Dissociation of Propyl Radicals and Other Reactions on a C₃H₇ Potential

In this article we make theoretical predictions of the thermal rate coefficients for a series of elementary reactions on a C₃H₇ potential. Perhaps most importantly, we study the association/dissociation reactions for <i>n</i>-C₃H₇ and <i>i</i>-C₃H₇ [CH₃ + C₂H₄(+M) ⇄ <i>n</i>-C₃H₇(+M), C₃H₆+ H­(+M) ⇄ <i>n</i>-C₃H₇(+M), and C₃H₆ + H­(+M) ⇄ <i>i</i>-C₃H₇(+M)], where <i>n</i>-C₃H₇ and <i>i</i>-C₃H₇ are the propyl radicals and C₃H₆ is propene. However, in order to provide more information for our kinetic model, we have also included analyses of the association/elimination reaction [C₃H₆ + H ⇄ CH₃ + C₂H₄] and the abstraction reactions [C₃H₆ + H ⇄ CH₂CHCH₂ + H₂, C₃H₆ + H ⇄ CH₃CCH₂ + H₂, C₃H₆ + H ⇄ CH₃CHCH + H₂, and CH₃ + C₂H₄ ⇄ CH₄ + C₂H₃]. The theory employs high-level electronic-structure methods to characterize the potential energy surface, conventional transition-state theory to calculate <i>k</i>(<i>T</i>) for the abstraction reactions, RRKM theory to calculate microcanonical, J-resolved rate coefficients for the dissociation processes, and master-equation methods to determine phenomenological rate coefficients <i>k</i>(<i>T</i>,<i>p</i>), for all of the nonabstraction reactions. The agreement between our theory and the experimental results available is remarkably good. The final results are cast in a form that is convenient for chemical kinetics modeling

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

Kinetics of Propargyl Radical Dissociation

Due to the prominent role of the propargyl radical for hydrocarbon growth within combustion environments, it is important to understand the kinetics of its formation and loss. The ab initio transition state theory-based master equation method is used to obtain theoretical kinetic predictions for the temperature and pressure dependence of the thermal decomposition of propargyl, which may be its primary loss channel under some conditions. The potential energy surface for the decomposition of propargyl is first mapped at a high level of theory with a combination of coupled cluster and multireference perturbation calculations. Variational transition state theory is then used to predict the microcanonical rate coefficients, which are subsequently implemented within the multiple-well multiple-channel master equation. A variety of energy transfer parameters are considered, and the sensitivity of the thermal rate predictions to these parameters is explored. The predictions for the thermal decomposition rate coefficient are found to be in good agreement with the limited experimental data. Modified Arrhenius representations of the rate constants are reported for utility in combustion modeling

Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen

State-of-the-art calculations of the C₂H₃O₂ potential energy surface are presented. A new method is described for computing the interaction potential for R + O₂ reactions. The method, which combines accurate determination of the quartet potential along the doublet minimum energy path with multireference calculations of the doublet/quartet splitting, decreases the uncertainty in the doublet potential and thence the rate constants by more than a factor of 2. The temperature- and pressure-dependent rate coefficients are computed using variable reaction coordinate transition-state theory, variational transition-state theory, and conventional transition-state theory, as implemented in a new RRKM/ME code. The main bimolecular product channels are CH₂O + HCO at lower temperatures and CH₂CHO + O at higher temperatures. Above 10 atm, the collisional stabilization of CH₂CHOO directly competes with these two product channels. CH₂CHOO decomposes primarily to CH₂O + HCO. The next two most significant bimolecular products are OCHCHO + H and ³CHCHO + OH, and not C₂H₂ + HO₂. C₂H₃ + O₂ will be predominantly chain branching above 1700 K. Uncertainty analysis is presented for the two most important transition states. The uncertainties in these two barrier heights result in a significant uncertainty in the temperature at which CH₂CHO + O overtakes all other product channels

Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen

State-of-the-art calculations of the C₂H₃O₂ potential energy surface are presented. A new method is described for computing the interaction potential for R + O₂ reactions. The method, which combines accurate determination of the quartet potential along the doublet minimum energy path with multireference calculations of the doublet/quartet splitting, decreases the uncertainty in the doublet potential and thence the rate constants by more than a factor of 2. The temperature- and pressure-dependent rate coefficients are computed using variable reaction coordinate transition-state theory, variational transition-state theory, and conventional transition-state theory, as implemented in a new RRKM/ME code. The main bimolecular product channels are CH₂O + HCO at lower temperatures and CH₂CHO + O at higher temperatures. Above 10 atm, the collisional stabilization of CH₂CHOO directly competes with these two product channels. CH₂CHOO decomposes primarily to CH₂O + HCO. The next two most significant bimolecular products are OCHCHO + H and ³CHCHO + OH, and not C₂H₂ + HO₂. C₂H₃ + O₂ will be predominantly chain branching above 1700 K. Uncertainty analysis is presented for the two most important transition states. The uncertainties in these two barrier heights result in a significant uncertainty in the temperature at which CH₂CHO + O overtakes all other product channels

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

Weakly bound free radicals have low-dissociation thresholds such that at high temperatures, time scales for dissociation and collisional relaxation become comparable, leading to significant dissociation during the vibrational–rotational relaxation process. Here we characterize this “prompt” dissociation of formyl (HCO), an important combustion radical, using direct dynamics calculations for OH + CH₂O and H + CH₂O (key HCO-forming reactions). For all other HCO-forming reactions, presumption of a thermal incipient HCO distribution was used to derive prompt dissociation fractions. Inclusion of these theoretically derived HCO prompt dissociation fractions into combustion kinetics models provides an additional source for H-atoms that feeds chain-branching reactions. Simulations using these updated combustion models are therefore shown to enhance flame propagation in 1,3,5-trioxane and acetylene. The present results suggest that HCO prompt dissociation should be included when simulating flames of hydrocarbons and oxygenated molecules and that prompt dissociations of other weakly bound radicals may also impact combustion simulations

Comment on “When Rate Constants Are Not Enough”

Comment on “When Rate Constants Are Not Enough