Determination Of Rate Coefficients Research Articles

Dynamic electrochemical impedance spectroscopy of quasi-reversible redox systems. Properties of the Faradaic impedance, and relations to those of voltammograms

By analysing the electrochemical impedance spectra (EIS) of quasi-reversible redox systems, the two elements of the Faradaic impedance: charge transfer resistance and the coupled Warburg-coefficient can be obtained at a given potential. The same applies also to DEIS (dynamic EIS) measurements, when high frequency impedance spectra are measured while the potential is scanned to simultaneously accomplish cyclic voltammetry or other transient measurements. In case of DEIS both the charge transfer resistance and the Warburg coefficient depend on the applied potential program, e.g. on scan-rate. A theory is presented, yielding a transformation by which this dependence can be eliminated. The proposed procedure yields two, scan-rate independent, hysteresis-free functions, which are closely related to the EIS results, and also to the functions which are the transformed forms of the cyclic voltammograms as suggested in T. Pajkossy, S. Vesztergom, Electrochim. Acta,297 (2019) 1121. To illustrate the properties of the transformations and the functions involved, numerical simulations are also presented. The theory opens a new route for the high-accuracy, fast determination of charge transfer rate coefficients of quasi-reversible redox systems by employing DEIS.

Anchor Effect in Polymerization Kinetics: Case of Monofunctionalized POSS

The effect of the anchoring group on the detailed polymerization kinetics was investigated using monomethacryloxy-heptaisobutyl POSS (1M-POSS). This compound was copolymerized with lauryl methacrylate (LM) as the base monomer, at various molar ratios. The process was initiated photochemically. The polymerization kinetics were followed by photo-DSC and photorheology while the polymers were characterized by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). For comparison, a methacrylate containing the branched siloxy-silane group (TSM) was also studied. It was found that the modifiers with a bulky substituent have a dual effect on the termination process: (i) At low concentrations, they increase the molecular mobility by increasing the free volume fraction, which leads to an acceleration of the termination and slows the polymerization; while (ii) at higher concentrations, they retard molecular motions due to the “anchor effect” that suppresses the termination, leading to acceleration of the polymerization. The anchor effect can also be considered from a different point of view: The possibility of anchoring a monomer with a long substituent (LM) around the POSS cage, which can further enhance propagation. These conclusions were derived based on kinetic results, determination of polymerization rate coefficients, and copolymer analysis.

Mechanism of surface reactions and dissolution of fluorite surface in an aqueous electrolyte solution

Environmental contextSolubility and dissolution rates of mineral surfaces depend on both the surface properties of the mineral and the composition of the aqueous solution. We investigated the link between the interfacial reactions and dissolution of a fluorite crystal. The study provides a detailed microscopic picture of the dissolution phenomena at the fluorite surface, and the results have wider application to general mineral dissolution processes taking place in the environment. AbstractDissolutions of the fluorite (111) crystallographic plane and fluorite (CaF2) colloidal particles were studied as a function of pH. The process was examined by measuring the concentration of released fluoride and calcium ions by ion-selective electrodes. Additionally, electrokinetic and inner surface potentials were measured by means of electrophoresis and a fluorite single crystal electrode respectively. The rate of fluorite dissolution was analysed assuming a reaction mechanism with a series of elementary steps, which included the reaction of surface groups with H+ ions, the formation of F− vacancies, the dissociation of surface groups and the release of calcium and fluoride ions into the interfacial region as well as the diffusion of ions from the interfacial region. The proposed reaction mechanism indicates that H+ ions play a necessary role in allowing the dissolution to take place, a concept not possible to confirm by looking at the overall equation of fluorite dissolution. The order of the total reaction with respect to H+ ions was found to be 0.37, which is in good accordance with the value derived from the reaction mechanism (1/3). The experimentally determined rate coefficient of fluorite dissolution was found to be kdis=9×10−6mol2/3dmm−2s−1.

