Radical Decomposition Research Articles

A theoretical investigation of decomposition and reactivity of the atmospheric C3F7OCH2O radical

The present theoretical investigation involves the ab initio quantum mechanical study of the decomposition and reactivity mechanism of the C3F7OCH2O radical that is formed from HFE-7000. Geometry of reactants, products and transition states were optimized at B3LYP and B3PW91 levels of theory with 6-311G(d,p) basis set. Five important pathways for decomposition and reactivity of C3F7OCH2O were investigated: reaction with atmospheric O2, reaction with atmospheric OH radical, C–O bond cleavage, H elimination and the migration of hydrogen from carbon to oxygen and then C–O bond cleavage with energy barriers of 4.3, 12.6, 17.1, 20.0 and 32.4kcalmol−1, respectively. Rate constants were calculated by utilizing the canonical transition state theory (CTST) in the range of 200–400K and the Arrhenius diagrams have been plotted. From the obtained results, it was concluded that reaction with atmospheric O2 is a dominant pathway for the consumption of the C3F7OCH2O radical in the atmosphere. Intrinsic reaction coordinate (IRC) calculation was performed to confirm the existence of transition state on the corresponding potential energy surface.

Pyrolysis Pathways of Sulfonated Polyethylene, an Alternative Carbon Fiber Precursor

Polyethylene is an emerging precursor material for the production of carbon fibers. Its sulfonated derivative yields ordered carbon when pyrolyzed under inert atmosphere. Here, we investigate its pyrolysis pathways by selecting n-heptane-4-sulfonic acid (H4S) as a model compound. Density functional theory and transition state theory were used to determine the rate constants of pyrolysis for H4S from 300 to 1000 K. Multiple reaction channels from two different mechanisms were explored: (1) internal five-centered elimination (Ei5) and (2) radical chain reaction. The pyrolysis of H4S was simulated with kinetic Monte Carlo (kMC) to obtain thermogravimetric (TGA) plots that compared favorably to experiment. We observed that at temperatures <550 K, the radical mechanism was dominant and yielded the trans-alkene, whereas cis-alkene was formed at higher temperatures from the internal elimination. The maximum rates of % mass loss became independent of initial ȮH radical concentration at 440-480 K. Experimentally, the maximum % mass loss occurred from 440 to 460 K (heating rate dependent). Activation energies derived from the kMC-simulated TGAs of H4S (26-29 kcal/mol) agreed with experiment for sulfonated polyethylene (~31 kcal/mol). The simulations revealed that in this region, decomposition of radical HOSȮ2 became competitive to α-H abstraction by HOSȮ2, making ȮH the carrying radical for the reaction chain. The maximum rate of % mass loss for internal elimination was observed at temperatures >600 K. Low-scale carbonization utilizes temperatures <620 K; thus, internal elimination will not be competitive. E(i)5 elimination has been studied for sulfoxides and sulfones, but this represents the first study of internal elimination in sulfonic acids.

Ab Initio Kinetics for the Decomposition of Hydroxybutyl and Butoxy Radicals ofn-Butanol

The decomposition kinetics of the hydroxybutyl and butoxy radicals (C4H9O) arising via H abstraction from n-butanol were studied theoretically with ab initio transition-state-theory-based master equation analyses. Stationary points on the C4H9O potential energy surface were calculated at either the RQCISD(T)/CBS//B3LYP/6-311++G(d,p) level or the RQCISD(T)/CBS//CASPT2/aug-cc-pVDZ level. Unimolecular pressure- and temperature-dependent rate coefficients were calculated over broad ranges of temperature (300-2500 K) and pressure (1.3 × 10(-3) to 10(2) atm) by solving the time-dependent multiple-well master equation. The "well merging" phenomenon was observed and analyzed for its influence on the branching ratios and rate coefficients. The theoretical predictions were compared with the available experimental and theoretical data and any discrepancies were analyzed. The predicted rate coefficients are represented with forms that may readily be used in combustion modeling of n-butanol.

