Pressure dependence of the reaction Cl + C2H2 over the temperature range 230 to 370 K

The pressure dependence of reaction (1), Cl + C₂H₂ + M → C₂H₂Cl + M, has been measured by a relative rate technique using the pressure independent abstraction reaction (2), Cl + C₂H₆ → C₂H₅ + HCl, as the reference. Values of k₁/k₂ were measured at pressures between 25 and 1300 torr at four temperatures ranging from 252 to 370 K, using air, N₂, or SF₆ diluent gases. Low pressure measurements (10–50 torr) were performed at 230 K. Assuming a temperature-independent center broadening factor of 0.6 in the Troe formalism and using the established value of k₂, these data can be used to determine the temperature dependent high and low pressure limiting rate constants over the range of conditions studied in air for reaction (1): k_∞(1) = 2.13 × 10^?10 (T/300)^?1.045 cm³/molecule-s; and k₀(1) = 5.4 × 10^?30 (T/300)^?2.09 cm⁶/molecule²-s. Use of these expressions yields rate constants with an estimated 20% accuracy including uncertainty in the reference reaction. The data indicate that the rate constant for a typical stratospheric condition at 30 km altitude is approximately 50% of that previously estimated.     

Characteristics of Argon Laminar DC Plasma Jet at Atmospheric Pressure

Argon DC plasma jets in stable laminar flow were generated at atmospheric pressure with a specially designed torch under carefully balanced generating conditions. Compared with turbulent jets of short length with expanded radial appearance and high working noise, the laminar jet could be 550 mm in length with almost unchanged diameter along the whole length and very low noise. At gas feeding rate of 120 cm³/s, the jet length increases with increasing arc current in the range of 70–200 A, and thermal efficiency decreases slightly at first and then leveled off. With increasing gas flow rate, thermal efficiency of the laminar jets increases and could reach about 40%, when the arc current is kept at 200 A. Gauge pressure distributions of the jets impinging on a flat plate were measured. The maximum gauge pressure value of a laminar jet at low gas feeding rate is much lower than that of a turbulent jet. The low pressure acting on the material surface is favorable for surface cladding of metals, whereas the high pressure associated with turbulent jets will break down the melt pool.     

A kinetic analysis of the photolysis of mixtures of acetone and propylene

The photolysis of mixtures of acetone and propylene at 308 nm has been studied kinetically from approximately 300 K to 580 K. Rate constants were calculated for the reactions at total pressures ranging from 110 torr to 750 torr and for [C₃H₆]/[(CH₃)₂CO] ratios in the range 0.03 to 3.3. No dependence of the rate constants on total pressure or on this concentration ratio could be detected within this range of conditions. The temperature dependence of the rate constants is given by © 1994 John Wiley & Sons, Inc.     

Theoretical study of the reaction of ethynyl radical with acetonitrile

A detailed computational study has been performed on the mechanism and kinetics of the C₂H + CH₃CN reaction. The geometries were optimized at the BHandHLYP/6–311G(d, p) level. The single-point energies were calculated using the BMC-CCSD, MC-QCISD and QCISD(T)/6–311+G(2df, 2pd) methods. Five mechanisms were investigated, namely, direct hydrogen abstraction, C-addition/elimination, N-addition/elimination, C₂H–to–CN substitution and H-migration. The kinetics of the title reaction were studied using TST and multichannel RRKM methodologies over a wide range of temperatures (150–3,000 K) and pressures (10^?4–10⁴ torr). The total rate constants show positive temperature dependence and pressure independence. At lower temperatures, the C-addition step is the most feasible channel to produce CH₃ and HCCCN. At higher temperatures, the direct hydrogen abstraction path is the dominant channel leading to C₂H₂ and CH₂CN. The calculated overall rate constants are in good agreement with the experimental data.     

