A shock tube study of methyl-methyl reactions between 1200 and 2400 K

The methyl-methyl reaction was studied in a shock tube using uv narrowline laser absorption to measure time-varying concentration profiles of CH₃. Methyl radicals were rapidly formed initially by pyrolysis of various precursors, azomethane, ethane, or methyl iodide, dilute in argon. The contributions of the various product channels, C₂H₆, C₂H₅ + H, C₂H₄ + H₂, and CH₂ + CH₄, were examined by varying reactant mixtures and temperature. The measured rate coefficients for recombination to C₂H₆ between 1200 and 1800 K are accurately fit using the unimolecular rate coefficients reported by Wagner and Wardlaw (1988). The rate coefficient for the C₂H₅ + H channel was found to be 2.4 (±0.5) × 10¹³ exp(?6480/T) [cm³/mol-s] between 1570 and 1780 K, and is in agreement with the value reported by Frank and Braun-Unkhoff (1988). No evidence of a contribution by the C₂H₄ + H₂ channel was found in ethane/methane/argon mixtures, although methyl profiles in these mixtures should be particularly sensitive to this channel. An upper limit of approximately 10¹¹ [cm³/mol-s] over the range 1700 to 2200 K was inferred for the rate coefficient of the C₂H₄ + H₂ channel. Between 1800 and 2200 K, methyl radicals are also rapidly removed by CH₃ + H ? ¹CH₂ + H₂. In this temperature range, the reverse reaction was found to have a rate coefficient of 1.3 (±0.3) × 10¹⁴ [cm³/mol-s], which is 1.8 times the room-temperature value. © 1995 John Wiley & Sons, Inc.     

A shock tube study of the reactions of CH with CO2 and O2

The formation and consumption of CH radicals during shock-induced pyrolysis of a few ppm ethane diluted in argon was measured by a ring-dye laser spectrometer. Absorption-over-time profiles, measured at a resonance line in the Q-branch of the A²Δ − X²Π band of CH at λ = 431.1311 nm, were recorded and transformed into CH concentrations by known absorption coefficients. By adding some hundred ppm of CO₂ or O₂ to the initial mixtures, the CH concentration profiles were significantly perturbed. Both the perturbed and unperturbed CH concentration profiles have been compared with calculations based on a reaction kinetic model. A sensitivity analysis revealed that the perturbation process was dominated by direct reactions of CH with the added molecules. By fitting calculated to observed CH profiles the following rate coefficients were obtained The experiments were performed in the temperature range between 2500 K and 3500 K. © 1996 John Wiley & Sons, Inc.     

A shock tube study of the reactions of the hydroxyl radical with several combustion species

Reactions of the hydroxyl radical, OH, with several organic species of interest in combustion chemistry have been studied near 1200 K and 1 atm in shock tube experiments in which UV absorption was used to monitor the OH concentration. Rate coefficients were measured for the reactions of OH with 2,3-dimethylbutane, isooctane, neooctane, ethylene, propylene, acetylene, formaldehyde, methanol, and ethanol. The values were found to be (in units of 10¹² cm³/mol-s): 21, 22, 18, 2.6, 9.6, 0.28, 12, 5.2, and 5.3. These measured values are compared with previous experimental results and, where appropriate, transition-state theory calculations.     

A shock tube study of the reactions of NH with NO,O2, and O

The reactions of NH(X³Σ⁻) with NO, O₂, and O have been studied in reflected and incident shock wave experiments. The source of NH in all the experiments was the thermal dissociation of isocyanic acid, HNCO. Time-histories of the NH(X³Σ) and OH(X²Π) radicals were measured behind the shock waves using cw, narrow-linewidth laser absorption at 336 nm and 307 nm, respectively. The second-order rate coefficients of the reactions: were determined to be: and cm³ mol⁻¹ s⁻¹, where ƒ and F define the lower and upper uncertainty limits, respectively. The branching fraction of channel defined as k_3b/k_3total, was determined to be 0.19 ± 0.10 over the temperature range of 2940 K to 3040 K.     