Degradation of Diclofenac by Advanced Oxidation and Reduction Processes: Kinetic Studies, Degradation Pathways and Toxicity Assessments

Many pharmaceutical compounds and metabolites are found in surface and ground waters suggesting their ineffective removal by conventional wastewater treatment technologies. Advanced oxidation/reduction processes (AO/RPs), which utilize free radical reactions to directly degrade chemical contaminants, are alternatives to traditional water treatment. This study reports the absolute rate constants for reaction of diclofenac sodium and model compound (2, 6-dichloraniline) with the two major AO/RP radicals: the hydroxyl radical (•OH) and hydrated electron (eaq−). The bimolecular reaction rate constants (M−1 s−1) for diclofenac for •OH was (9.29 ± 0.11) × 109, and for e−aq was (1.53 ± 0.03) ×109. To provide a better understanding of the decomposition of the intermediate radicals produced by hydroxyl radical reactions, transient absorption spectra are observed from 1 – 250 μs. In addition, preliminary degradation mechanisms and major products were elucidated using 60Co γ-irradiation and LC-MS. The toxicity of products was evaluated using luminescent bacteria. These data are required for both evaluating the potential use of AO/RPs for the destruction of these compounds and for studies of their fate and transport in surface waters where radical chemistry may be important in assessing their lifetime.

Kinetics of Peroxide Vulcanization of Natural Rubber

This study was undertaken to optimize the vulcanization conditions and explore the effect of residual peroxide in the peroxide vulcanization of natural rubber. The study was followed through the kinetics of the vulcanization reaction at various temperatures viz. 150,155,160 and 165°C. Dicumyl peroxide (DCP) was used as the crosslinking agent. The Monsanto Rheometer was used to investigate the different crosslinking stages and vulcanization kinetics. The thermal decomposition of peroxide followed a first order free radical decomposition reaction. Half-lives at various temperatures were determined. The percentage of residual peroxide was calculated from the cure kinetic data. The effect of residual peroxide on mechanical properties was studied at various peroxide levels and also by extending the cure time (from t90 to t95 and then to t100). Mechanical properties such as tensile strength, elongation at break, modulus and compression set (70 and 100°C) were measured. Excess peroxide was found to cause a high compression set at elevated temperature and the cure time was selected to achieve minimum residual peroxide in the product. Results indicate that peroxide concentration is the dominant factor controlling the crosslink density and hence the properties of the vulcanizates.

Thermal decomposition of CH3CHO studied by matrix infrared spectroscopy and photoionization mass spectroscopy

A heated SiC microtubular reactor has been used to decompose acetaldehyde and its isotopomers (CH(3)CDO, CD(3)CHO, and CD(3)CDO). The pyrolysis experiments are carried out by passing a dilute mixture of acetaldehyde (roughly 0.1%-1%) entrained in a stream of a buffer gas (either He or Ar) through a heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 50-200 Torr with the SiC tube wall temperature in the range 1200-1900 K. Characteristic residence times in the reactor are 50-200 μs after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. The reactor has been modified so that both pulsed and continuous modes can be studied, and results from both flow regimes are presented. Using various detection methods (Fourier transform infrared spectroscopy and both fixed wavelength and tunable synchrotron radiation photoionization mass spectrometry), a number of products formed at early pyrolysis times (roughly 100-200 μs) are identified: H, H(2), CH(3), CO, CH(2)=CHOH, HC≡CH, H(2)O, and CH(2)=C=O; trace quantities of other species are also observed in some of the experiments. Pyrolysis of rare isotopomers of acetaldehyde produces characteristic isotopic signatures in the reaction products, which offers insight into reaction mechanisms that occur in the reactor. In particular, while the principal unimolecular processes appear to be radical decomposition CH(3)CHO (+M) → CH(3) + H + CO and isomerization of acetaldehyde to vinyl alcohol, it appears that the CH(2)CO and HCCH are formed (perhaps exclusively) by bimolecular reactions, especially those involving hydrogen atom attacks.

Thermal decomposition of 1-pentyl radicals at high pressures and temperatures

Shock-tube studies at the National Institute of Standards and Technology (NIST) and the University of Illinois at Chicago (UIC) have been used to examine the decomposition of 1-pentyl radicals in argon between (833–1130)K and (100–5000)kPa. High pressure limiting values of the product branching ratios appear to be approached at the highest pressures studied. Results agree well with a ‘best-fit’ model previously developed [1] and are consistent with an energy transfer value 〈ΔEdown〉=(675±100)cm−1 at 1000K.

