Direct measurements of rate constants for the reactions of CH3 radicals with C2H6, C2H4, and C2H2 at high temperatures

Gas-phase synthesis of benzene via the propargyl radical self-reaction

Polycyclic aromatic hydrocarbons (PAHs) have been invoked in fundamental molecular mass growth processes in our galaxy. We provide compelling evidence of the formation of the very first ringed aromatic and building block of PAHs-benzene-via the self-recombination of two resonantly stabilized propargyl (C₃H₃) radicals in dilute environments using isomer-selective synchrotron-based mass spectrometry coupled to theoretical calculations. Along with benzene, three other structural isomers (1,5-hexadiyne, fulvene, and 2-ethynyl-1,3-butadiene) and o-benzyne are detected, and their branching ratios are quantified experimentally and verified with the aid of computational fluid dynamics and kinetic simulations. These results uncover molecular growth pathways not only in interstellar, circumstellar, and solar systems environments but also in combustion systems, which help us gain a better understanding of the hydrocarbon chemistry of our universe.

Substitution Reactions in the Pyrolysis of Acetone Revealed through a Modeling, Experiment, Theory Paradigm

The development of high-fidelity mechanisms for chemically reactive systems is a challenging process that requires the compilation of rate descriptions for a large and somewhat ill-defined set of reactions. The present unified combination of modeling, experiment, and theory provides a paradigm for improving such mechanism development efforts. Here we combine broadband rotational spectroscopy with detailed chemical modeling based on rate constants obtained from automated ab initio transition state theory-based master equation calculations and high-level thermochemical parametrizations. Broadband rotational spectroscopy offers quantitative and isomer-specific detection by which branching ratios of polar reaction products may be obtained. Using this technique, we observe and characterize products arising from H atom substitution reactions in the flash pyrolysis of acetone (CH₃C(O)CH₃) at a nominal temperature of 1800 K. The major product observed is ketene (CH₂CO). Minor products identified include acetaldehyde (CH₃CHO), propyne (CH₃CCH), propene (CH₂CHCH₃), and water (HDO). Literature mechanisms for the pyrolysis of acetone do not adequately describe the minor products. The inclusion of a variety of substitution reactions, with rate constants and thermochemistry obtained from automated ab initio kinetics predictions and Active Thermochemical Tables analyses, demonstrates an important role for such processes. The pathway to acetaldehyde is shown to be a direct result of substitution of acetone's methyl group by a free H atom, while propene formation arises from OH substitution in the enol form of acetone by a free H atom.

Selective Oxidative Cracking of n-Butane to Light Olefins over Hexagonal Boron Nitride with Limited Formation of COx

In recent years, hexagonal boron nitride (hBN) has emerged as an unexpected catalyst for the oxidative dehydrogenation of alkanes. Here, the versatility of hBN was extended to alkane oxidative cracking chemistry by investigating the production of ethylene and propylene from n-butane. Cracking selectivity was primarily controlled by the ratio of n-butane to O₂ within the reactant feed. Under O₂ -lean conditions, increasing temperature led to increased selectivity to ethylene and propylene and decreased selectivity to CO_x . In addition to surface-mediated chemistry, homogeneous gas-phase reactions likely contributed to the observed product distribution, and a reaction mechanism was proposed based on these observations. The catalyst showed good stability under oxidative cracking conditions for 100 h time-on-stream while maintaining high selectivity to ethylene and propylene.

Facile hydrogen atom abstraction and sulfide formation in a methyl-thiolate capped iron–sulfur–carbonyl cluster

SAM mediated methyl transfer and subsequent hydrogen atom abstraction are key steps in the biogenesis of nitrogenase. A model system was utilized to demonstrate facile C–H abstraction from a methyl-thiolate containing iron–sulfur cluster with TEMPO.

Shock Tube Measurement of the CH3 + C2H6 → CH4 + C2H5 Rate Constant

The rate constant for the CH₃ + C₂H₆ → CH₄ + C₂H₅ reaction was studied behind reflected shock waves at temperatures between 1369 and 1626 K and pressures from 8.6 to 47.4 atm in mixtures of methane, ethane, and argon. Ethylene time histories were measured using laser absorption of radiation from a carbon dioxide gas laser near 10.532 μm. The resulting rate constant data can be represented by the Arrhenius equation k (T) = 3.90 × 10¹³ exp(-16670 cal/mol/RT) cm³ mol^-1 s^-1. We believe this is the first study to extend experimental data for this rate constant to temperatures above 1400 K. The overall 2σ uncertainty of the current data is +18%/-21% resulting primarily from uncertainties associated with the influence of secondary reactions and the fitting of rapidly changing species time histories at the higher temperatures.

High-Temperature Unimolecular Decomposition of Diethyl Ether: Shock-Tube and Theory Studies

The unimolecular decomposition of diethyl ether (DEE; C₂H₅OC₂H₅) is considered to be initiated via a molecular elimination and a C-O and a C-C bond fission step: C₂H₅OC₂H₅ → C₂H₄ + C₂H₅OH (1), C₂H₅OC₂H₅ → C₂H₅ + C₂H₅O (2), and C₂H₅OC₂H₅ → CH₃ + C₂H₅OCH₂ (3). In this work, two shock-tube facilities were used to investigate these reactions via (a) time-resolved H-atom concentration measurements by H-ARAS (atomic resonance absorption spectrometry), (b) time-resolved DEE-concentration measurements by high repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS), and (c) product-composition measurements via gas chromatography/MS (GC/MS) after quenching the test gas. The experiments were conducted at temperatures ranging from 1054 to 1505 K and at pressures between 1.2 and 2.5 bar. Initial DEE mole fractions between 0.4 and 9300 ppm were used to perform the kinetics experiments by H-ARAS (0.4 ppm), GC/MS (200-500 ppm), and HRR-TOF-MS (7850-9300 ppm). The rate constants, k_total (k_total = k₁ + k₂ + k₃) derived from the GC/MS and HRR-TOF-MS experiments agree well with each other and can be described by the Arrhenius expression, k_total(1054-1467 K; 1.3-2.5 bar) = 10^12.81±0.22 exp(-240.27 ± 5.11 kJ mol^-1/RT) s^-1. From the H-ARAS experiments, overall rate constants for the bond fission channels, k₂₊₃ = k₂ + k₃ have been extracted. The k₂₊₃ data can be well described by the Arrhenius equation, k₂₊₃(1299-1505 K; 1.3-2.5 bar) = 10^14.43±0.33 exp(-283.27 ± 8.78 kJ mol^-1/RT) s^-1. A master-equation analysis was performed using CCSD(T)/aug-cc-pvtz//B3LYP/aug-cc-pvtz and CASPT2/aug-cc-pvtz//B3LYP/aug-cc-pvtz molecular properties and energies for the three primary thermal decomposition processes in DEE. The derived experimental data is very well reproduced by the simulations with the mechanism of this work. With regard to the branching ratios between bond fissions and elimination channels, uncertainties remain.