6 research outputs found
Shock Tube Investigation of CH<sub>3</sub> + CH<sub>3</sub>OCH<sub>3</sub>
The title reaction has been investigated in a diaphragmless
shock
tube by laser schlieren densitometry over the temperature range 1163β1629
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 CH<sub>3</sub> + CH<sub>3</sub>OCH<sub>3</sub> were obtained from simulations of the experimental
data yielding the following expression which is valid over the range
1100β1700 K: <i>k</i> = (10.19 Β± 3.0)<i>T</i><sup>3.78</sup>βexp<sup>(β4878/T)</sup> cm<sup>3</sup> mol<sup>β1</sup>s<sup>β1</sup>. 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), 713β740.
Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem.
Kinet.</i> <b>2000</b>, <i>32</i> (12), 741β759]
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), 1β18]
Shock Tube Investigation of CH<sub>3</sub> + CH<sub>3</sub>OCH<sub>3</sub>
The title reaction has been investigated in a diaphragmless
shock
tube by laser schlieren densitometry over the temperature range 1163β1629
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 CH<sub>3</sub> + CH<sub>3</sub>OCH<sub>3</sub> were obtained from simulations of the experimental
data yielding the following expression which is valid over the range
1100β1700 K: <i>k</i> = (10.19 Β± 3.0)<i>T</i><sup>3.78</sup>βexp<sup>(β4878/T)</sup> cm<sup>3</sup> mol<sup>β1</sup>s<sup>β1</sup>. 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), 713β740.
Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem.
Kinet.</i> <b>2000</b>, <i>32</i> (12), 741β759]
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), 1β18]
Shock Tube Investigation of CH<sub>3</sub> + CH<sub>3</sub>OCH<sub>3</sub>
The title reaction has been investigated in a diaphragmless
shock
tube by laser schlieren densitometry over the temperature range 1163β1629
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 CH<sub>3</sub> + CH<sub>3</sub>OCH<sub>3</sub> were obtained from simulations of the experimental
data yielding the following expression which is valid over the range
1100β1700 K: <i>k</i> = (10.19 Β± 3.0)<i>T</i><sup>3.78</sup>βexp<sup>(β4878/T)</sup> cm<sup>3</sup> mol<sup>β1</sup>s<sup>β1</sup>. 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), 713β740.
Curran, H. J.; Fischer, S. L.; Dryer, F. L. <i>Int. J. Chem.
Kinet.</i> <b>2000</b>, <i>32</i> (12), 741β759]
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), 1β18]
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 1β10 bar, 650β1300 K, and 1.4β2
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% CH<sub>3</sub>OCH<sub>3</sub>/Ar) was observed behind the reflected
shock (1400 < <i>T</i><sub>5</sub> < 1700 K, 3 < <i>P</i><sub>5</sub> < 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 CH<sub>3</sub>NO<sub>2</sub> 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 CH<sub>3</sub> and NO<sub>2</sub> 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 CH<sub>3</sub>NO<sub>2</sub>, and for the reaction of CH<sub>3</sub> with
NO<sub>2</sub>