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The unimolecular decomposition kinetics of methyl propanoate (MP), including the direct C−O/C−C bond fissions and molecular reaction channels, were studied by using high-level ab initio calculations and Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) theory. Four homolytic bond-fission and ten hydrogen transfer reactions of the MP unimolecular decomposition were identified. The phenomenological rate constants were determined using the RRKM/ME theory over a temperature range of 1000−2000 K and a pressure range of 0.01 atm to the high-pressure limit. At 1 atm, the branching ratios show that the dissociation reactions MP ↔ •CH 2 C( O)OCH 3 + CH 3 , MP ↔ CH 3 OC•( O) + C 2 H 5 and MP ↔ CH 3 CH 2 C( O)O• + CH 3 dominate MP pyrolysis over the temperature range of 1000−1500 K. Our calculated rate constants were adopted in a detailed kinetic model to reproduce the laser-absorption measured CO and CO 2 concentration time-histories during the pyrolysis of 0.2% MP/Ar in a shock tube from 1292−1551 K and at 1.6 atm. The updated mechanism accurately predicted the early-time CO and CO 2 formation over the entire temperature range. In particular, our mechanism well reproduced the CO 2 time-histories from the early-time formation to the final plateau level. [ABSTRACT FROM AUTHOR]

[Display omitted] • The CGA-ONIOM method was extended for rate constant calculation to prove it accuracy and efficiency in determining the thermodynamics and kinetics of large combustion reaction system. • A hybrid CGA-ONIOM-DFT framework was proposed to approach accurate kinetics of large reaction system with loose transition state at affordable cost. • The CGA-ONIOM-DFT framework was shown to be more efficient for larger reaction system. Accurate and efficient determination of thermodynamics and kinetics is long pursued but hindered by electronic energy calculations due to very high computational cost. In this work, we propose a Cascaded Group Additivity (CGA)-ONIOM-DFT framework targeting fast thermodynamic and kinetic predictions. We studied the reaction of cyclopentane with OH radical as a proof-of-principle demonstration. The CGA-ONIOM method accurately constructed the potential energy surface with a mean unsigned error of 0.20 kcal∙mol-1 compared to the full-level CCSD(T)/CBS results. The calculated rate constants agree well with the experimental data, suggesting the feasibility of using the CGA-ONIOM-DFT framework for combustion chemical kinetics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Solving the accurate energy barrier calculations for H-abstraction reactions (amines + H) at a low level HF method. • Providing the accurate reaction kinetics for amines combustion modeling. • Evans-Polanyi diagram for each sub-class and the corresponding rate rule are also performed for the 154 reactions. The H-abstraction reaction kinetics of primary, secondary and tertiary amines with H atom are computed using the isodesmic reaction method. The average unsigned deviation of energy barriers is 12.40 kcal mol−1 at the HF/6-31+G(d) level compared to the CCSD(T)/CBS(D-T-Q) method but reduced to 0.75 kcal mol−1 after corrections. The calculated rate constants of all reactions and corresponding rate rules are also presented. This work solves the accurate energy barrier calculations for larger molecules reaction amines + H at a low level HF method and also provides the accurate reaction kinetics for the critical important H-abstraction reactions of amines combustion modeling. [ABSTRACT FROM AUTHOR]

We report the first demonstration of heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for the simultaneous temperature and H2O concentration measurements in combustion environments. Two continuous-wave distributed-feedback quantum cascade lasers (DFB-QCLs) at 5.27 and 10.53 µm were used to exploit the strong H2O transitions (1897.52 and 949.53 cm−1) at high temperatures. The injection current of each QCL was modulated at sub-GHz or GHz to generate the three-tone radiation and the dispersion signal was detected by the radio-frequency down-conversion heterodyning. The peak-to-peak ratio of the two H2O dispersion spectra exhibits a monotonic relationship with temperature over the temperature range of 1000-3000 K, indicating the capability of performing two-line thermometry using laser dispersion spectroscopy. We measured the temperatures of CH4/air flames at different equivalence ratios (Φ = 0.8-1.2), yielding a good agreement with the corresponding thermocouple measurements. In addition, one-dimensional kinetic modeling coupled with a detailed chemical kinetic mechanism (GRI 3.0) was conducted to compare with the measured H2O concentrations using HPSDS. Finally, we demonstrated HPSDS is immune to optical power fluctuations by measuring the dispersion spectra at varied incident laser powers. [ABSTRACT FROM AUTHOR]

