5 research outputs found
A Shock-Tube Study of the CO + OH Reaction Near the Low-Pressure Limit
Rate coefficients for the reaction
between carbon monoxide and
hydroxyl radical were measured behind reflected shock waves over 700–1230
K and 1.2–9.8 bar. The temperature/pressure conditions correspond
to the predicted low-pressure limit of this reaction, where the channel
leading to carbon dioxide formation is dominant. The reaction rate
coefficients were inferred by measuring the formation of carbon dioxide
using quantum cascade laser absorption near 4.2 μm. Experiments
were performed under pseudo-first-order conditions with <i>tert</i>-butyl hydroperoxide (TBHP) as the OH precursor. Using ultraviolet
laser absorption by OH radicals, the TBHP decomposition rate was measured
to quantify potential facility effects under extremely dilute conditions
used here. The measured CO + OH rate coefficients are provided in
Arrhenius form for three different pressure ranges: <i>k</i><sub>CO+OH</sub>(1.2–1.6 bar) = (9.14 ± 2.17) ×
10<sup>–13</sup> exp(−(1265 ± 190)/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>; <i>k</i><sub>CO+OH</sub>(4.3–5.1 bar) = (8.70
± 0.84) × 10<sup>–13</sup> exp(−(1156 ±
83)/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>; and <i>k</i><sub>CO+OH</sub>(9.6–9.8
bar) = (7.48 ± 1.92) × 10<sup>–13</sup> exp(−(929
± 192)/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. The measured rate coefficients are found
to be lower than the master equation modeling results by Weston et
al. [<i>J. Phys. Chem. A</i>, <b>2013</b>, <i>117</i>, 821] at 819 K and in closer agreement with the expression
provided by Joshi and Wang [<i>Int. J. Chem. Kinet.</i>, <b>2006</b>, <i>38</i>, 57]
Rate Coefficients of the Reaction of OH with Allene and Propyne at High Temperatures
Allene (H<sub>2</sub>Cî—»Cî—»CH<sub>2</sub>; a-C<sub>3</sub>H<sub>4</sub>) and propyne (CH<sub>3</sub>Cî—¼CH; p-C<sub>3</sub>H<sub>4</sub>) are important species
in various chemical environments. In combustion processes, the reactions
of hydroxyl radicals with a-C<sub>3</sub>H<sub>4</sub> and p-C<sub>3</sub>H<sub>4</sub> are critical in the overall fuel oxidation system.
In this work, rate coefficients of OH radicals with allene (OH + H<sub>2</sub>CCCH<sub>2</sub> → products) and propyne
(OH + CH<sub>3</sub>CCH → products) were measured behind
reflected shock waves over the temperature range of 843–1352
K and pressures near 1.5 atm. Hydroxyl radicals were generated by
rapid thermal decomposition of <i>tert</i>-butyl hydroperoxide
((CH<sub>3</sub>)<sub>3</sub>–CO–OH), and monitored
by narrow line width laser absorption of the well-characterized <i>R</i><sub>1</sub>(5) electronic transition of the OH A–X
(0,0) electronic system near 306.7 nm. Results show that allene reacts
faster with OH radicals than propyne over the temperature range of
this study. Measured rate coefficients can be expressed in Arrhenius
form as follows: <i>k</i><sub>allene+OH</sub>(<i>T</i>) = 8.51(±0.03) × 10<sup>–22</sup><i>T</i><sup>3.05</sup> exp(2215(±3)/<i>T</i>), <i>T</i> = 843–1352 K; <i>k</i><sub>propyne+OH</sub>(<i>T</i>) = 1.30(±0.07) × 10<sup>–21</sup><i>T</i><sup>3.01</sup> exp(1140(±6)/<i>T</i>), <i>T</i> = 846–1335 K
High-Temperature Experimental and Theoretical Study of the Unimolecular Dissociation of 1,3,5-Trioxane
Unimolecular dissociation of 1,3,5-trioxane
was investigated experimentally
and theoretically over a wide range of conditions. Experiments were
performed behind reflected shock waves over the temperature range
of 775–1082 K and pressures near 900 Torr using a high-repetition
rate time of flight mass spectrometer (TOF-MS) coupled to a shock
tube (ST). Reaction products were identified directly, and it was
found that formaldehyde is the sole product of 1,3,5-trioxane dissociation.
