Limiting Rate Coefficients Research Articles

Bimolecular kinetics of Criegee intermediate (CH2OO) with 2-pentanone: experimental and theoretical analysis in atmospheric conditions.

Temperature-dependent kinetics of Criegee intermediate (CH2OO) with 2-pentanone were performed at 258-318 K and 50 Torr using pulsed laser photolysis-cavity ring-down spectroscopy (PLP-CRDS) technique. The measured room temperature rate coefficient was (3.84 ± 0.24) × 10-13 cm3 molecule-1 s-1. The reaction follows a negative temperature dependency, and the corresponding Arrhenius equation is k4 = (1.76 ± 0.36) × 10-15exp{(3.18 ± 0.11) kcal mol-1/RT}. The high-P limit rate coefficients obtained using CVT/SCT at CCSD(T)/aug-cc-pVTZ//B3LYP/6-311 + G(2df,2p) level of theory deviate from the experimental results, especially in the low-temperature region. At 258 K, the computed high-P limit rate coefficient exceeded the experimental value by more than a factor of four. Comparing all the reaction pathways, HCOOH was the major product formed in the title reaction. The atmospheric lifetime of 2-pentanone due to its reaction with CH2OO was calculated to be ~ 3000 days, rendering the reaction insignificant for inclusion in models of atmospheric 2-pentanone.

Reaction between Criegee Intermediate CH2OO and Isobutyraldehyde: Kinetics and Atmospheric Implications

AbstractIn the present work, we have investigated the 1,3‐cycloaddition reaction of isobutyraldehyde ((CH3)2CHCHO) with the simplest Criegee intermediate CH2OO using the OH laser‐induced fluorescence (LIF) method in a reaction tube. The experiments were conducted at temperatures and pressures ranging from 280 to 338 K and 5 to 150 Torr. We found that the bimolecular rate coefficient of the reaction is temperature‐ and pressure‐dependent. The measured rate coefficients at 100 Torr are (4.93±0.89), (4.46±0.80), (3.99±0.72), (3.56±0.64), (3.34±0.60), (2.98±0.54), (2.78±0.50)×10−12 cm3 molecule−1 s−1 at 280, 288, 298, 308, 318, 328, and 338 K, respectively. A high‐pressure limit rate coefficient of (4.47±0.80)×10−12 cm3 molecule−1 s−1 was obtained at 298 K by fitting the pressure‐dependent rate coefficients according to the Lindemann mechanism. The Arrhenius plot of the temperature‐dependent rate coefficients results in an activation energy of (−1.86±0.33) kcal mol−1 and a preexponential factor of (1.72±0.31)×10−13 cm3 molecule−1 s−1. In the atmosphere, the reaction of CH2OO with isobutyraldehyde may contribute to the sink of CH2OO in regions where isobutyraldehyde emission is intense under low temperature and low relative humidity conditions.

The gas-phase pyrolysis of cyclopropylamine. A computational study on the kinetics and reaction mechanism

Small amines represent an accessible model to understand the reactivity of the nitrogenous components of biomass during combustion and pyrolysis processes. In this work, the kinetics and mechanism of the gas-phase pyrolysis of cyclopropylamine, c-C3H5NH2, have been studied by ab initio and density functional calculations. From the derived information high-pressure limit rate coefficients were estimated using the transition state theory over the 400 – 1400 K temperature range. The results support a stepwise process, in which the initial ring-opening stage determines the rate through competitive direct, biradical and carbene mechanisms to give 1-aminopropene, CH3CH=CHNH2, and 1-propanimine, CH3CH2CH=NH, as reactive intermediates. The Arrhenius equation obtained at the G3(MP2)//B3LYP/6–311++G(3df,3pd) level of theory, log10 (koverall,∞/s-1) = (15.47 ± 0.11) – (57.68 ± 0.32) kcal mol-1 (2.303 RT)-1, agrees very well with the experimental expression determined by Parry and Robinson. According to these findings, the carbenic pathway is most important up to ∼500 K, above which the biradical mechanism predominates. Subsequent reaction steps include the addition of the CH3CH=CHNH2 and CH3CH2CH=NH species to another c-C3H5NH2 molecule to produce c-C3H5NHCH(NH2)CH2CH3, which afterwards decomposes into c-C3H5N=CHCH2CH3 and NH3. The results also reveal relatively high electronic barriers for the interconversion between the geometric isomers of CH3CH=CHNH2 and CH3CH2CH=NH, suggesting that the contribution of these processes is negligible. Further calculations of the title reaction in the presence of selected aliphatic amines RNH2 (R = Me, Et,n-Pr, i-Pr, allyl) confirm that the unimolecular fission of the ring is the rate determining step.

