Initiation Reactions in Acetylene Pyrolysis

In gas-phase combustion systems the interest in acetylene stems largely from its role in molecular weight growth processes. The consensus is that above 1500 K acetylene pyrolysis starts mainly with the homolytic fission of the C–H bond creating an ethynyl radical and an H atom. However, below ∼1500 K this reaction is too slow to initiate the chain reaction. It has been hypothesized that instead of dissociation, self-reaction initiates this process. Nevertheless, rigorous theoretical or direct experimental evidence is lacking, to an extent that even the molecular mechanism is debated in the literature. In this work we use rigorous ab initio transition-state theory master equation methods to calculate pressure- and temperature-dependent rate coefficients for the association of two acetylene molecules and related reactions. We establish the role of vinylidene, the high-energy isomer of acetylene in this process, compare our results with available experimental data, and assess the competition between the first-order and second-order initiation steps. We also show the effect of the rapid isomerization among the participating wells and highlight the need for time-scale analysis when phenomenological rate coefficients are compared to observed time scales in certain experiments

Evaluation of Rate Coefficients in the Gas Phase Using Machine-Learned Potentials

We assess the capability of machine-learned potentials to compute rate coefficients by training a neural network (NN) model and applying it to describe the chemical landscape on the C5H5 potential energy surface, which is relevant to molecular weight growth in combustion and interstellar media. We coupled the resulting NN with an automated kinetics workflow code, KinBot, to perform all necessary calculations to compute the rate coefficients. The NN is benchmarked exhaustively by evaluating its performance at the various stages of the kinetics calculations: from the electronic energy through the computation of zero point energy, barrier heights, entropic contributions, the portion of the PES explored, and finally the overall rate coefficients as formulated by transition state theory

Evaluation of Rate Coefficients in the Gas Phase Using Machine-Learned Potentials

We assess the capability of machine-learned potentials to compute rate coefficients by training a neural network (NN) model and applying it to describe the chemical landscape on the C5H5 potential energy surface, which is relevant to molecular weight growth in combustion and interstellar media. We coupled the resulting NN with an automated kinetics workflow code, KinBot, to perform all necessary calculations to compute the rate coefficients. The NN is benchmarked exhaustively by evaluating its performance at the various stages of the kinetics calculations: from the electronic energy through the computation of zero point energy, barrier heights, entropic contributions, the portion of the PES explored, and finally the overall rate coefficients as formulated by transition state theory

A Combined Experimental and Theoretical Study of the Reaction OH + 2‑Butene in the 400–800 K Temperature Range

We report a combined experimental and theoretical study of the OH + cis-2-butene and OH + trans-2-butene reactions at combustion-relevant conditions: pressures of 1–20 bar and temperatures of 400–800 K. We probe the OH radical time histories by laser-induced fluorescence and analyze these experimental measurements with aid from time-dependent master-equation calculations. Importantly, our investigation covers a temperature range where experimental data on OH + alkene chemistry in general are lacking, and interpretation of such data is challenging due to the complexity of the competing reaction pathways. Guided by theory, we unravel this complex behavior and determine the temperature- and pressure-dependent rate coefficients for the three most important OH + 2-butene reaction channels at our conditions: H abstraction, OH addition to the double bond, and back-dissociation of the OH–butene adduct

Sulfur (³P) Reaction with Conjugated Dienes Gives Cyclization to Thiophenes under Single Collision Conditions

We combine crossed-beam velocity map imaging with high-level ab initio/transition state theory modeling of the reaction of S(3P) with 1,3-butadiene and isoprene under single collision conditions. For the butadiene reaction, we detect both H and H2 loss from the initial adduct, and from reaction with isoprene, we see both H loss and methyl loss. Theoretical calculations confirm these arise following intersystem crossing to the singlet surface forming long-lived intermediates. For the butadiene reaction, these lose H2 to form thiophene as the dominant channel, H to form the detected 2H-thiophenyl radical, or ethene, giving thioketene. For isoprene, additional reaction products are suggested by theory, including the observed H and methyl loss radicals, but also methyl thiophene, thioformaldehyde, and thioketene. The results for S(3P) + 1,3-butadiene, showing direct cyclization to the aromatic product and yielding few bimolecular product channels, are in striking contrast to those for the analogous O(3P) reaction

Unconventional Peroxy Chemistry in Alcohol Oxidation: The Water Elimination Pathway

Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD­(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states

Unimolecular Reaction Pathways of a γ‑Ketohydroperoxide from Combined Application of Automated Reaction Discovery Methods