Detection of PLP Structure for Accurate Determination of Propagation Rate Coefficients over an Enhanced Range of PLP-SEC Conditions

The factors influencing the periodic structure in molar mass distributions (MMDs) generated in pulsed-laser polymerization (PLP) experiments are investigated to extend the range of operating conditions under which radical polymerization propagation rate coefficients (kp) can be reliably estimated. Specifically, it is shown how kp may be determined well into conditions corresponding to the so-called low and high termination rate limits. A new parameter x is introduced to provide a convenient measure of when PLP pseudostationary conditions approach the low (x ≤ 0.2) and high (x ≥ 5.0) termination rate limits. In addition, a simple transformation is proposed to detect the PLP structure obscured by the background of the distribution under limiting experimental conditions, with simulations confirming that the methodology provides an estimate of kp with reasonable accuracy. The usefulness of the technique is then demonstrated through application to several experimental distributions. The influences of chain tra...

Analysis of voltammograms of quasi-reversible redox systems: Transformation to potential program invariant form

A simple procedure has recently been suggested (T. Pajkossy, Electrochem. Comm.90 (2018) 69) by which various types of voltammograms, above all cyclic voltammograms, pertaining to partially diffusion controlled charge transfer reactions can be analysed. Using this procedure, from voltammograms taken with varied scan-rates or other-than-triangular waveforms two scan-rate independent, hysteresis-free functions can be calculated. One of them is the diffusion-free polarization curve; the other the semiintegrated form of the reversible voltammograms. Here we show the underlying theory in details, along with numerical simulations to highlight important properties of the transformation. The theory opens a new route for the determination of charge-transfer rate coefficients of quasi-reversible redox systems.

CFCs replacements: Reactivity and atmospheric lifetimes of a series of Hydrofluoroolefins towards OH radicals and Cl atoms

A study of the temperature dependence of the gas-phase reactions of OH radicals and Cl atoms with a series of hydrofluroolefins: 2-fluoropropene (CH3CFCH2), hexafluoroisobutylene ((CF3)2CCH2) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CFCHF) has been performed. The relative-rate technique was used to determine rate coefficients over the temperature range 287–313 K.This work constitutes the first temperature dependence study of OH radicals and Cl atoms with 2-fluoropropene and (E/Z)-1,2,3,3,3-pentafluoropropene and for the reaction of Cl with hexafluoroisobutylene. Atmospheric implications are discussed with particular reference to the rate coefficients obtained as a function of the temperature.

Comparison of Potential Energy Surface and Computed Rate Coefficients for Dissociation

Comparisons are made between potential energy surfaces (PESs) for and collisions and between rate coefficients for dissociation that were computed using the quasi-classical trajectory (QCT) method on these PESs. For , Laganà’s empirical London–Eyring–Polanyi–Sato surface is compared with one from NASA Ames Research Center based on ab initio quantum chemistry calculations. For , two ab initio PESs (from NASA Ames and from the University of Minnesota) are compared. These use different methods for computing the ground state electronic energy for but give similar results. Thermal dissociation rate coefficients, for the 10,000–30,000 K temperature range, have been computed using each PES, and the results are in excellent agreement. Quasi-stationary state (QSS) rate coefficients using both PESs have been computed at these temperatures using the direct molecular simulation method (DMS) of Schwartzentruber and coworkers. The QSS rate coefficients are up to a factor of 5 lower than the thermal ones, and the thermal and QSS values bracket the results of shock-tube experiments. It is concluded that the combination of ab initio quantum chemistry PESs and QCT calculations provides an attractive approach for the determination of accurate high-temperature rate coefficients for use in aerothermodynamics modeling.