Theoretical Study of Intramolecular H-Migration Reactions of Peroxyl Radicals of JP-10 (Exo-Tetrahydrodicyclopentadiene)

ABSTRACT Intramolecular H-migration reactions of peroxyl radicals of JP-10 (C10H15OO•) play an important role in developing the oxidation mechanism of JP-10 at low temperature. However, accurate rate coefficients for these reactions are lacking in the literature. Therefore, 33 H-migration reactions of C10H15OO•, which belong to six series of reactions, were investigated. Quantum chemical computations at the CBS-QB3 level reveal that C10H15OO• do not obey the rule that the energy barrier of 1,4-H migration reaction is higher than that of 1,5-H migration reaction because of the special steric cyclic structure of C10H15OO• and the ring strain in the transition state. The transition state theory and Rice-Ramsberger-Kassel-Marcus/Master-Equation theory were used to obtain high-pressure limit rate coefficients and pressure-dependent rate coefficients, respectively. The results show that H-migration reactions of C10H15OO• have obvious pressure-dependent effect. The most competitive reaction was obtained among 33 reactions. Moreover, branching ratios of each series of reactions were used as weight ratios to lump this series of reactions. Six series of reactions were lumped as six lumped reactions by the isomer lumping method. This will provide accurate rate coefficients for constructing a mechanism with the low number of species and suitability for computational fluid dynamics simulation.

Automated reaction kinetics and network exploration (Arkane): A statistical mechanics, thermodynamics, transition state theory, and master equation software

AbstractThe open‐source statistical mechanics software described here, Arkane–Automated Reaction Kinetics and Network Exploration–facilitates computations of thermodynamic properties of chemical species, high‐pressure limit reaction rate coefficients, and pressure‐dependent rate coefficient over multi‐well molecular potential energy surfaces (PES) including the effects of collisional energy transfer on phenomenological kinetics. Arkane can use estimates to fill in information for molecules or reactions where quantum chemistry information is missing. The software solves the internal energy master equation for complex unimolecular reaction systems. Inputs to the software include converged electronic structure computations performed by the user using a variety of supported software packages (Gaussian, Molpro, Orca, TeraChem, Q‐Chem, Psi4). The software outputs high‐pressure limit rate coefficients and pressure‐dependent phenomenological rate coefficients, as well as computed thermodynamic properties (enthalpy, entropy, and constant pressure heat capacity) with added energy corrections. Some of the key features of Arkane include treatment of 1D, 2D or ND hindered internal rotation modes, treatment of free internal rotation modes, quantum tunneling effect consideration, transition state theory (TST) and Rice‐Ramsperger‐Kassel‐Marcus (RRKM) rate coefficient computations, master equation solution with four implemented methods, inverse‐Laplace transform of high‐pressure limit rate coefficients into the energy domain, energy corrections based on bond‐additivity or isodesmic reactions, automated and efficient PES exploration, and PES sensitivity analysis. The present work describes the design of Arkane, how it should be used, and refers to the theory that it employs. Arkane is distributed via the RMG‐Py software suite (https://github.com/ReactionMechanismGenerator/RMG‐Py).

Experimental, theoretical, and astrochemical modelling investigation of the gas-phase reaction between the amidogen radical (NH2) and acetaldehyde (CH3CHO) at low temperatures.

The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH2 with acetaldehyde (CH3CHO) has been performed. Experiments were carried out using laser-flash photolysis and laser-induced fluorescence spectroscopy to create and monitor the temporal decay of NH2 in the presence of CH3CHO. Low temperatures relevant to the interstellar medium were achieved using a pulsed Laval nozzle expansion. Rate coefficients were measured over the temperature and pressure range of 29-107 K and 1.4-28.2 × 1016 molecules per cm3, with the reaction exhibiting a negative temperature dependence and a positive pressure dependence. The yield of CH3CO from the reaction has also been determined at 67.1 and 35.0 K, by observing OH produced from the reaction of CH3CO with added O2. Ab initio calculations of the potential energy surface (PES) were combined with Rice-Rampsberger-Kessel-Marcus (RRKM) calculations to predict rate coefficients and branching ratios over a broad range of temperatures and pressures. The calculated rate coefficients were shown to be sensitive to the calculated density of states of the stationary points, which in turn are sensitive to the inclusion of hindered rotor potentials for several of the vibrational frequencies. The experimentally determined rate coefficients and yields have been used to fit the calculated PES, from which low-pressure limiting rate coefficients relevant to the ISM were determined. These have been included in a single-point dark cloud astrochemical model, in which the reaction is shown to be a potential source of gas-phase CH3CO radicals under dark cloud conditions.

