (1), 1–18]

Single Pulse Shock Tube Study of Allyl Radical Recombination

The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube with gas chromatographic measurements at 1–10 bar, 650–1300 K, and 1.4–2 ms reaction times. 1,5-Hexadiene and allyl iodide were used as precursors. Simulation of the results using derived rate expressions from a complementary diaphragmless shock tube/laser schlieren densitometry study provided excellent agreement with precursor consumption and formation of all major stable intermediates. No significant pressure dependence was observed at the present conditions. It was found that under the conditions of these experiments, reactions of allyl radicals in the cooling wave had to be accounted for to accurately simulate the experimental results, and this unusual situation is discussed. In the allyl iodide experiments, higher amounts of allene, propene, and benzene were found at lower temperatures than expected. Possible mechanisms are discussed and suggest that iodine containing species are responsible for the low temperature formation of allene, propene, and benzene

Probing Combustion Chemistry in a Miniature Shock Tube with Synchrotron VUV Photo Ionization Mass Spectrometry

Tunable synchrotron-sourced photoionization time-of-flight mass spectrometry (PI-TOF-MS) is an important technique in combustion chemistry, complementing lab-scale electron impact and laser photoionization studies for a wide variety of reactors, typically at low pressure. For high-temperature and high-pressure chemical kinetics studies, the shock tube is the reactor of choice. Extending the benefits of shock tube/TOF-MS research to include synchrotron sourced PI-TOF-MS required a radical reconception of the shock tube. An automated, miniature, high-repetition-rate shock tube was developed and can be used to study high-pressure reactive systems (<i>T</i> > 600 K, <i>P</i> < 100 bar) behind reflected shock waves. In this paper, we present results of a PI-TOF-MS study at the Advanced Light Source at Lawrence Berkeley National Laboratory. Dimethyl ether pyrolysis (2% CH₃OCH₃/Ar) was observed behind the reflected shock (1400 < <i>T</i>₅ < 1700 K, 3 < <i>P</i>₅ < 16 bar) with ionization energies between 10 and 13 eV. Individual experiments have extremely low signal levels. However, product species and radical intermediates are well-resolved when averaging over hundreds of shots, which is ordinarily impractical in conventional shock tube studies. The signal levels attained and data throughput rates with this technique are comparable to those with other synchrotron-based PI-TOF-MS reactors, and it is anticipated that this high pressure technique will greatly complement those lower pressure techniques

Thermal Dissociation and Roaming Isomerization of Nitromethane: Experiment and Theory

The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH₃NO₂ decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH₃ and NO₂ radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH₃NO₂, and for the reaction of CH₃ with NO₂