Kinetics and Products of Vinyl + 1,3-Butadiene, a Potential Route to Benzene.

The reaction between vinyl radical, C2H3, and 1,3-butadiene, 1,3-C4H6, has long been recognized as a potential route to benzene, particularly in 1,3-butadiene flames, but the lack of reliable rate coefficients has hindered assessments of its true contribution. Using laser flash photolysis and visible laser absorbance (λ = 423.2 nm), we measured the overall rate coefficient for C2H3 + 1,3-C4H6, k1, at 297 K ≤ T ≤ 494 K and 4 ≤ P ≤ 100 Torr. k1 was in the high-pressure limit in this range and could be fit by the simple Arrhenius expression k1 = (1.1 ± 0.2) × 10(-12) cm(3) molecule(-1) s(-1) exp(-9.9 ± 0.6 kJ mol(-1)/RT). Using photoionization time-of-flight mass spectrometry, we also investigated the products formed. At T ≤ 494 K and P = 25 Torr, we found only C6H9 adduct species, while at 494 K ≤ T ≤ 700 K and P = 4 Torr, we observed ≤∼10% branching to cyclohexadiene in addition to C6H9. Quantum chemistry master-equation calculations using the modified strong collision model indicate that n-C6H9 is the dominant product at low temperature, consistent with our experimental results, and predict the rate coefficient and branching ratios at higher T where chemically activated channels become important. Predictions of k1 are in close agreement with our experimental results, allowing us to recommend the following modified Arrhenius expression in the high-pressure limit from 300 to 2000 K: k1 = 6.5 × 10(-20) cm(3) molecule(-1) s(-1) T(2.40) exp(-1.76 kJ mol(-1)/RT).

Reaction rate constant of CH2O + H = HCO + H2 revisited: a combined study of direct shock tube measurement and transition state theory calculation.

The rate constant of the H-abstraction reaction of formaldehyde (CH2O) by hydrogen atoms (H), CH2O + H = H2 + HCO, has been studied behind reflected shock waves with use of a sensitive mid-IR laser absorption diagnostic for CO, over temperatures of 1304-2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxane as the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintain relatively constant H atom concentrations, the current study significantly boosted the measurement sensitivity of the target reaction and suppressed the influence of interfering reactions. The measured CH2O + H rate constant can be expressed in modified Arrhenius from as kCH2O+H(1304-2006 K, 1 atm) = 1.97 × 10(11)(T/K)(1.06) exp(-3818 K/T) cm(3) mol(-1)s(-1), with uncertainty limits estimated to be +18%/-26%. A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12 level of theory, is in good agreement with the shock tube measurement and extended the temperature range of the current study to 200-3000 K, over which a modified Arrhenius fit of the rate constant can be expressed as kCH2O+H(200-3000 K) = 5.86 × 10(3)(T/K)(3.13) exp(-762 K/T) cm(3) mol(-1)s(-1).

Master equation modeling of the unimolecular decompositions of hydroxymethyl (CH2OH) and methoxy (CH3O) radicals to formaldehyde (CH2O) + H.

α-Hydroxyalkyl radical intermediates (RCHOH, R = H, CH3, etc.) are common to the combustion of nearly all oxygenated fuels. Despite their importance in modeling the combustion phenomena of these compounds through detailed kinetic models, the unimolecular decomposition kinetics remains uncertain for even the simplest α-hydroxyalkyl radical, hydroxymethyl (CH2OH). In this study, RRKM/master equation simulations were carried out for CH2OH decomposition to formaldehyde + H between N2 pressures of 0.01-100 atm and temperatures ranging from 1000 to 1800 K. These simulations were guided by methoxy (CH3O) decomposition calculations between pressures of 0.01-100 atm and temperatures ranging from 600 to 1200 K, in both helium and nitrogen. Excellent agreement of the methoxy results was observed for all regions where experimental data exist. Rates were parametrized as a function of both density and temperature within the Troe formalism. Temperature- and pressure-dependent uncertainty estimates are provided, with the largest source of uncertainty being tunneling contributions at very low pressures and at the lowest temperatures. In the regimes relevant to combustion, uncertainties range from factors of 1.4-2 for CH3O decomposition, and from 1.5-2.6 for CH2OH decomposition. The results of this study are expected to have an impact on the high temperature combustion modeling of methanol, as formation rates to CH2O + H from CH2OH are notably different from previous estimates under some conditions.