Gas‐phase rate coefficients for a series of alkyl cyclohexanes with OH radicals and Cl atoms

AbstractThe rate coefficients of the reactions of OH radicals and Cl atoms with three alkylcyclohexanes compounds, methylcyclohexane (MCH), trans‐1,4‐dimethylcyclohexane (DCH), and ethylcyclohexane (ECH) have been investigated at (293 ± 1) K and 1000 mbar of air using relative rate methods. A majority of the experiments were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC), a stainless steel chamber using in situ FTIR analysis and online gas chromatography with flame ionization detection (GC‐FID) detection to monitor the decay of the alkylcyclohexanes and the reference compounds. The studies were undertaken to provide kinetic data for calibrations of radical detection techniques in HIRAC. The following rate coefficients (in cm3 molecule−1 s−1) were obtained for Cl reactions: k(Cl+MCH) = (3.51 ± 0.37) × 10–10, k(Cl+DCH) = (3.63 ± 0.38) × 10−10, k(Cl+ECH) = (3.88 ± 0.41) × 10−10, and for the reactions with OH radicals: k(OH+MCH) = (9.5 ± 1.3) × 10–12, k(OH+DCH) = (12.1 ± 2.2) × 10−12, k(OH+ECH) = (11.8 ± 2.0) × 10−12. Errors are a combination of statistical errors in the relative rate ratio (2σ) and the error in the reference rate coefficient. Checks for possible systematic errors were made by the use of two reference compounds, two different measurement techniques, and also three different sources of OH were employed in this study: photolysis of CH3ONO with black lamps, photolysis of H2O2 at 254 nm, and nonphotolytic trans‐2‐butene ozonolysis. For DCH, some direct laser flash photolysis studies were also undertaken, producing results in good agreement with the relative rate measurements. Additionally, temperature‐dependent rate coefficient investigations were performed for the reaction of methylcyclohexane with the OH radical over the range 273‐343 K using the relative rate method; the resulting recommended Arrhenius expression is k(OH + MCH) = (1.85 ± 0.27) × 10–11 exp((–1.62 ± 0.16) kJ mol−1/RT) cm3 molecule−1 s−1. The kinetic data are discussed in terms of OH and Cl reactivity trends, and comparisons are made with the existing literature values and with rate coefficients from structure‐activity relationship methods. This is the first study on the rate coefficient determination of the reaction of ECH with OH radicals and chlorine atoms, respectively.

Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L2, L3) and Cyclic (D3, D4) Permethylsiloxanes.

Permethylsiloxanes are emitted into the atmosphere during production and use as personal care products, lubricants, and cleaning agents. The predominate atmospheric loss process for permethylsiloxanes is expected to be via gas-phase reaction with the OH radical. In this study, rate coefficients, k(T), for the OH radical gas-phase reaction with the two simplest linear and cyclic permethylsiloxanes were measured using a pulsed laser photolysis-laser induced fluorescence technique over the temperature range of 240-370 K and a relative rate method at 294 K: hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, L2), k1; octamethyltrisiloxane ([(CH3)3SiO]2Si(CH3)2, L3), k2; hexamethylcyclotrisiloxane ([-Si(CH3)2O-]3, D3), k3; and octamethylcyclotetrasiloxane ([-Si(CH3)2O-]4, D4), k4. The obtained k(294 K) values and temperature-dependence expressions for the 240-370 K temperature range are (cm3 molecule-1 s-1, 2σ absolute uncertainties): k1(294 K) = (1.28 ± 0.08) × 10-12, k1( T) = (1.87 ± 0.18) × 10-11 exp(-(791 ± 27)/ T); k2(294 K) = (1.72 ± 0.10) × 10-12, k2( T) = 1.96 × 10-13 (T/298)4.34 exp(657/ T); k3(294 K) = (0.82 ± 0.05) × 10-12, k3( T) = (1.29 ± 0.19) × 10-11 exp(-(805 ± 43)/ T); and k4(294 K) = (1.12 ± 0.10) × 10-12, k4( T) = (1.80 ± 0.26) × 10-11 exp(-(816 ± 43)/ T). The cyclic molecules were found to be less reactive than the analogous linear molecule with the same number of -CH3 groups, while the linear and cyclic permethylsiloxane reactivity both increase with the increasing number of CH3- groups. The present results are compared with previous rate coefficient determinations where available. The permethylsiloxanes included in this study are atmospherically short-lived compounds with estimated atmospheric lifetimes of 11, 8, 17, and 13 days, respectively.