Mechanistic insight into the cyclohexene epoxidation with VO(acac)2 and tert-butyl hydroperoxide

The epoxidation reaction of cyclohexene is investigated for the catalytic system vanadyl acetylacetonate (VO(acac)2) with tert-butyl hydroperoxide (TBHP) as oxidant with the aim to identify the most active species for epoxidation and to retrieve insight into the most plausible epoxidation mechanism. The reaction mixture is composed of various inactive and active complexes in which vanadium may either have oxidation state+IV or+V. Inactive species are activated with TBHP to form active complexes. After reaction with cyclohexene, each active species transforms back into an inactive complex that may be reactivated again. The reaction mixture is quite complex containing hydroxyl, acetyl acetonate, acetate, or a tert-butoxide anion as ligands, and thus, various ligand exchange reactions may occur among active and inactive complexes. Also, radical decomposition reactions allow transforming V+IV to V+V species. To obtain insight into the most abundant active complexes, each of previous transformation steps has been modeled through thermodynamic equilibrium steps. To unravel the nature of the most plausible epoxidation mechanism, first principle chemical kinetics calculations have been performed on all proposed epoxidation pathways. Our results allow to conclude that the concerted Sharpless mechanism is the preferred reaction mechanism and that alkylperoxo species V+IVO(L)(OOtBu) and V+VO(L1)(L2)(OOtBu) species are most abundant. At the onset of the catalytic cycle, vanadium+IV species may play an active role, but as the reaction proceeds, reaction mechanisms that involve vanadium+V species are preferred as the acetyl acetonate is readily oxidized. Additionally, an experimental IR and kinetic study has been performed to give a qualitative composition of the reaction mixture and to obtain experimental kinetic data for comparison with our theoretical values. The agreement between theory and experiment is satisfactory.

Radiation-induced destruction of hydroxyl-containing amino acids and dipeptides

The yields of molecular products resulting from radiolysis of hydroxyl-containing amino acids and dipeptides under various conditions were determined. The possibility of a new radiation-induced destruction pathway has been shown for serine and threonine, as well as for the dipeptides having residues of these amino acids at the N-terminal part of the respective molecule. This process includes formation of N-centered radicals from the starting molecules followed by their decomposition with elimination of side substituents. On radiolysis, serine and threonine were also shown to undergo free-radical destruction to form acetaldehyde and acetone, respectively. A mechanism has been proposed including consecutive stages of fragmentation of α-hydroxyl-containing carbon-centered radicals with elimination of ammonia and decomposition of the secondary radicals with elimination of CO2. The yields of CO2 obtained on radiolysis of serine and threonine were significantly higher (except for solutions at pH 12) than those for alanine and valine, which have no hydroxyl groups in their structures. The obtained data indicate that the hydroxyl-containing amino acids occupy a special place among other amino acids as regards the variety of radiation-induced reactions which they may undergo due to their structural features.

Flame studies of C2 hydrocarbons

The oxidation characteristics of C2 hydrocarbons were revisited in flames established in the counterflow configuration. Laminar flame speeds of ethane/air, ethylene/air, and acetylene/oxygen/nitrogen mixtures as well as extinction strain rates of non-premixed ethane/air flames were measured using digital particle image velocimetry. The experiments were modeled using three different kinetic models. While the experimental and computed laminar flame speeds agreed closely for all C2 hydrocarbons under fuel-lean conditions, notable discrepancies were identified under fuel-rich conditions. Using the computed flame structures, insight was provided into the controlling mechanisms that could be responsible for the observed discrepancies. More specifically, the uncertainties associated with the kinetics of the thermal decomposition of the ethyl radical were found to be a potential source of the observed discrepancies for ethane flames. It was shown also by using alternative rate constants for the ethyl radical decomposition, the rate of flame propagation and the extinction propensity are affected notably. Furthermore, the values of the branching ratio of acetylene consumption reactions involving atomic oxygen were found to have a significant effect on the propagation of rich acetylene flames.

The decomposition of 2-pentyl and 3-pentyl radicals

The isomerization and decomposition reactions of 2-pentyl and 3-pentyl radicals have been studied in a single-pulse shock tube over a temperature range of 973–1121K and pressures of 120–800kPa. The results represent the first direct study of the alkene product branching ratio resulting from the kinetics of the competition between isomerization and beta C–C bond scission for a secondary straight-chain alkyl radical at high temperatures. Such species are representative of intermediates important in the combustion of typical hydrocarbon fuels. In the present work, a small quantity of precursor (∼45μL/L) is used to thermally generate H atoms in the presence of excess (E)-2-pentene, leading to the radicals of interest via addition of H to the double bond. Decomposition of the chemically activated pentyl radicals results in the stable olefin products ethene, propene, and 1-butene, which are detected in postshock gas chromatographic analyses utilizing flame-ionization and mass-spectrometric detection. It is shown that the olefin product ratios can be related to the isomerization and decomposition reactions of the 2-pentyl and 3-pentyl radicals and the results are consistent with the existence of distinct non-overlapping cracking patterns for the two radicals. The data are compared with predictions made on the basis of a model developed from experiments on the decomposition of thermal (i.e. not chemically activated) 1-pentyl radicals. Good agreement is observed. In conjunction with an RRKM/Master Equation analysis, the results for 2-pentyl and 3-pentyl radicals are projected over a wide range of temperatures. In addition, the rate constants for addition of H atoms to the alternate double bond positions of (E)-2-pentene are derived relative to a standard reaction and absolute rate constants for these processes are reported.