The reactions of methyl radicals with atomic and molecular oxygen

The reaction of methyl radicals with atomic and molecular oxygen was studied with a photoionization mass spectrometer. The methyl radicals were generated by reacting oxygen atoms with ethylene in a fast-flow tube reactor. The rate constant for the reaction of methyl radicals with oxygen atoms was (1.0 ± 0.2) × 10^?10 cm³/molec · sec with no significant variation with temperature over the range of 259–341°K. The reaction of methyl radicals with molecular oxygen involves both a two-body reaction, having a rate constant \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c} {k_{{\rm 3a}} = (10^{- 12.54 \pm 0.35})\exp [(- 940 \pm 250)T^{- 1}]} & {{\rm cm}^{\rm 3} /{\rm molec} \cdot {\rm sec}} \end{array}$\end{document} and a three-body recombination having a negative temperature dependence. The methyl peroxy radical could be observed at its steady-state concentration. The rate constants determined at low pressures are compatible with the values determined at higher pressures by flash photolysis. Formaldehyde appears to be a major product of the two-body reaction of CH₃ with O₂, and also of the reaction of CH₃O₂ with oxygen atoms.     

A novel inductively coupled plasma/selected-ion flow tube mass spectrometer for the study of reactions of atomic and atomic oxide ions

A novel inductively coupled plasma/selected-ion flow tube (ICP/SIFT) mass spectrometer has been constructed for the study of the kinetics and product distributions of reactions of atomic and atomic oxide ions with neutral molecules. The ICP essentially provides a universal source for atomic ions. The operation of the instrument is demonstrated with prototype reactivity and kinetic measurements.     

Surface reactions of molecular and atomic oxygen with carbon phosphide films

The surface reactions of atomic and molecular oxygen with carbon phosphide films have been studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Carbon phosphide films were produced by ion implantation of trimethylphosphine into polyethylene. Atmospheric oxidation of carbon phosphide films was dominated by phosphorus oxidation and generated a carbon-containing phosphate surface film. This oxidized surface layer acted as an effective diffusion barrier, limiting the depth of phosphorus oxidation within the carbon phosphide film to < 3 nm. The effect of atomic oxygen (AO) exposure on this oxidized carbon phosphide layer was subsequently probed in situ using XPS. Initially AO exposure resulted in a loss of carbon atoms from the surface, but increased the surface concentration of phosphorus atoms as well as the degree of phosphorus oxidation. For more prolonged AO exposures, a highly oxidized phosphate surface layer formed that appeared to be inert toward further AO-mediated erosion. By utilizing phosphorus-containing hydrocarbon thin films, the phosphorus oxides produced during exposure to AO were found to desorb at temperatures >500 K under vacuum conditions. Results from this study suggest that carbon phosphide films can be used as AO-resistant surface coatings on polymers.     

The reaction of atomic oxygen and hydrogen with nitric acid

The reactions have been studied in a discharge-flow system. Kinetic studies were made using resonance fluorescence for the measurement of atom concentrations. Based on the rates of atom loss, the following upper limits were obtained for the rate constants: Observed reaction in the H? HNO₃ system is at least partially due to an autocatalytic chain removal of both reactants. Diagnostic tests have suggested that OH, NO₂, and NO₃ are the chain carriers.     

A shock tube study of the reaction of methyl radicals with hydroxyl radicals

The reaction of CH₃ with OH has been studied near 1200 K and 1 atmosphere pressure in shock tube experiments in which UV absorption was used to monitor [OH]. A rate coefficient of (1.1 ± 0.3) × 10¹³ cm³/mol-s was measured for removal of OH by CH₃. This measured value is compared with previous experimental data and calculations. Several possible reaction channels are discussed, and although products were not monitored, it seems probable, on the basis of other work and theoretical estimates, that the primary mechanism (?75%) for the removal of OH by CH₃ at these conditions is their combination to form CH₃OH. Rate coefficients of (5.3 ± 0.8) × 10¹² and (9.0 ± 1.4) × 10¹² cm³/mol-s were measured for the reactions of OH with acetone and ethane, respectively, at the same temperature and pressure.     