A self-consistent, multivariate method for the determination of gas-phase rate coefficients, applied to reactions of atmospheric VOCs and the hydroxyl radical

Abstract. Gas-phase rate coefficients are fundamental to understanding atmospheric chemistry, yet experimental data are not available for the oxidation reactions of many of the thousands of volatile organic compounds (VOCs) observed in the troposphere. Here, a new experimental method is reported for the simultaneous study of reactions between multiple different VOCs and OH, the most important daytime atmospheric radical oxidant. This technique is based upon established relative rate concepts but has the advantage of a much higher throughput of target VOCs. By evaluating multiple VOCs in each experiment, and through measurement of the depletion in each VOC after reaction with OH, the OH + VOC reaction rate coefficients can be derived. Results from experiments conducted under controlled laboratory conditions were in good agreement with the available literature for the reaction of 19 VOCs, prepared in synthetic gas mixtures, with OH. This approach was used to determine a rate coefficient for the reaction of OH with 2,3-dimethylpent-1-ene for the first time; k = 5.7 (±0.3) × 10−11 cm3 molecule−1 s−1. In addition, a further seven VOCs had only two, or fewer, individual OH rate coefficient measurements available in the literature. The results from this work were in good agreement with those measurements. A similar dataset, at an elevated temperature of 323 (±10) K, was used to determine new OH rate coefficients for 12 aromatic, 5 alkane, 5 alkene and 3 monoterpene VOC + OH reactions. In OH relative reactivity experiments that used ambient air at the University of York, a large number of different VOCs were observed, of which 23 were positively identified. Due to difficulties with detection limits and fully resolving peaks, only 19 OH rate coefficients were derived from these ambient air samples, including 10 reactions for which data were previously unavailable at the elevated reaction temperature of T = 323 (±10) K.

RAFT Polymerization of Tert-Butyldimethylsilyl Methacrylate: Kinetic Study and Determination of Rate Coefficients.

Well-defined poly(tert-butyldimethylsilyl methacrylate)s (TBDMSMA) were prepared by the reversible addition-fragmentation chain transfer (RAFT) process using cyanoisopropyl dithiobenzoate (CPDB) as chain-transfer agents (CTA). The experimentally obtained molecular weight distributions are narrow and shift linearly with monomer conversion. Propagation rate coefficients (kp) and termination rate coefficients (kt) for free radical polymerization of TBDMSMA have been determined for a range of temperature between 50 and 80 °C using the pulsed laser polymerization-size-exclusion chromatography (PLP-SEC) method and the kinetic method via steady-state rate measurement, respectively. The CPDB-mediated RAFT polymerization of TBDMSMA has been subjected to a combined experimental and PREDICI modeling study at 70 °C. The rate coefficient for the addition reaction to RAFT agent (kβ1, kβ2) and to polymeric RAFT agent (kβ) is estimated to be approximately 1.8 × 104 L·mol−1·s−1 and for the fragmentation reaction of intermediate RAFT radicals in the pre-equilibrium (k-β1, k-β2) and main equilibrium (k-β) is close to 2.0 × 10−2 s−1. The transfer rate coefficient (ktr) to cyanoisopropyl dithiobenzoate is found to be close to 9.0 × 103 L·mol−1·s−1 and the chain-transfer constant (Ctr) for CPDB-mediated RAFT polymerization of TBDMSMA is about 9.3.

Temperature-dependent rate coefficients for the reactions of the hydroxyl radical with the atmospheric biogenics isoprene, alpha-pinene and delta-3-carene