Side-chain degradation of perfluorosulfonic acid membranes: An ab initio study

Abstract

Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550–700 K) oxidation chemistry of isobutanol and tert-butanol

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4–5.1Torr) experiments at 550 and 700K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O2 by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the α-hydroxyalkyl radical with O2 is predominantly linked to chain-terminating formation of HO2. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O2 with β-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO2 elimination is not possible in the β-hydroxyalkyl+O2 reaction because of the absence of a beta C–H bond; this channel is available in the β-hydroxyalkyl+O2 reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700K.

N2O production rate of an enriched ammonia-oxidising bacteria culture exponentially correlates to its ammonia oxidation rate

The relationship between the ammonia oxidation rate (AOR) and nitrous oxide production rate (N2OR) of an enriched ammonia-oxidising bacteria (AOB) culture was investigated. The AOB culture was enriched in a nitritation system fed with synthetic anaerobic digester liquor. The AOR was controlled by adjusting the dissolved oxygen (DO) and pH levels and also by varying the initial ammonium (NH4+) concentration in batch experiments. Tests were also performed directly on the parent reactor where a stepwise decrease/increase in DO was implemented to alter AOR. The experimental data indicated a clear exponential relationship between the biomass specific N2OR and AOR. Four metabolic models were used to analyse the experimental data. The metabolic model formulated based on aerobic N2O production from the decomposition of nitrosyl radical (NOH) predicted the exponential correlation observed experimentally. The experimental data could not be reproduced by models developed on the basis of N2O production through nitrite (NO2−) and nitric oxide (NO) reduction by AOB.

Modulation of H2O2 production in vitro by low level magnetic fields

BackgroundIn this paper, we present data on the influence of low level magnetic fields (0.5 μT‐2 μT) on the modulation of hydrogen peroxide (H2O2) biogenesis in human fibrosarcoma cancer cell line HT1080, and bovine pulmonary artery endothelial cells (PAEC) compared to cells exposed to geomagnetic field (control; 45 μT‐60 μT).MethodsThe extracellular quantification of the production of H2O2 may be a direct measure of the production and decomposition of superoxide radical. H2O2 can freely pass through cell membranes and can be detected outside the cell. For the quantification of H2O2 we used a method that has been successfully applied before. This method involves the oxidation of Amplex Red by H2O2 in the presence of horseradish peroxidase (HRP). Tri‐axial Helmholtz coils established the control static magnetic field. Low level fields were implemented by a μ–metal cylinder. The background time‐varying magnetic field was measured by induction with a sensor comprised of a set of square perpendicular coils.ResultsReduction of the Earth's magnetic field suppressed H2O2 production in cancer cells and PAEC. The addition of catalase and superoxide dismutase (SOD) mimetic MnTBAP inhibited the magnetic field effect on H2O2 production.ConclusionBecause most cancer cells undergo increased oxidative stress and ROS production due to excessive metabolic requirements to support an increased rate of cell division, it is reasonable to consider investigating a therapeutic that might target cancer cells through a free radical mechanism. Whether radio frequency exposure can influence growth rates and H2O2 production remains open and we propose to investigate it in the future. Modulating ROS production by magnetic fields may open new venues of biomedical research and therapeutic strategies.