Pressure Dependence of Rate Coefficients of Unimolecular and Chemical Activation Reactions Connected to the Potential Energy Wells of Si2H2Cl4, Si2Cl6, and Si2Cl4 via Rice-Ramsperger-Kassel-Marcus Calculations.

Rate coefficients for elementary reactions connected to the potential energy wells of Si2H2Cl4, Si2Cl6, and Si2Cl4, which are important Si2 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through the Rice-Ramsperger-Kassel-Marcus theory under various conditions of temperature and pressure. The optimized structures and vibrational frequencies of the reactants, products, and transition state were obtained using (U)B3LYP/6-31+G(d,p), and the single-point energies of the optimized structures were recalculated using the coupled cluster method with single and double excitations plus triple perturbation (U)CCSD(T) with complete basis set extrapolation. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall-off regime under subatmospheric to moderately high-pressure conditions so that it is expected that accurate modeling of the gas phase in the chlorosilane CVD reactor requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high-pressure limit rate coefficients. The rate coefficients determined here were expressed through Chebyshev coefficients and also modified Arrhenius parameters to be used in simulations of systems under a wide range of temperature and pressure conditions.

A theoretical kinetic study of the reactions of NH2 radicals with methanol and ethanol and their implications in kinetic modeling

AbstractAmino (NH2) radicals play a central role in the pyrolysis and oxidation of ammonia. Several reports in the literature highlight the importance of the reactions of NH2 radicals with fuel in NH3‐dual‐fuel combustion. Therefore, we investigated the reactions of NH2 radicals with methanol (CH3OH) and ethanol (C2H5OH) theoretically. We explored the various reaction pathways by exploiting CCSD(T)/cc‐pV(T, Q)Z//M06‐2X/aug‐cc‐pVTZ level of theory. The reaction proceeds via complex formation at the entrance and exit channels in an overall exothermic process. We used canonical transition state theory to obtain the high‐pressure limiting rate coefficients for various channels over the temperature range of 300–2000 K. We discerned the role of various channels in the potential energy surface (PES) of NH2 + CH3OH/C2H5OH reactions. For both reactions, the hydrogen abstraction pathway at the OH‐site of alcohols plays a minor role in the entire T‐range investigated. By including the title reactions into an extensive kinetic model, we demonstrated that the reaction of NH2 radicals with alcohols plays a paramount role in accurately predicting the low‐temperature oxidation kinetics of NH3‐alcohols dual fuel systems (e.g., shortening the ignition delay time). On the contrary, these reactions have negligible importance for high‐temperature oxidation kinetics of NH3‐alcohol blends (e.g., not affecting the laminar flame speed). In addition, we calculated the rate coefficients for NH2 + CH4 = CH3 + NH3 reaction that are in excellent agreement with the experimental data.

Rate Coefficients for OH + NO (+N2) in the Fall-off Regime and the Impact of Water Vapor

The termolecular, association reaction between OH and NO is a source of nitrous acid (HONO), an important atmospheric trace gas. Rate coefficients for the title reaction as recommended by evaluation panels differ substantially at the temperatures and pressures that prevail in the Earth’s boundary layer where the reaction is in the fall-off regime between low- and high-pressure limiting rate coefficients. Using pulsed laser methods for generation and detection of OH, we have reinvestigated the kinetics of the title reaction at pressures of 22–743 Torr (1 Torr = 1.333 hPa) and temperatures (273, 298, and 333 K) in pure N2 and in N2–H2O bath gases. In situ optical absorption measurements were used to rule out any bias due to NO2 or HONO impurities. Our rate coefficients (k1) in N2 bath gas are parametrized in terms of low-pressure (k0) and high-pressure (k∞) rate coefficients and a fall-off parameter (FC) with k1,0N2 = 7.24 × 10–31 (T/300 K)−2.17 cm6 molecule–2 s–1, k1,∞ = 3.3 × 10–12 (T/300 K)−0.3 cm3 molecule–1 s–1, and FC = 0.53. Used with the “Troe” expression for termolecular reactions, these parameters accurately reproduce the current data in the fall-off regime and also capture literature rate coefficients at extrapolated temperatures. The presence of water vapor was found to enhance the rate coefficients of the title reaction significantly. The low-pressure limiting rate coefficient in H2O bath gas is a factor 5–6 larger than in N2, at room temperature (k1,0H2O = 4.55 × 10–30 (T/300 K)−4.85 cm6 molecule–2 s–1) indicating that H2O is much more efficient in quenching the association complex HONO* through collisional energy transfer. Based on measurements in N2–H2O mixtures, a parametrization of k1 including both N2 and H2O as third-body quenchers was derived. Neglecting the effect of H2O results, e.g., in an underestimation of k1 by >10% in the tropical boundary layer.