Abstract. Pulsed laser methods for OH generation and detection were used to study atmospheric degradation reactions for three important biogenic gases: OH + isoprene (Reaction R1), OH +α-pinene (Reaction R2) and OH + Δ-3-carene (Reaction R3). Gas-phase rate coefficients were characterized by non-Arrhenius kinetics for all three reactions. For (R1), k1 (241–356 K) = (1.93±0.08) × 10−11exp{(466±12)∕T} cm3 molecule−1 s−1 was determined, with a room temperature value of k1 (297 K) = (9.3±0.4) × 10−11 cm3 molecule−1 s−1, independent of bath-gas pressure (5–200 Torr) and composition (M = N2 or air). Accuracy and precision were enhanced by online optical monitoring of isoprene, with absolute concentrations obtained via an absorption cross section, σisoprene = (1.28±0.06) × 10−17 cm2 molecule−1 at λ = 184.95 nm, determined in this work. These results indicate that significant discrepancies between previous absolute and relative-rate determinations of k1 result in part from σ values used to derive the isoprene concentration in high-precision absolute determinations.Similar methods were used to determine rate coefficients (in 10−11 cm3 molecule−1 s−1) for (R2)–(R3): k2 (238–357 K) = (1.83±0.04) × exp{(330±6)∕T} and k3 (235–357 K) = (2.48±0.14) × exp{(357±17)∕T}. This is the first temperature-dependent dataset for (R3) and enables the calculation of reliable atmospheric lifetimes with respect to OH removal for e.g. boreal forest springtime conditions. Room temperature values of k2 (296 K) = (5.4±0.2) × 10−11 cm3 molecule−1 s−1 and k3 (297 K) = (8.1±0.3) × 10−11 cm3 molecule−1 s−1 were independent of bath-gas pressure (7–200 Torr, N2 or air) and in good agreement with previously reported values. In the course of this work, 184.95 nm absorption cross sections were determined: σ = (1.54±0.08) × 10−17 cm2 molecule−1 for α-pinene and (2.40±0.12) × 10−17 cm2 molecule−1 for Δ-3-carene.

A method for retrieving char oxidation kinetic data from reacting particle trajectories in a novel test facility

In this paper, a new method for retrieving char combustion rates is presented. The method is based on an observation that the curvature of a trajectory of a freely falling and reacting char particle in a horizontal laminar flow changes due to the change of the particles mass. An experimental facility was designed and built allowing for recording reacting particle trajectories and determining oxidation kinetics. A brief description of the experimental facility and measurement procedure is given. In this procedure, a model of a freely falling particle in a laminar horizontal flow was developed. The model comprises a set of ordinary differential equations that predict a particle trajectory for given rate constants. The trajectory predicted by the model is then fitted to the measured trajectories by changing the rate constants. The best fit corresponds to the mean rate constant. Moreover, measurement results for Janina coal char combustion in air at various temperatures are presented. The key input parameters of the model are identified by a global sensitivity analysis, and the uncertainty in the results is quantified. The method is validated by comparison with TGA results obtained from the literature. Finally, the experimental data are used to determine rate coefficients of a kinetic-diffusion surface combustion model. The presented methodology is suitable to determine the kinetic data, however its further development is required in order to improve the accuracy of the measurements and include the more complex physico-chemical processes that occur during carbon conversion.

Rotationally inelastic collisions of excited NaK and NaCs molecules with noble gas and alkali atom perturbers.

We report measurements of rate coefficients at T ≈ 600 K for rotationally inelastic collisions of NaK molecules in the 2(A)1Σ+ electronic state with helium, argon, and potassium atom perturbers. Several initial rotational levels J between 14 and 44 were investigated. Collisions involving molecules in low-lying vibrational levels (v = 0, 1, and 2) of the 2(A)1Σ+ state were studied using Fourier-transform spectroscopy. Collisions involving molecules in a higher vibrational level, v = 16, were studied using pump/probe, optical-optical double resonance spectroscopy. In addition, polarization spectroscopy measurements were carried out to study the transfer of orientation in these collisions. Many, but not all, of the measurements were carried out in the "single-collision regime" where more than one collision is unlikely to occur within the lifetime of the excited molecule. The analysis of the experimental data, which is described in detail, includes an estimate of effects of multiple collisions on the reported rate coefficients. The most significant result of these experiments is the observation of a strong propensity for ΔJ = even transitions in collisions involving either helium or argon atoms; the propensity is much stronger for helium than for argon. For the initial rotational levels studied experimentally, almost all initial orientation is preserved in collisions of NaK 2(A)1Σ+ molecules with helium. Roughly between 1/3 and 2/3 of the orientation is preserved in collisions with argon, and almost all orientation is destroyed in collisions with potassium atoms. Complementary measurements on rotationally inelastic collisions of NaCs 2(A)1Σ+ with argon do not show a ΔJ = even propensity. The experimental results are compared with new theoretical calculations of collisions of NaK 2(A)1Σ+ with helium and argon. The calculations are in good agreement with the absolute magnitudes of the experimentally determined rate coefficients and accurately reproduce the very strong propensity for ΔJ = even transitions in helium collisions and the less strong propensity for ΔJ = even transitions in argon collisions. The calculations also show that collisions with helium are less likely to destroy orientation than collisions with argon, in agreement with the experimental results.