Shock Tube Explorations of Roaming Radical Mechanisms: The Decompositions of Isobutane and Neopentane

The thermal decompositions of isobutane and neopentane have been studied using both shock tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate constants for these molecules have been measured at high temperatures (1260-1566 K) behind reflected shock waves using high-sensitivity H-ARAS detection. The two major dissociation channels at high temperature are iso-C(4)H(10) → CH(3) + i-C(3)H(7) (1a) and neo-C(5)H(12) → CH(3) + t-C(4)H(9) (2a). Ultrahigh-sensitivity ARAS detection of H-atoms produced from the rapid decomposition of the product radicals, i-C(3)H(7) in (1a) and t-C(4)H(9) in (2a), through i-C(3)H(7) + M → H + C(3)H(6) + M (3a) and t-C(4)H(9) + M → H + i-C(4)H(8) + M (4a) allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, which were observed to be equivalent in both systems, k(1a)/k(total) and k(2a)/k(total) = 0.79 ± 0.05. Theoretical analyses indicate that in isobutane, the non-H-atom fraction has two contributions, the dominant fraction being due to the roaming radical mechanism leading to molecular products through iso-C(4)H(10) → CH(4) + C(3)H(6) (1b) with k(1b)/k(total) = 0.16, and a minor fraction that involves the isomerization of i-C(3)H(7) to n-C(3)H(7) that then subsequently forms methyl radicals, i-C(3)H(7) + M → n-C(3)H(7) + M → CH(3) + C(2)H(4) + M (3b). In contrast to isobutane, in neopentane, the contribution to the non-H-atom fraction is exclusively through the roaming radical mechanism that leads to neo-C(5)H(12) → CH(4) + i-C(4)H(8) (2b) with k(2b)/k(total) = 0.21. These quantitative measurements of larger contributions from the roaming mechanism for larger molecules are in agreement with the qualitative theoretical arguments that suggest long-range dispersion interactions (which become increasingly important for larger molecules) may enhance roaming.

Oxidation of Human Growth Hormone by Oxygen-Centered Radicals: Formation of Leu-101 Hydroperoxide and Tyr-103 Oxidation Products

Human growth hormone (hGH) was exposed to oxygen-centered radicals generated through the thermolysis of AAPH in the presence of dioxygen. Such conditions mimic oxidative processes which protein pharmaceuticals can encounter during formulation in the presence of polysorbates. We detected the oxidation of Met to Met sulfoxide, the formation of protein carbonyls, the oxidation of Tyr to dityrosine and several additional Tyr oxidation products, the conformation-dependent oxidation of Trp, and the site-specific formation of protein hydroperoxides. The sensitivity of Met oxidation correlates with their solvent accessible surface, i.e. the yields of MetSO decreased in the order Met-14 > Met-125 > Met-170. Trp oxidation in native hGH was negligible, but was enhanced through denaturation. Dityrosine formed predominantly intramolecularly but did not contribute significantly to protein cross-linking. Hydroperoxides formed selectively on Leu-101 and were generated specifically by alkoxyl radicals, generated through the decomposition of peroxyl radicals. Tyr-103 was converted into a series of oxidation products characterized by mass shifts of Tyr + 14 Da and Tyr + 16 Da.

Experimental and modeling study of the effects of adding oxygenated fuels to premixed n-heptane flames

The effects of methanol, dimethoxymethane (DMM), and dimethylcarbonate (DMC) on laminar premixed low pressure (30Torr) n-heptane flames were investigated by using synchrotron photoionization and molecular-beam mass spectrometry (PI–MBMS) techniques. The overall C/O ratio was maintained constant (0.507) and the equivalence ratio was kept around 1.6 for all the tested flames. The composition of unburned mixtures was adjusted such that the post-flame temperatures were nearly equivalent for all the test conditions. Mole fraction profiles of major and intermediate species were derived and compared among the flames. Parallel computations were performed based on a modified model, and the predicted concentrations of flame species agree reasonably well with the measured results. Early production of CO2 was observed in the DMC-doped flame. Reaction flux analysis suggested that it was caused by the decomposition of CH3OCO radical, DMC molecule and CH3OCOO radical. As oxygenated fuels were added, the concentrations of most C1C5 hydrocarbon intermediates were reduced while that of benzene (C6H6) also decreased apparently, and the extent of benzene reduction showed little difference among the oxygenate-doped flames. Reaction flux analysis indicated that, in all the tested flames, the primary pathway leading from small aliphatics to C6H6 was through C3+C3 reactions, including the self-recombination reaction of propargyl radical (C3H3) and the cross reaction between C3H3 and allyl radical (a-C3H5). Considering that the temperatures of the tested flames were almost equivalent, the reduction of C6H6 concentration when doped with oxygenated fuels should be resulted from the reduced concentrations of its precursors. Furthermore, concentrations of certain oxygenated intermediates were also examined. The concentration of formaldehyde (CH2O) was found to increase when flames were doped with oxygenated fuels, while those of acetaldehyde (CH3CHO) and vinyl alcohol (C2H3OH) were nearly equivalent for all the flames. Methyl formate (CH3OCHO) was detected only in the DMM-doped flame, which was attributed to the efficient CH3OCHO formation pathway through the decomposition of CH3OCHOCH3 radical in the flame.