A shock tube study of the reaction of the hydroxyl radical with propane

The rate coefficient for the reaction of the hydroxyl radical, OH, with propane has been measured at 1220 K in shock tube experiments, and a value of (1.58 ± 0.24) × 10¹³ cm³/mol s was obtained. This measured value is compared with previous experimental results and a transition-state theory calculation.     

A shock tube study of the pyrolysis of NO2

NO₂ concentration profiles in shock-heated NO₂/Ar mixtures were measured in the temperature range of 1350–2100 K and pressures up to 380 atm using Ar⁺ laser absorption at 472.7 nm, IR emission at 6.25±0.25 μm, and visible emission at 300–600 nm. In the course of this study, the absorption coefficient of NO₂ at 472.7 nm was measured at temperatures from 300 K to 2100 K and pressures up to 75 atm. Rate coefficients for the reactions NO₂+M→NO+O+M (1), NO₂+NO₂→2NO+O₂ (2a), and NO₂+NO₂→NO₃+NO (2b) were derived by comparing the measured and calculated NO₂ profiles. For reaction (1), the following low- and high-pressure limiting rate coefficients were inferred which describe the measured fall-off curves in Lindemann form within 15% [FORMULA] The inferred rate coefficient at the low- pressure limit, k_1o, is in good agreement with previous work at higher temperatures, but the energy of activation is lower by 20 kJ/mol than reported previously. The pressure dependence of k₁ observed in the earlier work of Troe [1] was confirmed. The rate coefficient inferred for the high pressure limit, k_1∞, is higher by a factor of two than Troe's value, but in agreement with data obtained by measuring specific energy-dependent rate coefficients. For the reactions (2a) and (2b), least-squares fits of the present data lead to the following Arrhenius expressions: [FORMULA] For reaction (2), the new data agree with previously recommended values of k_2a and k_2b, although the present study suggests a slightly higher preexponential factor for k_2a. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 483–493, 1997.     

Reaction of 1-substituted 3-aminoquinolinediones with isocyanic and isothiocyanic acid

1-Substituted-3-aminoquinoline-2,4(1H,3H)-diones react with potassium cyanate or potassium thiocyanate in boiling acetic acid to give ureido- or thioureidooxindoles, spiro-oxindoles and dihydroimidazoquinolones. However, if the starting compounds are substituted with a benzyl group at position 3, a C-debenzylation proceeds to give imidazoquinolones. According to a proposed reaction mechanism, a molecular rearrangement of the primarily formed mono-substituted urea or thiourea takes place. All compounds were characterized by ¹H, ¹³C and IR spectroscopy and MS data.     

A multi-dimensional microcanonical Monte Carlo study of S0 → T1 intersystem crossing of isocyanic acid

Science China Chemistry - A general formula for the multi-dimensional Monte Carlo microcanonical nonadiabatic rate constant expressed in configuration space is applied to calculate the rate of...     

High-temperature kinetics of the reactions of SO2 and SO3 with atomic oxygen

A nozzle-beam-skimmer sampling system is used to measure species concentration profiles for a lean one-dimensional premixed CO? O₂? Ar flame, into which small amounts of sulfur dioxide are introduced. The net formation rate for sulfur trioxide is obtained from the flux fraction profile for this species. The kinetic scheme is then utilized, along with the measured temperature profiles, to evaluate the rate coefficients k₁ and k₂ over the temperature range of 1435–1850 K. The most satisfactory agreement between the measured net formation rate for SO₃ and that calculated on the basis of reactions (1) and (2) is obtained with the rate coefficients Reactions (1) and (2) are found to be nearly balanced in a substantial region of the flame. Here the data are more sensitive to the difference in activation energies, as opposed to a particular value for either. Implications of this observation on the uncertainty of the deduced temperature dependence for each reaction are discussed, as are some of the procedures used in the data analysis.     

