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Kinetic studies and computational modeling of atomic chlorine reactions in the gas phase.
The gas phase reactions of atomic chlorine with hydrogen sulfide, ammonia, benzene, and ethylene are investigated using the laser flash photolysis / resonance fluorescence experimental technique. In addition, the kinetics of the reverse processes for the latter two elementary reactions are also studied experimentally. The absolute rate constants for these processes are measured over a wide range of conditions, and the results offer new accurate information about the reactivity and thermochemistry of these systems. The temperature dependences of these reactions are interpreted via the Arrhenius equation, which yields significantly negative activation energies for the reaction of the chlorine atom and hydrogen sulfide as well as for that between the phenyl radical and hydrogen chloride. Positive activation energies which are smaller than the overall endothermicity are measured for the reactions between atomic chlorine with ammonia and ethylene, which suggests that the reverse processes for these reactions also possess negative activation energies. The enthalpies of formation of the phenyl and β-chlorovinyl are assessed via the third-law method. The stability and reactivity of each reaction system is further rationalized based on potential energy surfaces, computed with high-level ab initio quantum mechanical methods and refined through the inclusion of effects which arise from the special theory of relativity. Large amounts of spin-contamination are found to result in inaccurate computed thermochemistry for the phenyl and ethyl radicals. A reformulation of the computational approach to incorporate spin-restricted reference wavefunctions yields computed thermochemistry in good accord with experiment. The computed potential energy surfaces rationalize the observed negative temperature dependences in terms of a chemical activation mechanism, and the possibility that an energized adduct may contribute to product formation is investigated via RRKM theory
Experimental and Computational Studies of the Kinetics of the Reaction of Atomic Hydrogen with Methanethiol
Article on experimental and computational studies of the kinetics of the reaction of atomic hydrogen with methanethiol
Dehydration of Isobutanol and the Elimination of Water from Fuel Alcohols
Rate coefficients for the dehydration
of isobutanol have been determined
experimentally from comparative rate single pulse shock tube measurements
and calculated via multistructural transition state theory (MS-TST).
They are represented by the Arrhenius expression, <i>k</i>(isobutanol â isobutene + H<sub>2</sub>O)<sub>experimental</sub> = 7.2 Ă 10<sup>13</sup>âexpÂ(â35300 K/<i>T</i>) s<sup>â1</sup>. The theoretical work leads to
the high pressure rate expression, <i>k</i>(isobutanol â
isobutene + H<sub>2</sub>O)<sub>theory</sub> = 3.5 Ă 10<sup>13</sup>âexpÂ(â35400 K/<i>T</i>) s<sup>â1</sup>. Results are thus within a factor of 2 of each other. The experimental
results cover the temperature range 1090â1240 K and pressure
range 1.5â6 atm, with no discernible pressure effects. Analysis
of these results, in combination with earlier single pulse shock tube
work, made it possible to derive the governing factors that control
the rate coefficients for alcohol dehydration in general. Alcohol
dehydration rate constants depend on the location of the hydroxyl
group (primary, secondary, and tertiary) and the number of available
H-atoms adjacent to the OH group for water elimination. The position
of the H-atoms in the hydrocarbon backbone appears to be unimportant
except for highly substituted molecules. From these correlations,
we have derived <i>k</i>(isopropanol â propene +
H<sub>2</sub>O) = 7.2 Ă 10<sup>13</sup>âexpÂ(â33000
K/<i>T</i>) s<sup>â1</sup>. Comparison of experimental
determination with theoretical calculations for this dehydration,
and those for ethanol show deviations of the same magnitude as for
isobutanol. Systematic differences between experiments and theoretical
calculations are common
New Pathways for Formation of Acids and Carbonyl Products in Low-Temperature Oxidation: The Korcek Decomposition of Îł-Ketohydroperoxides
We present new reaction pathways relevant to low-temperature oxidation in gaseous and condensed phases. The new pathways originate from Îł-ketohydroperoxides (KHP), which are well-known products in low-temperature oxidation and are assumed to react only via homolytic OâO dissociation in existing kinetic models. Our ab initio calculations identify new exothermic reactions of KHP forming a cyclic peroxide isomer, which decomposes via novel concerted reactions into carbonyl and carboxylic acid products. Geometries and frequencies of all stationary points are obtained using the M06-2X/MG3S DFT model chemistry, and energies are refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. Thermal rate coefficients are computed using variational transition-state theory (VTST) calculations with multidimensional tunneling contributions based on small-curvature tunneling (SCT). These are combined with multistructural partition functions (Q[superscript MSâT]) to obtain direct dynamics multipath (MP-VTST/SCT) gas-phase rate coefficients. For comparison with liquid-phase measurements, solvent effects are included using continuum dielectric solvation models. The predicted rate coefficients are found to be in excellent agreement with experiment when due consideration is made for acid-catalyzed isomerization. This work provides theoretical confirmation of the 30-year-old hypothesis of Korcek and co-workers that KHPs are precursors to carboxylic acid formation, resolving an open problem in the kinetics of liquid-phase autoxidation. The significance of the new pathways in atmospheric chemistry, low-temperature combustion, and oxidation of biological lipids are discussed.United States. Dept. of Energy. Office of Basic Energy Sciences (Energy Frontier Research Center for Combustion Science. Grant DE-SC0001198)University of Minnesota. Supercomputer InstitutePacific Northwest National Laboratory (U.S.) Molecular Science Computing Facilit