Pressure Dependence and Branching Ratios in the Decomposition of 1-Pentyl Radicals: Shock Tube Experiments and Master Equation Modeling

The decomposition and intramolecular H-transfer isomerization reactions of the 1-pentyl radical have been studied at temperatures of 880 to 1055 K and pressures of 80 to 680 kPa using the single pulse shock tube technique and additionally investigated with quantum chemical methods. The 1-pentyl radical was generated by shock heating dilute mixtures of 1-iodopentane and the stable products of its decomposition have been observed by postshock gas chromatographic analysis. Ethene and propene are the main olefin products and account for >97% of the carbon balance from 1-pentyl. Also produced are very small amounts of (E)-2-pentene, (Z)-2-pentene, and 1-butene. The ethene/propene product ratio is pressure dependent and varies from about 3 to 5 over the range of temperatures and pressures studied. Formation of ethene and propene can be related to the concentrations of 1-pentyl and 2-pentyl radicals in the system and the relative rates of five-center intramolecular H-transfer reactions and β C-C bond scissions. The 3-pentyl radical, formed via a four-center intramolecular H transfer, leads to 1-butene and plays only a very minor role in the system. The observed (E/Z)-2-pentenes can arise from a small amount of beta C-H bond scission in the 2-pentyl radical. The current experimental and computational results are considered in conjunction with relevant literature data from lower temperatures to develop a consistent kinetics model that reproduces the observed branching ratios and pressure effects. The present experimental results provide the first available data on the pressure dependence of the olefin product branching ratio for alkyl radical decomposition at high temperatures and require a value of <ΔE(down)(1000 K)> = (675 ± 100) cm(-1) for the average energy transferred in deactivating collisions in an argon bath gas when an exponential-down model is employed. High pressure rate expressions for the relevant H-transfer reactions and β bond scissions are derived and a Rice Ramsberger Kassel Marcus/Master Equation (RRKM/ME) analysis has been performed and used to extrapolate the data to temperatures between 700 and 1900 K and pressures of 10 to 1 × 10(5) kPa.

Determination of rate parameters of cyclohexane and 1-hexene decomposition reactions

Peukert et al. recently published (Int. J. Chem. Kinet. 2010; 43: 107–119) the results of a series of shock tube measurements on the thermal decomposition of cyclohexane (c-C6H12) and 1-hexene (1-C6H12). The experimental data included 16 and 23 series, respectively, of H-atom profiles measured behind reflected shock waves by applying the ARAS technique (temperature range 1250–1550 K, pressure range 1.48–2.13 bar). Sensitivity analysis carried out at the experimental conditions revealed that the rate coefficients of the following six reactions have a high influence on the simulated H-atom profiles: R1: c-C6H12 = 1-C6H12, R2: 1-C6H12 = C3H5 + C3H7, R4: C3H5 = aC3H4 + H; R5: C3H7 = C2H4 + CH3; R6: C3H7 = C3H6 + H; R8: C3H5 + H = C3H6. The measured data of Peukert et al. were re-analysed together with the measurement results of Fernandes et al. (J. Phys. Chem. A 2005; 109: 1063–1070) for the rate coefficient of reaction R4, the decomposition of allyl radicals. The optimization resulted in the following Arrhenius parameters: R1: A = 2.441 × 1019, E/R = 52,820; R2: A = 3.539 × 1018, E/R = 42,499; R4: A = 8.563 × 1019, n = −3.665, E/R = 13,825 (high pressure limit); R4: A = 7.676 × 1031n = −3.120, E/R = 40,323 (low pressure limit); R5: A = 3.600 × 1012, E/R = 10699; R6: A = 1.248 × 1017, E/R = 28,538; R8: A = 6212 × 1013, E/R = −970. The rate parameters above are in cm3, mol, s, and K units. Data analysis resulted in the covariance matrix of all these parameters. The standard deviations of the rate coefficients were converted to temperature dependent uncertainty parameter f(T). These uncertainty parameters were typically f = 0.1 for reaction R1, f = 0.1–0.3 for reaction R2, below 0.5 for reaction R8 in the temperature range of 1250–1380 K, and above 1 for reactions R4–R6.