It remains a long-standing challenge to predict the reaction kinetics of large complex hydrocarbon system from first principles. Exo -tricyclo[5.2.1.02,6decane, which is further called tricyclodecane and which is the major component in JP-10 aviation fuel, and therefore its reaction with hydroxyl radical is very significant. We report high-level theoretical calculations of the rate constants of tricyclodecane + OH for the first time. We show that the exchange-correlation hybrid functional M06-2X with MG3S basis set provides an accurate and efficient calculation of the barrier heights and reaction energies. A total of 20 reaction pathways for hydrogen abstractions from chair and boat tricyclodecane conformations are considered for direct dynamics. Multi-structural canonical variational transition state theory with small-curvature tunneling (MS-CVT/SCT) is adopted to calculate the rate constants of the title reaction at 200–2000 K. The theoretical results obtained using MS-CVT/SCT with curvilinear coordinates are in fairly good agreement with the experimental measurements. We also determine the branching fractions as a function of temperature and find the tertiary abstraction from tricyclodecane is dominant. This work highlights the significance of modern DFT methods for studying combustion kinetics of large fuel molecules. [ABSTRACT FROM AUTHOR]

• Explored various isomerization and decomposition pathways for MA, MB and MC radicals, considering not only conventional C–C and C–H β- scission reactions but also including C–O β- scission reactions. • High-level theoretical calculations are conducted to obtain the rate constants for isomerization and decomposition reactions for MA, MB and MC radicals over wide temperature and pressure ranges and the thermodynamics of involved species. • Improved species concentration prediction by updating the calculated rate constants into the kinetic models. Reaction kinetics of radical isomerization and decomposition of three small unsaturated methyl esters of methyl acrylate (MA, C 4 H 6 O 2), methyl butenoate (MB, C 5 H 8 O 2) and methyl crotonate (MC, C 5 H 8 O 2) are systematically studied with high-level quantum chemistry computation. MB and MC are isomers with the C=C double bonds at different positions. The potential energy profiles for these reactions are obtained at the DLPNO-CCSD(T)/CBS(T-Q)//M062X/ma-TZVP level of theory, and the thermodynamics of the involved species are derived at the CCSD(T)/CBS(T-Q) level of theory using the atomization enthalpy method. Our results show that isomerization reactions with stable cyclic transition states have lower reaction barrier heights, making them energetically and kinetically favored. For decomposition reactions, C–C β- scission reactions are more energetically favorable than C–H β- scission reactions, and the C–O β- scission reactions to produce HCHO are the most energetically favorable. The related rate constants are calculated via solving the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation at 300–2500 K and over a pressure range of 0.01–100 atm along with the high-pressure limit. It is shown that isomerization reaction dominates at low and intermediate temperatures; while as the temperature increases, the importance of decomposition reaction becomes apparent. Pressure is found to have a significant impact on the studied reactions. Additionally, the computed rate constants of this work agree well with the available theoretical results but differ significantly from the estimated results in the literatures. Kinetic reaction mechanisms are further updated, and the results show that through incorporating the presently computed rate constants and thermodynamics, the experimental species concentration distributions can be better reproduced. This work provides the necessary rate constants and thermodynamics for the kinetic model construction of MA, MB, and MC and are expected to help establish more accurate chemical kinetic models of small unsaturated methyl esters, enriching our understanding of the combustion chemistry of biodiesel. [ABSTRACT FROM AUTHOR]

2-Ethylfuran (EF2), having similar thermophysical properties as gasoline, can be produced from biomass and has received increasing attention in recent years. The reaction of EF2 with OH radical plays a fundamental role in combustion chemistry of furan fuels. This work provides a comprehensive investigation on the reaction kinetics of the title reaction. The performance of several density functional theory methods is evaluated against the DLPNO CCSD(T)/CBS(T-Q) benchmark method. The results show that M08-HX/jun-cc-pVTZ is the best one for both abstraction and addition reactions with the lowest average unsigned deviations of 0.61 and 0.63 kcal mol−1, respectively. Multi-structural variational transition state theory combined with small-curvature tunneling approximation (MS-CVT/SCT) is employed to calculate the high-pressure limit (HPL) rate constants over a wide temperature range of 500−2000 K. The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) method is used to obtain the pressure-dependent rate constants for addition reactions at several selected pressures from 0.01 to 100 atm. We have measured the rate constants of EF2 + OH reaction behind reflected shock wave at temperatures of 985−1342 K and pressure around 1 bar. The measured rate constants exhibit a positive temperature dependence and may be described by a three-parameter Arrhenius expression: k = 1.28 × 10 − 10 × T 6.89 exp (5318 T) (cm3 mol−1 s–1). The calculated results are in good agreement with measurements and the total rate constants exhibit a non-monotonic temperature dependence due to the competition between the abstraction and addition channels. At the HPL, the addition reaction is dominant, and the branching ratio is larger than abstraction reaction at 500–1800 K and is 0.92 at 500 K. Finally, to evaluate the performance of our calculations on the ignition delay time of EF2, three available kinetic models (Somers et al., Xu et al. and Li et al.) of EF2 are updated and the results show that the predictions of Li's model are improved. [ABSTRACT FROM AUTHOR]