Mechanistic modeling of the wall reactions in the pyrolysis of pentachloroethane

The thermal dehydrochlorination C₂HCl₅ → C₂Cl₄ + HCl has been studied in a static system between 565 and 645 K at pressures ranging from 5 to 21 torr. The course of the reaction was followed by measuring the pressure rise in the conditioned quartz reaction vessel and by analyzing the products by gas chromatography. The observed experimental results and data from the literature for flow systems can be explained quantitatively in terms of a radical reaction model involving heterogeneous chain initiation and termination steps. The rate constants have been deduced for reactions of Cl, Cl₂, and C₂HCl₅ over reactor walls covered with a pyrolytic carbon film and for reactions of adsorbed Cl atoms. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 322–330, 2002     

Lyman-α absorption photometry at high pressure and atom density kinetic results for H recombination

Atomic absorption and fluorescence spectrophotometry have been routinely used in kinetic investigations as probes of relative, rather than absolute, atom concentration. The calibration of a Lyman-α photometer for measurement of absolute hydrogen atom concentrations at levels [H] ι ≤ 1.8 × 10¹⁴ atoms/cm² and total pressure of 1.5 torr He is described. The photometer is characterized in terms of a two-level emission source and an absorption region in which only Doppler broadening of the transition is considered. The modifications due to pressure broadening by high pressures (500 ≤ P ≤ 1500 torr) in the absorption region are discussed in detail. Application of the technique is reported for the recombination of hydrogen atoms in the presence of six nonreactive heat bath gases. Experiments were performed in a static reaction cell at pressures of 500–1500 torr of heat bath gas, and hydrogen atoms were produced by Hg (³P₁) photosensitization of H₂. The technique is critically evaluated and the mechanistic implications of the hydrogen atom recombination results are examined. The measured room temperature recombination rate constants in H₂, He, Ne, Ar, Kr, and N₂ are 8.5 ± 1.2, 6.9 ± 1.5, 5.9 ± 1.5, 8.0 ± 0.8, 10.2 ± 0.9, and 9.6 ± 1.4, respectively, where the units are 10³³ cm⁶/molec² · sec.     

An investigation of temperature and pressure dependence of the reaction of CF2ClO2 radicals with nitrogen dioxide by flash photolysis and time resolved mass spectrometry

Rate coefficients of the termolecular reaction were determined over the temperature range of 248–324 K and at pressures from 1 to 10 torr by time resolved mass spectrometry. CF₂ClO₂ radicals were generated by flash photolysis of CF₂ClBr in the presence of oxygen. Their rate of decay was measured by following (CF₂O₂)⁺ fragment ions formed in the ion source. With 2 to 40 mtorr of NO₂ present the dominant removal pathway is addition to form the peroxynitrate. The third order rate coefficients are wholly within the falloff over the experimental pressure range, and have a negative temperature coefficient. Rate constants for the reverse reaction, the unimolecular dissociation of CF₂ClO₂NO₂, D. Koppenkastrop and F. Zabel, Int. J. Chem. Kinet., 23 , 1 (1991) were employed to calculate equilibrium constants, from which a 600 torr value of the combination rate coefficient was obtained at each temperature. From the temperature dependence of the equilibrium constants, the average enthalpy of reaction over the experimental temperature range was found to be 106.1 ± 0.3 kJ/mol. Extrapolation of the combination rate constants to lower and higher pressures, and estimates of low and high pressure limiting rate coefficients, k₀ and k_∞ were done by nonlinear least squares fit of the experimental data, using the empirical F_c equations developed by Troe.     

Reactions of the copper dimer,Cu2, in the gas phase

Reactions of Cu₂ with several small molecules have been studied in the gas phase, under thermalized conditions at room temperature, in a fast-flow reactor. They fall into one of two categories. Cu₂ does not react with O₂, N₂O, N₂, H₂, and CH₄ at pressures up to 6 torr. This implies bimolecular rate constants of less than 5 × 10^?15 cm³ s^?1 at 6 torr He. Cu₂ reacts with CO, NH₃, C₂H₄, and C₃H₆ in a manner characteristic of association reactions. Second-order rate constants for all four of these reagents are dependent on total pressure. The reactions with CO, NH₃, and C₂H₄ are in their low pressure limit at up to 6 torr He buffer gas pressure. The reaction with C₃H₆ begins to show fall-off behavior at pressures above 3 torr. Limiting low-pressure, third-order rate constants are 0.66 ± 0.10, 8.8 ± 1.2, 9.3 ± 1.4, and 85 ± 15 × 10^?30 cm⁶ s^?1 in He for CO, NH₃, C₂H₄, and C₃H₆, respectively. Modeling studies of these rate constants imply that the association complexes are bound by at least 20 kcal mol^?1 in the case of C₂H₄ and C₃H₆ and at least 25 kcal mol^?1 in the other cases. The implications of these results for Cu-ligand bonding are developed in comparison with existing work on the interactions of these ligands with Cu atoms, larger clusters, and surfaces. © 1994 John Wiley & Sons, Inc.     