Uncertainty quantification of a newly optimized methanol and formaldehyde combustion mechanism

A detailed reaction mechanism for methanol combustion that is capable of describing ignition, flame propagation and species concentration profiles with high accuracy has been developed. Starting from a modified version of the methanol combustion mechanism of Li et al. (2007) and adopting the H2/CO base chemistry from the joint optimized hydrogen and syngas combustion mechanism of Varga et al. (2016), an optimization of 57 Arrhenius parameters of 17 important elementary reactions was performed, using several thousand indirect measurement data points, as well as direct and theoretical determinations of reaction rate coefficients as optimization targets. The final optimized mechanism was compared to 18 reaction mechanisms published in recent years, with respect to their accuracy in reproducing the available indirect experimental data for methanol and formaldehyde combustion. The utilized indirect measurement data, in total 24,900 data points in 265 datasets, include measurements of ignition delay times, laminar burning velocities and species profiles captured using a variety of experimental techniques. In addition to new best fit values for all rate parameters, the covariance matrix of the optimized parameters, which provides a quantitative description of the temperature-dependent ranges of uncertainty for the optimized rate coefficients, was calculated. These posterior uncertainty limits are much narrower than the prior limits in the temperature range for which experimental data are available. The uncertainty of the self-reaction of HȮ2 radicals and important H-atom abstraction reactions from the methanol molecule are discussed in detail.

Accurate Determination of Tunneling-Affected Rate Coefficients: Theory Assessing Experiment.

The thermal rate coefficients of a prototypical bimolecular reaction are determined on an accurate ab initio potential energy surface (PES) using ring polymer molecular dynamics (RPMD). It is shown that quantum effects such as tunneling and zero-point energy (ZPE) are of critical importance for the HCl + OH reaction at low temperatures, while the heavier deuterium substitution renders tunneling less facile in the DCl + OH reaction. The calculated RPMD rate coefficients are in excellent agreement with experimental data for the HCl + OH reaction in the entire temperature range of 200-1000 K, confirming the accuracy of the PES. On the other hand, the RPMD rate coefficients for the DCl + OH reaction agree with some, but not all, experimental values. The self-consistency of the theoretical results thus allows a quality assessment of the experimental data.

Kinetic Studies of Nitrate Radicals: Flash Photolysis at 193 nm

Nitrate radicals, NO3, were produced for the first time by 193 nm laser flash photolysis of N2O5 and HNO3. Detection was achieved due to NO3's strong absorption at 622.7 nm confirmed by measurements of the absorption spectrum in the range of 617–625 nm using both NO3 precursors. Time-resolved kinetic studies allowed the determination of room temperature rate coefficients for the reactions of NO3 with 2-methylbut-2-ene and NO2 of (1.28 ± 0.11) × 10−11 and (8.4 ± 1.2) × 10−13 cm3 molecule−1 s−1, respectively. The rate coefficients compare well to previous measurements with alternative techniques, suggesting that the reported method is valid and may be applied in follow-up studies. The rate coefficient for 2-methylbut-2-ene is compared to previous measurements and predictions for the alkene as well as the related alkenol. The new data are consistent with a previously suggested deactivation of the reactive site of the double bond if adjacent to an OH group. A calculated atmospheric lifetime for 2-methylbut-2-ene with respect to NO3-initiated oxidation of less than 3 min suggests predominant removal by NO3 in the atmosphere.