A combined photoionization time-of-flight mass spectrometry and laser absorption spectrometry flash photolysis apparatus for simultaneous determination of reaction rates and product branching
© 2018 Author(s). In recent years, predictions of product branching for reactions of consequence to both combustion and atmospheric chemistry have outpaced validating experiments. An apparatus is described that aims to fill this void by combining several well-known experimental techniques into one: flash photolysis for radical generation, multiple-pass laser absorption spectrometry (LAS) for overall kinetics measurements, and time-resolved photoionization time-of-flight mass spectrometry (PI TOF-MS) for product branching quantification. The sensitivity of both the LAS and PI TOF-MS detection techniques is shown to be suitable for experiments with initial photolytically generated radical concentrations of âŒ1 Ă 1012 molecules cm-3. As it is fast (ÎŒs time resolution) and non-intrusive, LAS is preferred for accurate kinetics (time-dependence) measurements. By contrast, PI TOF-MS is preferred for product quantification because it provides a near-complete picture of the reactor composition in a single mass spectrum. The value of simultaneous LAS and PI TOF-MS detection is demonstrated for the chemically interesting phenyl radical + propene system
New Pathways for Formation of Acids and Carbonyl Products in Low-Temperature Oxidation: The Korcek Decomposition of ÎłâKetohydroperoxides
We
present new reaction pathways relevant to low-temperature oxidation
in gaseous and condensed phases. The new pathways originate from Îł-ketohydroperoxides
(KHP), which are well-known products in low-temperature oxidation
and are assumed to react only via homolytic OâO dissociation
in existing kinetic models. Our <i>ab initio</i> calculations
identify new exothermic reactions of KHP forming a cyclic peroxide
isomer, which decomposes via novel concerted reactions into carbonyl
and carboxylic acid products. Geometries and frequencies of all stationary
points are obtained using the M06-2X/MG3S DFT model chemistry, and
energies are refined using RCCSDÂ(T)-F12a/cc-pVTZ-F12 single-point
calculations. Thermal rate coefficients are computed using variational
transition-state theory (VTST) calculations with multidimensional
tunneling contributions based on small-curvature tunneling (SCT).
These are combined with multistructural partition functions (Q<sup>MSâT</sup>) to obtain direct dynamics multipath (MP-VTST/SCT)
gas-phase rate coefficients. For comparison with liquid-phase measurements,
solvent effects are included using continuum dielectric solvation
models. The predicted rate coefficients are found to be in excellent
agreement with experiment when due consideration is made for acid-catalyzed
isomerization. This work provides theoretical confirmation of the
30-year-old hypothesis of Korcek and co-workers that KHPs are precursors
to carboxylic acid formation, resolving an open problem in the kinetics
of liquid-phase autoxidation. The significance of the new pathways
in atmospheric chemistry, low-temperature combustion, and oxidation
of biological lipids are discussed
New Pathways for Formation of Acids and Carbonyl Products in Low-Temperature Oxidation: The Korcek Decomposition of ÎłâKetohydroperoxides
We
present new reaction pathways relevant to low-temperature oxidation
in gaseous and condensed phases. The new pathways originate from Îł-ketohydroperoxides
(KHP), which are well-known products in low-temperature oxidation
and are assumed to react only via homolytic OâO dissociation
in existing kinetic models. Our <i>ab initio</i> calculations
identify new exothermic reactions of KHP forming a cyclic peroxide
isomer, which decomposes via novel concerted reactions into carbonyl
and carboxylic acid products. Geometries and frequencies of all stationary
points are obtained using the M06-2X/MG3S DFT model chemistry, and
energies are refined using RCCSDÂ(T)-F12a/cc-pVTZ-F12 single-point
calculations. Thermal rate coefficients are computed using variational
transition-state theory (VTST) calculations with multidimensional
tunneling contributions based on small-curvature tunneling (SCT).
These are combined with multistructural partition functions (Q<sup>MSâT</sup>) to obtain direct dynamics multipath (MP-VTST/SCT)
gas-phase rate coefficients. For comparison with liquid-phase measurements,
solvent effects are included using continuum dielectric solvation
models. The predicted rate coefficients are found to be in excellent
agreement with experiment when due consideration is made for acid-catalyzed
isomerization. This work provides theoretical confirmation of the
30-year-old hypothesis of Korcek and co-workers that KHPs are precursors
to carboxylic acid formation, resolving an open problem in the kinetics
of liquid-phase autoxidation. The significance of the new pathways
in atmospheric chemistry, low-temperature combustion, and oxidation
of biological lipids are discussed