The Photochemistry of Diazomethane the Reaction of Methylene with Ammonia and Methylamine

In the photolysis of diazomethane in the presence of ammonia, the ratio of the products, [CH₃NH₂/[C₂H₄], has been studied as a function of pressure, temperature and composition of diazomethane-ammonia mixture, respectively. This ratio increases with increase in total pressure at low pressures and approaches a constant at high pressures. In the diazomethane-methylamine system, the ratio of products, [CH₃CH₂NH₂]/[(CH₃)₂NH], is independent of the pressure in the range from 23 to 500 torr. The rates of insertion of singlet methylene into C–H bond and N–H bond of CH₃NH₂ are found to be about the same.     

Kinetics of the reaction CH + N2 rarr; [M]⇒[M] products in the range 10–620 torr and 298–1059 K

The gas-phase reaction of CH(X² Π) radicals with molecular nitrogen was studied in the temperature range 298–1059 K at total pressures between 10 and 620 torr. CH radicals were generated by excimer laser photolysis of CHCIBr₂ at 248 nm and were detected by laser-induced fluorescence. The investigated reaction shows a strong temperature and pressure dependence. At pressures of 20, 100, and 620 torr the Arrhenius plots exhibit a strong decrease of the rate constant with increasing temperature. The rate constant is well described by, with E₀ in kJ/mol. The pressure dependence was studied at temperatures of 298, 410, 561, and 750 K. The rate constants for each temperature were fitted by the Troe formalism. From the calculated values of k₀ and k_infinity, the Arrhenius expressions, were obtained with E₀ (k₀) and E_A (k_infinity) in units of kJ/mol. Within the range of 298–750 K the temperature dependence of the broadening factor is well described by F_c = 0.029 + (173.3/T). © 1996 John Wiley & Sons, Inc.     

Thermal decomposition of ethanol. 4. Ab initio chemical kinetics for reactions of H atoms with CH3CH2O and CH3CHOH radicals

The potential energy surfaces of H-atom reactions with CH(3)CH(2)O and CH(3)CHOH, two major radicals in the decomposition and oxidation of ethanol, have been studied at the CCSD(T)/6-311+G(3df,2p) level of theory with geometric optimization carried out at the BH&HLYP/6-311+G(3df,2p) level. The direct hydrogen abstraction channels and the indirect association/decomposition channels from the chemically activated ethanol molecule have been considered for both reactions. The rate constants for both reactions have been calculated at 100-3000 K and 10(-4) Torr to 10(3) atm Ar pressure by microcanonical VTST/RRKM theory with master equation solution for all accessible product channels. The results show that the major product channel of the CH(3)CH(2)O + H reaction is CH(3) + CH(2)OH under atmospheric pressure conditions. Only at high pressure and low temperature, the rate constant for CH(3)CH(2)OH formation by collisonal deactivation becomes dominant. For CH(3)CHOH + H, there are three major product channels; at high temperatures, CH(3)+CH(2)OH production predominates at low pressures (P < 100 Torr), while the formation of CH(3)CH(2)OH by collisional deactivation becomes competitive at high pressures and low temperatures (T < 500 K). At high temperatures, the direct hydrogen abstraction reaction producing CH(2)CHOH + H(2) becomes dominant. Rate constants for all accessible product channels in both systems have been predicted and tabulated for modeling applications. The predicted value for CH(3)CHOH + H at 295 K and 1 Torr pressure agrees closely with available experimental data. For practical modeling applications, the rate constants for the thermal unimolecular decomposition of ethanol giving key accessible products have been predicted; those for the two major product channels taking place by dehydration and C-C breaking agree closely with available literature data.     

The influence of flow rate and pressure on the excitation conditions in low-pressure microwave induced plasmas

Using a double probe technique, electron temperatures and electron concentrations together with spectral line intensities have been measured in low-pressure microwave induced plasmas at various pressures and flow rates of monoatomic and polyatomic support gases. For two distinct pressures viz. 0.2 and 1.0 torr (0.267–1.333 mbar) the flow rate has been independently varied.

Measurements of spectral-line intensities in the absence and presence of the probes demonstrate that the probes exert little or no influence upon the plasma conditions. The results show that when low-pressure microwave induced plasmas in flow systems are applied for quantitative analytical purposes exact specification of both flow rate and pressure of the carrier gas is required.     

