Search

ABSTRACT Presented here are computed rates for the thermal unimolecular decomposition of a variety of alkoxy radicals with four- and five-carbon length backbones. Three classes of molecules are examined: alkoxy radicals with saturated hydrocarbon backbones, those with alcohol functional groups, and those with carbonyl functional groups. The chosen species represent many of those found during the combustion of fossil fuels as well as bio-derived alternatives. Density functional theory calculations were benchmarked against higher level coupled cluster calculations and used to explore the potential energy surfaces of these systems. Transition state theory was used to calculate high-pressure limit rate coefficients of all radical intermediates in the regimes relevant to atmospheric chemistry and combustion. We show that the assumption that alkoxy radicals quickly decompose via β-scission to aldehydes and other radicals is not valid for some of the alkoxy radicals investigated in this work. We further illustrate how intra-H migrations in larger alkoxy radicals with carbonyl and alcohol functional groups can dominate unimolecular decomposition under combustion and atmospheric relevant conditions. Finally, we discuss why carbonyl groups can increase or decrease intra-H migration barriers depending on their location relative to the transferring H-atom. [ABSTRACT FROM AUTHOR]

Thereaction between vinyl radical, C2H3,and 1,3-butadiene, 1,3-C4H6, has long been recognizedas a potential route to benzene, particularly in 1,3-butadiene flames,but the lack of reliable rate coefficients has hindered assessmentsof its true contribution. Using laser flash photolysis and visiblelaser absorbance (λ = 423.2 nm), we measured the overall ratecoefficient for C2H3+ 1,3-C4H6, k1, at 297 K ≤ T≤ 494 K and 4 ≤ P≤100 Torr. k1was in the high-pressurelimit in this range and could be fit by the simple Arrhenius expression k1= (1.1 ± 0.2) × 10–12cm3molecule–1s–1exp(−9.9 ± 0.6 kJ mol–1/RT). Using photoionization time-of-flight massspectrometry, we also investigated the products formed. At T≤ 494 K and P= 25 Torr, we foundonly C6H9adduct 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 calculationsusing the modified strong collision model indicate that n-C6H9is the dominant product at low temperature,consistent with our experimental results, and predict the rate coefficientand branching ratios at higher Twhere chemicallyactivated channels become important. Predictions of k1are in close agreement with our experimental results,allowing us to recommend the following modified Arrhenius expressionin the high-pressure limit from 300 to 2000 K: k1= 6.5 × 10–20cm3molecule–1s–1T2.40exp(−1.76 kJ mol–1/RT). [ABSTRACT FROM AUTHOR]

The rate constant of the H-abstractionreaction of formaldehyde(CH2O) by hydrogen atoms (H), CH2O + H = H2+ HCO, has been studied behind reflected shock waves withuse of a sensitive mid-IR laser absorption diagnostic for CO, overtemperatures of 1304–2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxaneas the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintainrelatively constant H atom concentrations, the current study significantlyboosted the measurement sensitivity of the target reaction and suppressedthe 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 × 1011(T/K)1.06exp(−3818 K/T) cm3mol–1s–1, with uncertainty limits estimated to be +18%/–26%.A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12level of theory, is in good agreement with the shock tube measurementand extended the temperature range of the current study to 200–3000K, over which a modified Arrhenius fit of the rate constant can beexpressed as kCH2O+H(200–3000K) = 5.86 × 103(T/K)3.13exp(−762 K/T) cm3mol–1 s–1. [ABSTRACT FROM AUTHOR]

ABSTRACT The unimolecular decomposition of two radical isomers of C2H5O (CH3CH2O/ethoxy, CH3CHOH/α-hydroxyethyl) are investigated by means of Rice-Ramsperger-Kassel-Marcus/master equation simulations in helium and nitrogen bath gases on an accurate one-dimensional potential energy surface. For ethoxy, simulations are carried out between temperatures of 406 and 1200 K and pressures of 0.001 and 100 atm. For CH3CHOH, simulations are carried out between temperatures of 800 and 1500 K and pressures of 0.001 and 100 atm. Results are compared with available experimental data, with good agreement. The dominant product of α-hydroxyethyl decomposition is CH3CHO + H, with C2H3OH + H and CH3 + CH2O, being minor channels. Rate coefficients are strongly dependent on temperature and pressure and are recommended with attendant uncertainty factor estimates. The relative roles of vinyl alcohol and acetaldehyde in the context of combustion chemistry are also discussed. [ABSTRACT FROM AUTHOR]