Weakly Bound Free Radicals in Combustion: "Prompt" Dissociation of Formyl Radicals and Its Effect on Laminar Flame Speeds

Solving the OH + Glyoxal Problem: A Complete Theoretical Description of Post-Transition-State Energy Deposition in Activated Systems

Activated chemistry in coupled reaction systems has broadened our understanding of the chemical kinetics. In the case of intermediates formed in gas phase abstraction reactions (e.g., OH + HC(O)C(O)H (glyoxal) →HC(O)CO + H2O), it is particularly crucial to understand how the reaction energy is partitioned between product species as this determines the propensity for a given product to undergo "prompt" dissociation (e.g., HC(O)CO → HCO + CO) before the excess reaction energy is removed. An example of such an activated system is the OH + glyoxal + O2 coupled reaction system. In this work, we develop a molecular dynamics pipeline, which, combined with a master equation analysis, accurately models previous experimental measurements. This new work resolves previous complexities and discrepancies from earlier master equation modeling for this reaction system. The detailed molecular dynamics approach employed here is a powerful new tool for modeling challenging activated reaction systems.

Insight into the high-temperature oxidation kinetics of acetylene: A first-principles molecular dynamics study

The study on high-temperature oxidation kinetics and kinetic modeling of acetylene (C2H2) has significant importance for its engineering applications. In this paper, the first-principles molecular dynamics method is used to simulate the C2H2 oxidation under high temperatures for the first time. Our results show that there are 38 intermediates and 225 elementary reactions in the process of C2H2 oxidation. The formation mechanisms of "prompt" CO2, as well as gas pollutants CHOCHO and HCOOH are revealed in depth. Four intermediates, CHCHO, CHOCO, CHOCHO and HCOOH, which have significant controversy in current kinetic models, are verified. And a new intermediate, CHOCO2, is discovered. Meanwhile, our simulation also shows that radicals, such as HO2, OH, O, etc. play a key role in promoting the oxidation of intermediates in the early stage of C2H2 oxidation. Combined with quantum chemical calculations, a detailed chemical kinetic model of C2H2/air (FP-C2H2) is built and verified by simulating ignition delay time, species concentration in the flow reactor and premixed laminar flame speed. Our studies provide novel insight for understanding the complex chemical reaction kinetics, and environmental and human health threats from air pollutant formation during C2H2 combustion.

Automated Generation of a Compact Chemical Kinetic Model for n-Pentane Combustion

We have employed automated mechanism generation tools to construct a detailed chemical kinetic model for combustion of n-pentane, as a step toward the generation of compact kinetic models for larger alkanes. Pentane is of particular interest as a prototype for combustion of alkanes and as the smallest paraffin employed as a hybrid rocket fuel, albeit at cryogenic conditions. A reaction mechanism for pentane combustion thus provides a foundation for modeling combustion of extra-large alkanes (paraffins) that are of more practical interest as hybrid rocket fuels, for which manual construction becomes infeasible. Here, an n-pentane combustion kinetic model is developed using the open-source software package Reaction Mechanism Generator (RMG). The model was generated and tested across a range of temperatures (650 to 1350 K) and equivalence ratios (0.5, 1.0, 2.0) at pressures of 1 and 10 atm. Available thermodynamic and kinetic databases were incorporated wherever possible. Predictions using the mechanism were validated against the published laminar burning velocities (Su) and ignition delay times (IDT) of n-pentane. To improve the model performance, a comprehensive analysis, including reaction path and sensitivity analyses of n-pentane oxidation, was performed, leading us to modify the thermochemistry and rate parameters for a few key species and reactions. These were combined as a separate data set, an RMG library, that was then used during mechanism generation. The resulting model predicted IDTs as accurately as the best manually constructed mechanisms, while remaining much more compact. It predicted flame speeds to within 10% of published experimental results. The degree of success of automated mechanism generation for this case suggests that it can be extended to construct reliable and compact models for combustion of larger n-alkanes, particularly when using this mechanism as a seed submodel.

