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

Thermal Dissociation and Roaming Isomerization of Nitromethane: Experiment and Theory

The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH₃NO₂ decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH₃ and NO₂ radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH₃NO₂, and for the reaction of CH₃ with NO₂