A theoretical analysis of the reaction between hydrogen atoms and isocyanic acid

Using BAC-MP4 potential-surface parameters, supplemented by an MP2 normal-mode analysis at one transition state, and statistical theoretical methods, we have computed thermal rate coefficients for the reactions, and Over the entire temperature range considered, 300 K < T < 3300 K, reaction (2) is the dominant product channel. The theoretical predictions are in excellent agreement with the experimental results available for k₂ and k_?1, the rate coefficient for the reverse of reaction (1). Modified Arrhenius expressions are given for k₁, k_?1, and k₂. In addition, we identify and discuss a weakness in utilizing a Hartree-Fock normal-mode analysis in the prediction of k₂. The present result for k₂ is much smaller than that used in the initial modeling of the RAPRENO_x process. The implications of this are discussed.     

Dissociation of multiply ionized isocyanic acid through electron impact

The dissociation of singly to triply ionized isocyanic acid (HNCO) has been investigated by two- and three-dimensional covariance mapping techniques through electron impact ionization at an electron energy of 200 eV. The absolute cross sections for the various dissociation channels of up to triply ionized HNCO have been measured. The HNCO dications dissociate mostly into ion pairs, while the HNCO trications dissociate mostly into ion triples, both through all the possible bond cleavages and charge allocations. Some major ion-pair dissociation channels of HNCO2+ are supposed to be sequential dissociation through initial charge separation. The metastable decay traces caused by HNCO(2+)-->H(+)+NCO+ and HNCO(+)-->HCO(+)+N have been observed on the covariance map.     

A shock tube and theoretical study on the pyrolysis of 1,4-dioxane

The dissociation of 1, 2 and 4% 1,4-dioxane dilute in krypton was studied in a shock tube using laser schlieren densitometry, LS, for 1550-2100 K with 56 ± 4 and 123 ± 3 Torr. Products were identified by time-of-flight mass spectrometry, TOF-MS. 1,4-dioxane was found to initially dissociate via C-O bond fission followed by nearly equal contributions from pathways involving 2,6 H-atom transfers to either the O or C atom at the scission site. The 'linear' species thus formed (ethylene glycol vinyl ether and 2-ethoxyacetaldehyde) then dissociate by central fission at rates too fast to resolve. The radicals produced in this fission break down further to generate H, CH(3) and OH, driving a chain decomposition and subsequent exothermic recombination. High-level ab initio calculations were used to develop a potential energy surface for the dissociation. These results were incorporated into an 83 reaction mechanism used to simulate the LS profiles with excellent agreement. Simulations of the TOF-MS experiments were also performed with good agreement for consumption of 1,4-dioxane. Rate coefficients for the overall initial dissociation yielded k(123Torr) = (1.58 ± 0.50) × 10(59) × T(-13.63) × exp(-43970/T) s(-1) and k(58Torr) = (3.16 ± 1.10) × 10(79) × T(-19.13) × exp(-51326/T) s(-1) for 1600 < T < 2100 K.     

A shock tube study of cyclopentane and cyclohexane ignition at elevated pressures

Ignition delay times for cyclopentane/air and cyclohexane/air mixtures were measured in a shock tube at temperatures of 847–1379 K, pressures of 11–61 atm, and equivalence ratios of ? = 1.0, 0.5, and 0.25. Ignition times were determined using electronically excited OH emission monitored through the shock tube endwall and piezoelectric pressure measurements made in the shock tube sidewall. The dependence of ignition time on pressure, temperature, and equivalence ratio is quantified and correlations for ignition time formulated. Measured ignition times are compared to kinetic modeling predictions from four recently published mechanisms. The data presented provide a database for the validation of cycloalkane kinetic mechanisms at the elevated pressures found in practical combustion engines. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 624–634, 2008