The reactions of atomic hydrogen and active nitrogen with hydrogen azide

Detection of atoms by mass spectrometry has been used to study the reactions of hydrogen azide, HN₃, with H atoms and active nitrogen, in a fast flow reactor at pressures of about 1 torr. Stoichiometry and products of the H + HN₃ reaction have been determined and the rate constant of the initial step, assumed to be H + HN₃ → NH₂ + N₂, was found to be 2.54 × 10^?11 exp (?4600/RT) cm³ molecule^?1 s^?1, in the temperature range of 300–460K. The formation of NH₃ and H₂ products has been discussed from the different secondary steps which may occur in the mechanism. For the reaction of active nitrogen with HN₃, evidence has been found for the participation of excited nitrogen molecules produced by a microwave discharge through molecular nitrogen. The influence of excited nitrogen molecules has been reduced by lowering the gas flow velocity. It was then possible to study the N + HN₃ reaction for which the rate constant of the initial step was found to be 4.9 × 10^?15 cm³ molecule^?1 s^?1 at room temperature. Finally, the occurrence of these elementary reactions has been discussed in the mechanism of the decomposition flame of HN₃.     

Rate constants for the reaction of atomic and molecular bromine with gallium arsenide

The reaction of the (100) face of a gallium arsenide single crystal with atomic and molecular bromine has been studied in a discharge flow system at temperatures between 100 and 225°C and pressures between 0.1 and 40 torr. The reaction with Br₂ was found to be first order in Br₂ only at pressures below 1 torr. Temperature dependence studies in the linear range gave the activation energy and preexponential factor for the rate controlling reaction in the low pressure regime. The results are summarized in the following Arrhenius equation: Deviations from linearity at high pressures are discussed in terms of two alternative mechanisms. The reaction of GaAs with atomic bromine was also studied as a function of temperature, and found to have a temperature dependence described by the following Arrhenius equation:      

Study of the HNO + HNO and HNO + NO reactions by intracavity laser spectroscopy

Flash photolysis of CH₃CHO and H₂CO in the presence of NO has been investigated by the intracavity laser spectroscopy technique. The decay of HNO formed by the reaction HCO + NO → HNO + CO was studied at NO pressures of 6.8–380 torr. At low NO pressure HNO was found to decay by the reaction HNO + HNO → N₂O + H₂O. The rate constant of this reaction was determined to be k₁ = (1.5 ± 0.8) × 10^?15 cm³/s. At high NO pressure the reaction HNO + NO → products was more important, and its rate constant was measured to be k₂ = (5 ± 1.5) × 10^?19 cm³/s. NO₂ was detected as one of the products of this reaction. Alternative mechanisms for this reaction are discussed.     

Diffusion of hydrogen atoms in spherical vessels

A previously developed model for active species concentration profiles in infinite cylindrical systems has been extended to include the spherical system. The model couples the processes of diffusion to and reaction at the wall. Predictions of time buildup under conditions of homogeneous production by light, and time decay after extinguishing the light source, are made for H atoms. Such predictions require a knowledge of the wall recombination coefficient and the binary diffusion coefficient for H in heat bath gas. The model is experimentally tested by measuring the first-order decay constants of H at room temperature in various pressures (10-1500 torr) of six heat bath gases. The atomic concentration is monitored by Lyman-α absorption photometry. The results show good agreement with model predictions in the various heat bath gases up to ～400 torr and depend only on one parameter,γ, the recombi-nation coefficient. This should be contrasted with the earlier work where slight variation in γ was invoked. The rate constants at pressures higher than 400 torr are consistently higher than model predictions.     

Experimental measurements and kinetic modeling of CO/H2/O2/NOx conversion at high pressure

This paper presents results from lean CO/H₂/O₂/NO_x oxidation experiments conducted at 20–100 bar and 600–900 K. The experiments were carried out in a new high‐pressure laminar flow reactor designed to conduct well‐defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H₂/O₂, CO/CO₂, and NO_x chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS‐QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO₂ ? H₂O + O₂, CO + OH ? [HOCO] ? CO₂ + H, HOCO + OH ? CO + H₂O₂, NO₂ + H₂ ? HNO₂ + H, NO₂ + HO₂ ? HONO/HNO₂ + O₂, and HNO₂(+M) ? HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H₂/O₂, H₂/O₂/NO₂, and CO/H₂O/O₂ at 780–1100 K and 1–10 bar. Moreover, introduction of the reaction CO + H₂O₂ → HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454–480, 2008     

