Theoretical Kinetics Analysis for (H) over dot Atom Addition to 1,3-Butadiene and Related Reactions on the (C) over dot4H7 Potential Energy Surface

The oxidation chemistry of the simplest conjugated hydrocarbon, 1,3-butadiene, can provide a first step in understanding the role of polyunsaturated hydrocarbons in combustion and, in particular, an understanding of their contribution toward soot formation. On the basis of our previous work on propene and the butene isomers (1-, 2-, and isobutene), it was found that the reaction kinetics of (H) over dot-atom addition to the C=C double bond plays a significant role in fuel consumption kinetics and influences the predictions of high-temperature ignition delay times, product species concentrations, and flame speed measurements. In this study, the rate constants and thermodynamic properties for (H) over dot-atom addition to 1,3-butadiene and related reactions on the (C) over dot(4)H(7) potential energy surface have been calculated using two different series of quantum chemical methods and two different kinetic codes. Excellent agreement is obtained between the two different kinetics codes. The calculated results including zero-point energies, single-point energies, rate constants, barrier heights, and thermochemistry are systematically compared among the two quantum chemical methods. 1-Methylallyl ((C) over dot(4)H(7)1-3) and 3-buten-1-yl ((C) over dot(4)H(7)1-4) radicals and C2H4 + (C) over dot(2)H(3) are found to be the most important channels and reactivity-promoting products, respectively. We calculated that terminal addition is dominant (>80%) compared to internal (H) over dot-atom addition at all temperatures in the range 298-2000 K. However, this dominance decreases with increasing temperature. The calculated rate constants for the bimolecular reaction C4H6 + (H) over dot -> products and C2H4 + (C) over dot(2)H(3) -> products are in excellent agreement with both experimental and theoretical results from the literature. For selected C-4 species, the calculated thermochemical values are also in good agreement with literature data. In addition, the rate constants for H atom abstraction by (H) over dot atoms have also been calculated, and it is found that abstraction from the central carbon atoms is the dominant channel (>70%) at temperatures in the range of 298-2000 K. Finally, by incorporating our calculated rate constants for both (H) over dot atom addition and abstraction into our recently developed 1,3-butadiene model, we show that laminar flame speed predictions are significantly improved, emphasizing the value of this study.