Kinetics of OH + SO<sub>2</sub> + M: temperature-dependent rate coefficients in the fall-off regime and the influence of water vapour

Abstract. The OH-initiated oxidation of SO2 is the dominant, first step in the transformation of this atmospherically important trace gas to particulate sulfate, and accurate rate coefficients for the title reaction under all atmospheric conditions (pressures, temperatures, and humidity) are required to assess its role in, for example, new particle formation. Prior to this study, no temperature-dependent data were available in the fall-off regime for atmospherically relevant bath gases. We thus address an important omission in the kinetic database for this reaction and highlight significant discrepancies in recommended parameterizations. In this work, generation of OH via pulsed laser photolysis at 248 and 351 nm was coupled to its detection by laser-induced fluorescence to obtain rate coefficients (k1) for the title reaction at pressures of 14–742 Torr (1 Torr = 1.333 hPa) and temperatures of 220–333 K in N2 bath gas. In situ SO2 concentrations, central to accurate kinetic measurements under pseudo-first-order conditions, were measured by optical absorption. Under the conditions of the present study, the termolecular reaction between OH and SO2 is in the fall-off regime, and we parameterized the rate coefficients in N2 in terms of low- (k1,0) and high-pressure (k1,∞) limiting rate coefficients and a broadening factor (FC) to obtain k1,0N2=3.03×10-31 (T/300 K)−4.10 cm6 molecule−2 s−1, k1,∞=2.00×10-12 cm3 molecule−1 s−1, and FC=0.58. The effects of water vapour on the title reaction were explored through measurements in N2–H2O mixtures at 273, 298, and 333 K using the same experimental methods. The rate coefficients are significantly enhanced by the presence of water vapour with k1,0H2O=1.65×10-30 (T/300 K)−4.90 cm6 molecule−2 s−1, which indicates that H2O is a factor >5 more efficient in quenching the HOSO2* association complex than N2. A model-based comparison of our rate coefficients and parameterization with previous literature measurements and recommendations of evaluation panels are presented and discussed. The use of the new parameterization instead of the IUPAC or NASA evaluations, particularly after including H2O as a third-body quencher, leads to a significant (10 %–30 %) reduction in the lifetime of SO2 in some parts of the atmosphere and increases the H2SO4/SO2 ratio concomitantly.

The merit of pressure dependent kinetic modelling in steam cracking.

Renewable cracking feedstocks from plastic waste and the need for novel reactor designs related to electrification of steam crackers drives the development of accurate and fundamental kinetic models for this process, despite its large scale implementation for more than half a century. Pressure dependent kinetics have mostly been omitted in fundamental steam cracking models, while they are crucial in combustion models. Therefore, we have assessed the importance of pressure dependent kinetics for steam cracking via in-depth modelling and experimental studies. In particular we have studied the influence of considering fall-off on the product yields for ethane and propane steam cracking. A high-pressure limit fundamental kinetic model is generated, based on quantum chemical data and group additive values, and supplemented with literature values for pressure dependent kinetic parameters for β-scission reactions and homolytic bond scissions of C2 and C3 species. Model simulations with high-pressure limit rate coefficients and pressure dependent kinetics are compared to new experimental measurements. Steam cracking experiments for pure ethane and propane feeds are performed on a tubular bench-scale reactor at 0.17 MPa and temperatures ranging from 1058 to 1178 K. All important product species are identified using a comprehensive GC × GC-FID/q-MS. For homolytic bond scissions, the inclusion of pressure dependent kinetics has a significant effect on the conversion profile for ethane steam cracking. On the other hand, pressure dependence of C2 β-scissions significantly influences conversion and product species profiles for both ethane and propane steam cracking. The C3 β-scissions pressure dependence has a negligible effect in ethane steam cracking, while for propane steam cracking the effect is non-negligible on the product species profiles.