A comprehensive kinetic mechanism for CO,CH2O,and CH3OH combustion

New experimental profiles of stable species concentrations are reported for formaldehyde oxidation in a variable pressure flow reactor at initial temperatures of 850–950 K and at constant pressures ranging from 1.5 to 6.0 atm. These data, along with other data published in the literature and a previous comprehensive chemical kinetic model for methanol oxidation, are used to hierarchically develop an updated mechanism for CO/H₂O/H₂/O₂, CH₂O, and CH₃OH oxidation. Important modifications include recent revisions for the hydrogen–oxygen submechanism (Li et al., Int J Chem Kinet 2004, 36, 565), an updated submechanism for methanol reactions, and kinetic and thermochemical parameter modifications based upon recently published information. New rate constant correlations are recommended for CO + OH = CO₂ + H ( R23 ) and HCO + M = H + CO + M ( R24 ), motivated by a new identification of the temperatures over which these rate constants most affect laminar flame speed predictions (Zhao et al., Int J Chem Kinet 2005, 37, 282). The new weighted least‐squares fit of literature experimental data for ( R23 ) yields k₂₃ = 2.23 × 10⁵T^1.89exp(583/T) cm³/mol/s and reflects significantly lower rate constant values at low and intermediate temperatures in comparison to another recently recommended correlation and theoretical predictions. The weighted least‐squares fit of literature results for ( R24 ) yields k₂₄ = 4.75 × 10¹¹T^0.66exp(?7485/T) cm³/mol/s, which predicts values within uncertainties of both prior and new (Friedrichs et al., Phys Chem Chem Phys 2002, 4, 5778; DeSain et al., Chem Phys Lett 2001, 347, 79) measurements. Use of either of the data correlations reported in Friedrichs et al. (2002) and DeSain et al. (2001) for this reaction significantly degrades laminar flame speed predictions for oxygenated fuels as well as for other hydrocarbons. The present C₁/O₂ mechanism compares favorably against a wide range of experimental conditions for laminar premixed flame speed, shock tube ignition delay, and flow reactor species time history data at each level of hierarchical development. Very good agreement of the model predictions with all of the experimental measurements is demonstrated. © 2007 Wiley Periodicals, Inc. 39: 109–136, 2007     

Chemical ionization over a wide range of pressures. 1. Mass spectra of dimethyl esters of maleic and fumaric acids

Conclusions By using chemical ionization over a wide range of pressures, from 0.01 torr to atmospheric pressure, and also by selecting the reagent gas, different mass spectra of isomers can be obtained, which are suitable for their reliable identification.Under ionization conditions at atmospheric pressure in helium (reagent ions [H(H₂O)_n]⁺), peaks of cluster ions [MGH]⁺ and [2M+1]⁺ are observed in the spectrum of dimethyl fumarate, which are absent in the case of the cis-isomer.Under the conditions of chemical ionization at a normal pressure (0.4-0.2 torr) of the reagent gases Me₃CH, n-C₇H₁₆ and at an ionic source temperature of 50°C, a stereospecific fragmentation of dimethyl maleate [MH]⁺-MeOH is observed, which is absent in the case of the the trans-isomer.In the chemical ionization spectra at reduced pressure of the reagent gases MeOH, EtOH, i-PrOH (0.01 torr), a peak of the cluster ion [MGH]⁺ is observed for dimethyl fumarate, which is absent in the spectra of the cis-isomer.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 815–819, April, 1985.     

Kinetics of the reactions Br + NO2 + M and I + NO2+ M

The reactions Br + NO₂ + M → BrNO₂ + M (1) and I + NO₂ + M → INO₂ + M (2) have been studied at low pressure (0.6-2.2 torr) at room temperature and with helium as the third body by the discharge-flow technique with EPR and mass spectrometric analysis of the species. The following third order rate constants were found k₁₍₀₎ = (3.7 ± 0.7) × 10^?31 and k₂₍₀₎ = (0.95 ± 0.35) × 10^?31 (units are cm⁶ molecule^?2 s^?1). The secondary reactions X + XNO₂ → X₂ + NO₂ (X = Br, I) have been studied by mass spectrometry and their rate constants have been estimated from product analysis and computer modeling.