Ketohydroperoxides are important in liquid-phase autoxidation and in gas-phase partial oxidation and pre-ignition chemistry, but because of their low concentration, instability, and various analytical chemistry limitations, it has been challenging to experimentally determine their reactivity, and only a few pathways are known. In the present work, 75 elementary-step unimolecular reactions of the simplest γ-ketohydroperoxide, 3-hydroperoxypropanal, were discovered by a combination of density functional theory with several automated transition-state search algorithms: the Berny algorithm coupled with the freezing string method, single- and double-ended growing string methods, the heuristic KinBot algorithm, and the single-component artificial force induced reaction method (SC-AFIR). The present joint approach significantly outperforms previous manual and automated transition-state searches – 68 of the reactions of γ-ketohydroperoxide discovered here were previously unknown and completely unexpected. All of the methods found the lowest-energy transition state, which corresponds to the first step of the Korcek mechanism, but each algorithm except for SC-AFIR detected several reactions not found by any of the other methods. We show that the low-barrier chemical reactions involve promising new chemistry that may be relevant in atmospheric and combustion systems. Our study highlights the complexity of chemical space exploration and the advantage of combined application of several approaches. Overall, the present work demonstrates both the power and the weaknesses of existing fully automated approaches for reaction discovery which suggest possible directions for further method development and assessment in order to enable reliable discovery of all important reactions of any specified reactant(s)

Low-Temperature Combustion Chemistry of <i>n-</i>Butanol: Principal Oxidation Pathways of Hydroxybutyl Radicals

Reactions of hydroxybutyl radicals with O₂ were investigated by a combination of quantum-chemical calculations and experimental measurements of product formation. In pulsed-photolytic Cl-initiated oxidation of <i>n</i>-butanol, the time-resolved and isomer-specific product concentrations were probed using multiplexed tunable synchrotron photoionization mass spectrometry (MPIMS). The interpretation of the experimental data is underpinned by potential energy surfaces for the reactions of O₂ with the four hydroxybutyl isomers (1-hydroxybut-1-yl, 1-hydroxybut-2-yl, 4-hydroxybut-2-yl, and 4-hydroxybut-1-yl) calculated at the CBS-QB3 and RQCISD(T)/cc-pV∞Z//B3LYP/6-311++G­(d,p) levels of theory. The observed product yields display substantial temperature dependence, arising from a competition among three fundamental pathways: (1) stabilization of hydroxybutylperoxy radicals, (2) bimolecular product formation in the hydroxybutyl + O₂ reactions, and (3) decomposition of hydroxybutyl radicals. The 1-hydroxybut-1-yl + O₂ reaction is dominated by direct HO₂ elimination from the corresponding peroxy radical forming butanal as the stable coproduct. The chemistry of the other three hydroxybutylperoxy radical isomers mainly proceeds via alcohol-specific internal H-atom abstractions involving the H atom from either the −OH group or from the carbon attached to the −OH group. We observe evidence of the recently reported water elimination pathway (Welz et al. <i>J. Phys. Chem. Lett.</i> <b>2013</b>, <i>4</i> (3), 350–354) from the 4-hydroxybut-2-yl + O₂ reaction, supporting its importance in γ-hydroxyalkyl + O₂ reactions. Experiments using the 1,1-<i>d</i>₂ and 4,4,4-<i>d</i>₃ isotopologues of <i>n</i>-butanol suggest the presence of yet unexplored pathways to acetaldehyde

Pressure-Dependent Competition among Reaction Pathways from First- and Second‑O₂ Additions in the Low-Temperature Oxidation of Tetrahydrofuran

We report a combined experimental and quantum chemistry study of the initial reactions in low-temperature oxidation of tetrahydrofuran (THF). Using synchrotron-based time-resolved VUV photoionization mass spectrometry, we probe numerous transient intermediates and products at <i>P</i> = 10–2000 Torr and <i>T</i> = 400–700 K. A key reaction sequence, revealed by our experiments, is the conversion of THF-yl peroxy to hydroperoxy-THF-yl radicals (QOOH), followed by a second O₂ addition and subsequent decomposition to dihydrofuranyl hydroperoxide + HO₂ or to γ-butyrolactone hydroperoxide + OH. The competition between these two pathways affects the degree of radical chain-branching and is likely of central importance in modeling the autoignition of THF. We interpret our data with the aid of quantum chemical calculations of the THF-yl + O₂ and QOOH + O₂ potential energy surfaces. On the basis of our results, we propose a simplified THF oxidation mechanism below 700 K, which involves the competition among unimolecular decomposition and oxidation pathways of QOOH

Comment on “When Rate Constants Are Not Enough”

Comment on “When Rate Constants Are Not Enough