Theoretical study on isomerization, decomposition and ring-closure reaction kinetics of methyl pentanoate radicals

A comprehensive study on the isomerization, decomposition and ring-closure reaction kinetics, as well as thermodynamic properties of methyl pentanoate radicals were carried out in this work. The M06–2X/cc-pVTZ level of theory was employed to optimize geometries and analyze frequencies for all the stationary points on the potential energy surfaces. The CCSD(T) method with two basis sets, cc-pVDZ and cc-pVTZ, was used to calculate single point energies, which were further extrapolated to the complete basis set (CBS) limit to construct the potential energy surfaces. The energy calculation results indicated that isomerization of δ-R to α-R, decomposition of γ-R to CH2C(=O)OCH3 and propene, and ring-closure reaction of δ-R to 1-methoxycyclopentanoxyl radical are the most energetically favored reactions among the studied isomerization, decomposition and ring-closure reaction channels, respectively. High pressure limit, and temperature and pressure dependent rate coefficients were determined by solving the one-dimensional energy-dependent master equations with Tsinghua University Minnesota Master Equation program (TUMME). The rate coefficients and branching ratios were found to be significantly affected by temperature and pressure. The decomposition reactions yielding small unsaturated esters or small ester radicals dominate at high temperatures, while the isomerization reactions proceeding via five- and six-membered ring saddle points, as well as the ring-closure reactions play more important roles at low temperatures. Significant discrepancies are observed between the theoretically calculated rate coefficients and estimated results. The thermochemical properties of methyl pentanoate radicals were calculated using isodesmic reaction method and atomization method together with the multistructural torsional anharmonicity partition functions. This study provides accurate kinetic and thermodynamic data on methyl pentanoate radicals, which are expected to advance our understanding of combustion chemistry of methyl pentanoate and larger methyl esters.

TU16. BRAINXCAN IDENTIFIES BRAIN FEATURES ASSOCIATED WITH BEHAVIORAL AND PSYCHIATRIC TRAITS USING LARGE SCALE GENETIC AND IMAGING DATA

The recombination and dissociation of the resonantly stabilized 2-methylallyl radical has been studied in a diaphragmless shock tube by laser schlieren densitometry (LS) over temperatures of 700–1350 K and pressures of 60–260 Torr. Both 2,5-dimethyl-1,5-hexadiene and the new low temperature precursor 3-methylbut-3-enyl nitrite were used to generate 2-methylallyl radicals under these conditions. Rate coefficients were obtained for dissociation of the precursors, recombination of 2-methylallyl, and dissociation of 2-methylallyl by simulation of the LS profiles. The experiments are complemented by a priori theoretical calculations for both the recombination and dissociation of 2-methylallyl. The experimental results and theoretical predictions are in excellent agreement with one another. The calculated high pressure limit rate coefficient for recombination of 2-methylallyl is log(k1) = 14.737−0.641logT+251.39/(2.303×T) and that for dissociation of 2-methylallyl is log(k3) = 11.100−1.2295logT−28545/(2.303×T). The uncertainties in k1 and k3 are estimated as factors of 1.5. Rate coefficients are provided over a broad range of pressures for chemical kinetic modeling.

Thermal Decomposition of 2-Methyltetrahydrofuran behind Reflected Shock Waves over the Temperature Range of 1179–1361 K