A Combined Experimental and Modeling Study on Isopropyl Nitrate Pyrolysis

Alkyl nitrates thermally decompose by homolytic cleavage of the weak nitrate bond at very low temperatures (e.g., around 500 K at reaction times of a few seconds). This provides the opportunity to study the subsequent chemistry of the initially formed radical (or its subsequent pyrolysis products, if unstable) and nitrogen dioxide at such mild conditions. In this work this idea is applied to isopropyl nitrate (iPN) pyrolysis, which is studied in a tubular reactor at atmospheric pressure, temperatures ranging from 373 to 773 K, and residence times of around 2 s. At the experimental conditions, iPN decomposition starts at 473 K with O-N bond fission producing isopropoxy radical (i-C₃H₇O) and NO₂. i-C₃H₇O is rapidly converted to acetaldehyde (CH₃CHO), which is the most abundant product detected, and methyl radicals. Other major products detected are formaldehyde (CH₂O), methanol (CH₃OH), nitromethane (CH₃NO₂), NO, methane, formamide (CHONH₂), and methyl nitrite (CH₃ONO). Four literature nitrogen chemistry models─three of those augmented with iPN specific reactions─have been tested for their ability to predict the iPN decomposition and product profiles. The mechanism by the Curran group performs best, but it still underpredicts the observed high formaldehyde and methanol yields. A rate analysis indicates that the branching ratio of the reaction between methyl radicals and nitrogen dioxide is of significant importance. Based on recent theoretical and experimental data, new rate expressions for the two reactions CH₃ + NO₂ → CH₃O + NO and CH₃ + NO₂ + He → CH₃ONO₂ + He are calculated and incorporated in the kinetic models. It is shown that this change clearly improves the predictions, although additional work is needed to achieve good agreement between calculated and measured species profiles.

Concluding Remarks on Faraday Discussion on Unimolecular Reactions

This Faraday Discussion, marking the centenary of Lindemann’s explanation of the pressure-dependence of unimolecular reactions, presented recent advances in measuring and computing collisional energy transfer efficiencies, microcanonical rate coefficients, pressure-dependent...

Dissociation-Induced Depletion of High-Energy Reactant Molecules as a Mechanism for Pressure-Dependent Rate Constants for Bimolecular Reactions

In 1922, Lindemann proposed the now-well-known mechanism for pressure-dependent rate constants for unimolecular reactions: reactant molecules with sufficiently high energies dissociate more quickly than collisions can reestablish the Boltzmann distribution...

Spiers Memorial Lecture: theory of unimolecular reactions

One hundred years ago, at an earlier Faraday Discussion meeting, Lindemann presented a mechanism that provides the foundation for contemplating the pressure dependence of unimolecular reactions. Since that time, our...

Accurate Rate Constants for the Forward and Reverse H + CO ↔ HCO Reactions at the High-Pressure Limit

The forward and reverse H + CO ↔ HCO reactions are important for combustion chemistry and have been studied from a wide variety of theoretical and experimental techniques. However, most of the chemical kinetic investigations concerning these processes are focused on low pressures or fall-off regions. Hence, a high-level electronic structure treatment was employed here in order to provide accurate rate constant values for these reactions at the high-pressure limit along temperatures from 50 to 4000 K. In relative terms, the variational effects on rate constants are shown to be almost as important at high temperatures as quantum tunneling corrections at the lowest temperatures investigated. The activation energies fitted by using modified and traditional Arrhenius' equations for the forward rate constants from 298 to 2000 K are, respectively, equal to 2.64 and 3.89 kcal mol^-1, while similar fittings provided, respectively, 1.96 and 3.22 kcal mol^-1, considering now forward rate constants from a temperature range of 298-373 K. This last activation energy determination (3.22 kcal mol^-1) is in better agreement with the commonly referenced experimental value of 2.0 ± 0.4 kcal mol^-1, also obtained from traditional fittings in the range 298-373 K, than the value attained from a broader temperature range fitting (3.89 kcal mol^-1). However, some additional care must be considered along these comparisons once the experimental reaction rate measurements have been done for the trimolecular H + CO + M → HCO + M reaction instead. Anyway, the usage of appropriate temperature ranges is fundamental for reliable activation energy comparisons, although the remaining deviation between theory and experiment is still large and is possibly caused by the different pressure regimes assessed in each case. Finally, we roughly estimated that the high-pressure limit for the HCO decomposition into H and CO can be achieved along temperatures ranging from ∼246 and ∼255 K downward, respectively, at pressures of 1.1 and 9.6 atm, although further experiments should be carried out in order to improve these estimates. On the other hand, pressures larger than 9.8 × 10⁴ atm are required for the aforementioned dissociation reaction to attain the high-pressure limit at 700 K. Therefore, the rate constants determined here are probably applicable in combustion studies at lower temperatures.

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.

