4 research outputs found

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

State-of-the-art
calculations of the C<sub>2</sub>H<sub>3</sub>O<sub>2</sub> potential
energy surface are presented. A new method
is described for computing the interaction potential for R + O<sub>2</sub> 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 CH<sub>2</sub>O +
HCO at lower temperatures and CH<sub>2</sub>CHO + O at higher temperatures.
Above 10 atm, the collisional stabilization of CH<sub>2</sub>CHOO
directly competes with these two product channels. CH<sub>2</sub>CHOO
decomposes primarily to CH<sub>2</sub>O + HCO. The next two most significant
bimolecular products are OCHCHO + H and <sup>3</sup>CHCHO + OH, and
not C<sub>2</sub>H<sub>2</sub> + HO<sub>2</sub>. C<sub>2</sub>H<sub>3</sub> + O<sub>2</sub> 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 CH<sub>2</sub>CHO + 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 C<sub>2</sub>H<sub>3</sub>O<sub>2</sub> potential
energy surface are presented. A new method
is described for computing the interaction potential for R + O<sub>2</sub> 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 CH<sub>2</sub>O +
HCO at lower temperatures and CH<sub>2</sub>CHO + O at higher temperatures.
Above 10 atm, the collisional stabilization of CH<sub>2</sub>CHOO
directly competes with these two product channels. CH<sub>2</sub>CHOO
decomposes primarily to CH<sub>2</sub>O + HCO. The next two most significant
bimolecular products are OCHCHO + H and <sup>3</sup>CHCHO + OH, and
not C<sub>2</sub>H<sub>2</sub> + HO<sub>2</sub>. C<sub>2</sub>H<sub>3</sub> + O<sub>2</sub> 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 CH<sub>2</sub>CHO + O overtakes all other product channels

### Weakly Bound Free Radicals in Combustion: â€śPromptâ€ť 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 vibrationalâ€“rotational relaxation process. Here
we characterize this â€śpromptâ€ť dissociation of formyl
(HCO), an important combustion radical, using direct dynamics calculations
for OH + CH<sub>2</sub>O and H + CH<sub>2</sub>O (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 CH<sub>3</sub>NO<sub>2</sub> 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 CH<sub>3</sub> and NO<sub>2</sub> 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 CH<sub>3</sub>NO<sub>2</sub>, and for the reaction of CH<sub>3</sub> with
NO<sub>2</sub>