The thermal unimolecular decomposition of 2-methyltetrahydrofuran (2-MTHF) was studied behind reflected shock waves in a single-pulse shock tube over the temperature range of 1179-1361 K and pressure range of 9-17 atm. Methane, ethylene, ethane, 1,3-butadiene, propylene, acetaldehyde, and acetylene were identified as products in the decomposition of 2-MTHF. A reaction scheme was proposed to explain the mechanism for the observed products. The experimentally determined rate coefficients were best fit to an Arrhenius expression for the overall decomposition and is represented as ktotalexp(1179-1361 K) = (3.23 ± 0.59) × 1011 s-1 exp(-51.3 ± 1.4 kcal mol-1/RT). Quantum chemistry methods were used to calculate the energetics and kinetics of various possible unimolecular dissociation pathways involved in the thermal decomposition of 2-MTHF. The initial decomposition of 2-MTHF occurs predominantly via ring-methyl (C-CH3) single bond fission, leading to the formation of tetrahydrofuran (C4H7O) radical, and methyl radical was found to be the major reaction compared to all the possible initial bond fission, ring opening, and molecular elimination channels. The temperature-dependent rate coefficients for the unimolecular dissociation of 2-MTHF were calculated using the RRKM (Rice-Ramsperger-Kassel-Marcus) theory in combination with the CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of electronic structure calculations over the temperature range of 800-1500 K. The computed high-pressure limiting rate coefficients for the initial decomposition of 2-MTHF through C-CH3 single bond fission channel were found to be ∼2 times higher in the temperatures between 800 and 900 K, and above this temperature, they agree well with the values reported in the literature.

Pressure dependence of rate coefficients of unimolecular and chemical activation reactions connected to the potential energy wells of chlorinated monosilanes by RRKM calculations

AbstractRate coefficients for elementary reactions connected to the potential energy wells of SiHCl3, SiH2Cl2, SiHCl2, and SiH3Cl, which are important Si1 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through Rice–Ramsperger–Kassel–Marcus (RRKM) theory for various conditions of temperature and pressure. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall‐off regime under subatmospheric to moderately high‐pressure conditions, so that it is expected that accurate modeling of the gas phase in chlorosilane CVD reactors requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high‐pressure limit rate coefficients. The rate coefficients determined here were tabulated using Chebyshev coefficients and can be used in simulations of systems under a wide range of temperature and pressure conditions.

An experimental and theoretical kinetic study of the reactions of hydroxyl radicals with tetrahydrofuran and two deuterated tetrahydrofurans

Rate coefficient expressions for the reactions of OH radicals with tetrahydrofuran (THF) (1), 2,2,5,5-tetradeutero-tetrahydrofuran (THF–d4) (2) and perdeuterated-tetrahydrofuran (THF–d8) (3) were determined from relative kinetic experiments over the temperature range T = 260–360 K displaying a small negative T-dependence. The following rate coefficients were determined at T = 298 K in 10–11 cm3 molecule–1 s−1: k1 = (1.73 ± 0.23), k2 = (0.85 ± 0.11) and k3 = (0.81 ± 0.12). The experimental k (T) values agree reasonably well with the high-pressure limit rate coefficients obtained from conventional transition state theory using input data from CCSD(T)/cc-pV(T,Q)Z//MP2/aug-cc-pVDZ computations. Standard enthalpies of formation for THF and two product furanyl radicals were also computed and compared with literature.

Kinetic Study of the Reactions of AlO and OAlO Relevant to Planetary Mesospheres

Aluminum atoms are injected into planetary upper atmospheres by meteoric ablation. Rapid oxidation of Al by O2 to form AlO is then likely to be followed by reactions with O3, O2, and CO2 to form larger oxides and carbonates, which can also be reduced by atomic O and CO. The reactions listed below were investigated experimentally using both pulsed laser photolysis of an Al precursor in a slow flow reactor, and pulsed laser ablation of an Al target in a fast flow tube, with laser-induced fluorescence detection of AlO. The experimental results were interpreted using electronic structure theory calculations and Rice–Ramsperger–Kassel–Markus theory. The low pressure limiting rate coefficients for the two recombination reactions are: log10(krec,0 (AlO + O2 + N2, 192–812 K)) = −35.137 + 6.1052 log10(T) – 1.4089 (log10(T))2 and log10(krec,0(AlO + CO2 + N2, 193–813 K)) = −38.736 + 8.7342 log10(T) – 2.0202 (log10(T))2 cm6 molecule–2 s–1, with a ±20% uncertainty over the experimental temperature range. The following bimolecular reactions were also studied at 295 K: k(AlO + O3 → OAlO + O2) = (1.25 ± 0.19) × 10–10; k(AlO + CO → Al + CO2) = (1.95 ± 0.35) × 10–12; k(OAlO + CO → AlO + CO2) = (2.6 ± 0.7) × 10–11 and k(OAlO + O → AlO + O2) = (1.9 ± 0.8) × 10–10 cm3 molecule–1 s–1. In the terrestrial atmosphere between 65 and 110 km, AlO is mostly removed by recombination with O2 below 85 km, and reaction with O3 above 90 km. On Mars, recombination with CO2 is much more important than with O2, although reduction of AlO by CO should maintain a significant density of Al atoms. Here we show that in both atmospheres, AlOH is likely to be an important reservoir.