Reaction rate coefficients were extracted by the best fit to the experimentally
measured concentration–time histories. Additionally, high-level
quantum chemical and RRKM calculations were employed to study the
falloff behavior of 1,3,5-trioxane dissociation. Molecular geometries
and frequencies of all species were obtained at the B3LYP/cc-pVTZ,
MP2/cc-pVTZ, and MP2/aug-cc-pVDZ levels of theory, whereas the single-point
energies of the stationary points were calculated using coupled cluster
with single and double excitations including the perturbative treatment
of triple excitation (CCSDÂ(T)) level of theory. It was found that
the dissociation occurs via a concerted mechanism requiring an energy
barrier of 48.3 kcal/mol to be overcome. The new experimental data
and theoretical calculations serve as a validation and extension of
kinetic data published earlier by other groups. Calculated values
for the pressure limiting rate coefficient can be expressed as log<sub>10</sub> <i>k</i><sub>∞</sub> (s<sup>–1</sup>) = [15.84 – (49.54 (kcal/mol)/2.3<i>RT</i>)] (500–1400
K)
Theoretical Study of the Reaction Kinetics of Atomic Bromine with Tetrahydropyran
A detailed theoretical analysis of
the reaction of atomic bromine
with tetrahydropyran (THP, C<sub>5</sub>H<sub>10</sub>O) was performed
using several ab initio methods and statistical rate theory calculations.
Initial geometries of all species involved in the potential energy
surface of the title reaction were obtained at the B3LYP/cc-pVTZ level
of theory. These molecular geometries were reoptimized using three
different meta-generalized gradient approximation (meta-GGA) functionals.
Single-point energies of the stationary points were obtained by employing
the coupled-cluster with single and double excitations (CCSD) and
fourth-order Møller–Plesset (MP4 SDQ) levels of theory.
The computed CCSD and MP4Â(SDQ) energies for optimized structures at
various DFT functionals were found to be consistent within 2 kJ mol<sup>–1</sup>. For a more accurate energetic description, single-point
calculations at the CCSDÂ(T)/CBS level of theory were performed for
the minimum structures and transition states optimized at the B3LYP/cc-pVTZ
level of theory. Similar to other ether + Br reactions, it was found
that the tetrahydropyran + Br reaction proceeds in an overall endothermic
addition–elimination mechanism via a number of intermediates.
However, the reactivity of various ethers with atomic bromine was
found to vary substantially. In contrast with the 1,4-dioxane + Br
reaction, the chair form of the addition complex (<i>c</i>-C<sub>5</sub>H<sub>10</sub>O–Br) for THP + Br does not need
to undergo ring inversion to form a boat conformer (<i>b</i>-C<sub>4</sub>H<sub>8</sub>O<sub>2</sub>–Br) before the intramolecular
H-shift can occur to eventually release HBr. Instead, a direct, yet
more favorable route was mapped out on the potential energy surface
of the THP + Br reaction. The rate coefficients for all relevant steps
involved in the reaction mechanism were computed using the energetics
of coupled cluster calculations. On the basis of the results of the
CCSDÂ(T)/CBS//B3LYP/cc-pVTZ level of theory, the calculated overall
rate coefficients can be expressed as <i>k</i><sub>ov.,calc.</sub>(<i>T</i>) = 4.60 × 10<sup>–10</sup> expÂ[−20.4
kJ mol<sup>–1</sup>/(<i>RT</i>)] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> for the temperature range of
273–393 K. The calculated values are found to be in excellent
agreement with the experimental data published previously
Measurements of Positively Charged Ions in Premixed Methane-Oxygen Atmospheric Flames
<p>Cations and anions are formed as a result of chemi-ionization processes in combustion systems. Electric fields can be applied to reduce emissions and improve combustion efficiency by active control of the combustion process. Detailed flame ion chemistry models are needed to understand and predict the effect of external electric fields on combustion plasmas. In this work, a molecular beam mass spectrometer (MBMS) is utilized to measure ion concentration profiles in premixed methane–oxygen argon burner-stabilized atmospheric flames. Lean and stoichiometric flames are considered to assess the dependence of ion chemistry on flame stoichiometry. Relative ion concentration profiles are compared with numerical simulations using various temperature profiles, and good qualitative agreement was observed for the stoichiometric flame. However, for the lean flame, numerical simulations misrepresent the spatial distribution of selected ions greatly. Three modifications are suggested to enhance the ion mechanism and improve the agreement between experiments and simulations. The first two modifications comprise the addition of anion detachment reactions to increase anion recombination at low temperatures. The third modification involves restoring a detachment reaction to its original irreversible form. To our knowledge, this work presents the first detailed measurements of cations and flame temperature in canonical methane–oxygen-argon atmospheric flat flames. The positive ion profiles reported here may be useful to validate and improve ion chemistry models for methane-oxygen flames.</p