Direct and relative rate coefficients for the gas-phase reaction of OH radicals with 2-methyltetrahydrofuran at room temperature

2-Methyltetrahydrofuran (2-MTHF) is a renewable green solvent, a platform molecule for organic syntheses and a potential automotive fuel or fuel additive that can be produced efficiently from lignocellulosic biomass. Its reaction with OH radicals is important concerning both of its atmospheric fate and combustion. The kinetics of the overall reaction OH + 2-MTHF → products (1) was studied at room temperature (T = 298 ± 1 K) using both direct and relative kinetic methods. The low pressure fast discharge flow (DF) experiments coupled with resonance fluorescence detection of OH have provided the direct rate coefficient of k 1 (298 K, 2.64 mbar He) = (1.21 ± 0.14 (±2σ)) × 10−11 cm3 molecule−1 s−1. The relative-rate experiments with gas-chromatographic analysis were performed at higher pressure in synthetic air using the reaction OH + cyclohexane (c-C6H12) → products (2) as reference. The determined rate coefficient ratio k 1/k 2 = 3.80 ± 0.17 (±2σ) has been converted to k 1(298 K, 1030 mbar air) = (2.65 ± 0.55 (±2σ)) × 10−11 cm3 molecule−1 s−1. This k 1 value agrees reasonably well with the only other rate coefficient reported which was determined also at higher pressures. Comparison with the DF result indicates a possible pressure dependence of the reaction. Impact of 2-MTHF on the chemistry of the atmosphere has been briefly discussed.

Development of an Ethanol Combustion Mechanism Based on a Hierarchical Optimization Approach

A detailed reaction mechanism for ethanol combustion was developed for describing ignition, flame propagation, and species concentration profiles with high accuracy. Starting from a modified version of the ethanol combustion mechanism of Saxena and Williams (Proc. Combust. Inst. 2007, 31, 1149–1156) and adopting the H2/CO base chemistry from the joint optimized hydrogen and syngas combustion mechanism of Varga et al. (Int. J. Chem. Kinet. 2016, 48, 407–422), an optimization of 54 Arrhenius parameters of 16 important elementary C1/C2 reactions was performed using several thousand direct and indirect measurement data points as well as the results of theoretical determinations of reaction rate coefficients. The final optimized mechanism was compared to 16 reaction mechanisms that have been used for the simulation of ethanol combustion with respect to the accuracy in reproducing the available experimental data, including measurements of ignition delay times in shock tubes (444 data points in 39 data sets) and rapid compression machines (20/3), laminar burning velocity measurements (1011/124), and species profiles measured using flow reactors (1750/23), jet-stirred reactors (398/6) and shock tubes (8871/14). In addition to providing best fitted values for 54 Arrhenius parameters, the covariance matrix of the optimized parameters was calculated, which provides a description of the temperature-dependent ranges of uncertainty for each of the optimized rate coefficients.

Reactivity of OH and CH3OH Between 22 and 64 K: Modelling the Gas Phase Production of CH3O in Barnard 1B.

In the last years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than previously thought. One example is the reaction between OH and CH3OH, which has been recently found to be accelerated at low temperatures yielding CH3O as main product. This finding opened the question of whether the CH3O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH3OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH3O with little success. Here we report for the first time rate coefficients for the gas-phase reaction of OH and CH3OH down to a temperature of 22 K, very close to those in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis-laser induced fluorescence technique were used to determine rate coefficients in the temperature range 22-64 K. The temperature dependence obtained in this work can be expressed as k(22-64 K) = (3.6 ± 0.1) × 10-12(T/300 K)-(1.0±0.2) cm3 molecule-1 s-1. Implementing this expression in a chemical model of a cold dense cloud results in CH3O/CH3OH abundance ratios similar or slightly lower than the value of ∼ 3 × 10-3 observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CH3OH is an important contributor to the formation of interstellar CH3O. The role of grain-surface processes in the formation of CH3O, although it cannot be fully neglected, remains controversial.