A high temperature shock tube study of phenyl recombination reaction using laser absorption spectroscopy

The chemistry of first aromatic ring, i.e., benzene (C6H6) and phenyl radial (C6H5), plays a key role in the growth of polycyclic aromatic hydrocarbons (PAHs) and ultimately soot formation. In this work, the self-recombination reaction of phenyl radicals was investigated over the temperature range of 950–1300 K and pressures near 1 atm by employing shock tube and laser absorption diagnostics. Phenyl radical was generated by the rapid thermal unimolecular dissociation of nitrosobenzene (C6H5NO), a clean precursor of C6H5 radical. The reaction progress was monitored by detecting C6H5 and NO simultaneously using visible laser absorption near 445 nm and mid-IR laser absorption near 5.517 µm, respectively. For the reaction C6H5NO → C6H5 + NO (R1), our data show an excellent agreement with earlier reports. The high-pressure limiting rate coefficient, by combining all available data, may be expressed as k1∞(T(K))=3.2×1066T−15.2e−37743/T s−1. This work reports the temperature dependence of the absorption cross-section of phenyl radical at 445 nm for first time. Our experiments indicate that the self-reaction of phenyl radicals yielding biphenyl, C6H5 + C6H5 → C6H5C6H5 (R2a), is a major channel. The rate coefficients of reaction (R2a) show a weak temperature dependence with an average value of k2a = (6.91 ± 0.42) × 1012 cm3 mol−1 s−1 in the temperature range of 950–1300 K. Our measured data, k2a(T, P = 1.1–1.5 atm), are found to be close to the high-pressure limiting rate coefficients. Combining with the literature low-temperature data, the self-recombination reaction of phenyl radicals may be expressed as k2a∞(T=300−1450K)=2.8×1017T−1.44e−540/T cm3 mol−1 s−1. The measurements of this study represent the first high-temperature direct experimental determination of the rate coefficients of this important prototype aromatic radical-radical reaction.

Shock Tube Laser Schlieren Study of the Pyrolysis of Isopropyl Nitrate.

The decomposition of isopropyl nitrate was measured behind incident shock waves using laser schlieren densitometry in a diaphragmless shock tube. Experiments were conducted over the temperature range of 700-1000 K and at pressures of 71, 126, and 240 Torr. Electronic structure theory and RRKM Master Equation methods were used to predict the decomposition kinetics. RRKM/ME parameters were optimized against the experimental data to provide an accurate prediction over a broader range of conditions. The initial decomposition i-C3H7ONO2 ⇌ i-C3H7O + NO2 has a high-pressure limit rate coefficient of 5.70 × 1022T-1.80 exp[-21287.5/T] s-1. A new chemical kinetic mechanism was developed to model the chemistry after the initial dissociation. A new shock tube module was developed for Cantera, which allows for arbitrarily large mechanisms in the simulation of laser schlieren experiments. The present work is in good agreement with previous experimental studies.

Direct measurements of C3F7I dissociation rate constants using a shock tube ARAS technique

AbstractThis work presents the first direct experimental study on the thermal unimolecular decomposition of n‐C3F7I. Experiments were performed behind incident and reflected shock waves using the atomic resonance absorption spectroscopy (ARAS) technique on a resonant line of atomic iodine at 183.04 nm. The reaction C3F7I + Ar → C3F7 + I + Ar (1) was studied at specific temperature (800–1200 K) and pressure (0.6–8.3 bar) ranges. Under experimental conditions, the obtained values of the rate constant at temperatures below 950 K are close to the high‐pressure limit; however, considering theoretical calculations, the influence of pressure on the rate constant at elevated temperatures remains noticeable. The resulting value of the experimental rate constant of reaction 1 is presented in the following Arrhenius form: urn:x-wiley:05388066:media:kin21244:kin21244-math-0001Experimental data were found to correlate with the results of the Rice–Ramsperger–Kassel–Marcus –master equation analysis based on quantum‐chemical calculations. The following low‐ and high‐pressure limiting rate coefficients were obtained over the temperature range T = 300–3000 K: urn:x-wiley:05388066:media:kin21244:kin21244-math-0002with the center broadening factor Fc = 0.119.