Nonthermal rate constants for CH4 * + X → CH3 + HX, X = H, O, OH, and O2

Quasiclassical trajectories are used to compute nonthermal rate constants, k^*, for abstraction reactions involving highly-excited methane CH₄ ^* and the radicals H, O, OH, and O₂. Several temperatures and internal energies of methane, E_vib, are considered, and significant nonthermal rate enhancements for large E_vib are found. Specifically, when CH₄ ^* is internally excited close to its dissociation threshold (E_vib ≈ D₀ = 104 kcal/mol), its reactivity with H, O, and OH is shown to be collision-rate-limited and to approach that of comparably-sized radicals, such as CH₃, with k^* > 10^-10 cm³ molecule^-1 s^-1. Rate constants this large are more typically associated with barrierless reactions, and at 1000 K, this represents a nonthermal rate enhancement, k^*/k, of more than two orders of magnitude relative to thermal rate constants k. We show that large nonthermal rate constants persist even after significant internal cooling, with k^*/k > 10 down to E_vib ≈ D₀/4. The competition between collisional cooling and nonthermal reactivity is studied using a simple model, and nonthermal reactions are shown to account for up to 35%-50% of the fate of the products of H + CH₃ = CH₄ ^* under conditions of practical relevance to combustion. Finally, the accuracy of an effective temperature model for estimating k^* from k is quantified.

Deciphering Stability of Five-Membered Heterocyclic Radicals: Balancing Act Between Delocalization and Ring Strain

Computational studies on five-membered heterocycles with single heteroatom and their isomeric dehydro-borole 1a-1c, cyclopentadiene 2a-2c, pyrrole 3a-3c, furan 4b-4c, phosphole 5a-5c, and thiophene 6b-6c radicals have been carried out. Geometrical aspects through ground state electronic structures and stability aspects using bond dissociation energies (BDE) and radical stabilization energies (RSE) have been envisaged in this regard. Spin densities, electrostatic potentials (ESP), and natural bond orbital (NBO) analysis unveiled the extent of spin delocalization. The estimated nucleus-independent chemical shifts (NICS) values revealed the difference in aromaticity characteristics of radicals. Particularly the heteroatom centered radicals exhibit odd electron π-delocalized systems with a quasi-antiaromatic character. Various factors such as, the relative position of the radical center with respect to heteroatoms, resonance, ring strain and orbital interactions influence the stability that follows the order: heteroatom centered > β-centered > α-centered radicals. Among the influences of various factors, we confirmed the existence of a competition between delocalization and the ring strain, and the interplay of both decides the overall stability order.

Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room-Temperature Flow Reactor

Chirped-pulse Fourier transform millimeter-wave spectroscopy is a potentially powerful tool for studying chemical reaction dynamics and kinetics. Branching ratios of multiple reaction products and intermediates can be measured with unprecedented chemical specificity; molecular isomers, conformers, and vibrational states have distinct rotational spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide photoproducts in a flow tube reactor at ambient temperature of 295 K and pressures of 1-10 μbar. This in situ and time-resolved experiment illustrates the utility of this novel approach to investigating chemical reaction dynamics and kinetics. Following 193 nm photodissociation of CH₂CHCN, we observe rotational relaxation of energized HCN, HNC, and HCCCN photoproducts with 10 μs time resolution and sample the vibrational population distribution of HCCCN. The experimental branching ratio HCN/HCCCN is compared with a model based on RRKM theory using high-level ab initio calculations, which were in turn validated by comparisons to Active Thermochemical Tables enthalpies.

Discussion of the Separation of Chemical and Relaxational Kinetics of Chemically Activated Intermediates in Master Equation Simulations

Chemical activation of intermediates, such as hydrogen abstraction products, is emerging as a basis for a fully new reaction type: hot β-scission. While for thermally equilibrated intermediates chemical kinetics are typically orders of magnitude slower than relaxational kinetics, chemically activated intermediates raise the issue of inseparable chemical and relaxational kinetics. Here, this separation problem is discussed in the framework of master equation simulations, proposing three cases often encountered in chemistry: insignificant chemical activation, predominant chemical activation, and the transition between these two limits. These three cases are illustrated via three example systems: methoxy (CH₃Ȯ), diazenyl (ṄNH), and methyl formate radicals (CH₃OĊO). For diazenyl, it is found that hot β-scission fully replaces the sequence of hydrogen abstraction and β-scission of thermally equilibrated diazenyl. Building on the example systems, a rule of thumb is proposed that can be used to intuitively judge the significance of hot β-scission: if the reverse hydrogen abstraction barrier height is comparable to or larger than the β-scission barrier height, hot β-scission should be considered in more detail.

Dehydro-oxazole, thiazole and imidazole radicals: insights into the electronic structure, stability and reactivity aspects

Computations on the effect of heteroatoms in the structural, stability and reactivity aspects of the oxazole, thiazole and imidazole radicals.