In this study, the reactions of Ċ5H9 radicals are theoretically investigated, with a particular emphasis on hydrogen atom addition reactions to 1,3-pentadiene (C5H8) to form Ċ5H9 radicals, although the subsequent isomerization and decomposition reactions of the Ċ5H9 radicals are also of direct relevance to the radicals formed from the pyrolysis and oxidation of species including pentene and cyclopentane. Moreover, H-atom abstraction reactions by hydrogen atoms from 1,3-pentadiene are also investigated. The geometries and frequencies of 63 potential energy surface (PES) minima and 88 transition states are optimized at the ωB97XD/aug-cc-pVTZ level of theory. Spin-unrestricted open-shell single-point energies for all the species are calculated at the CCSD­(T)/aug-cc-pVTZ level of theory with basis set corrections from MP2/aug-cc-pVXZ (where X = T and Q). A one-dimensional hindered rotor treatment is employed for torsional modes, with the M06-2X/6-311++G­(D,P) method used to compute the potential energy as a function of the dihedral angle. The high-pressure limiting rate constants and the thermochemical properties for C5 species are calculated using the Master Equation System Solver (MESS) with conventional transition-state theory and comparisons made with existing available literature data. A hydrogen atom can add to the terminal carbon atom of 1,3-pentadiene to form the 2,4-Ċ5H9 radical and/or the internal carbon atoms to form 2,5-Ċ5H9, 1,4-Ċ5H9, and 1,3-Ċ5H9 radicals. Among the four entrance channels for Ḣ atom addition reactions, the formation of 2,4-Ċ5H9 and 1,3-Ċ5H9 radicals is more exothermic in comparison to the other Ċ5H9 isomers (2,5-Ċ5H9, 1,4-Ċ5H9) because of the resonantly stabilized allylic structure. Consequently, the formation of the former is generally dominant in terms of barrier heights. Ḣ atom addition reactions to 1,3-pentadiene are compared to available C3–C5 alkenes and dienes, with external addition calculated to be kinetically favored over internal addition. However, the correlation between heats of formation and energy barriers for Ḣ atom addition to 1,2-dienes is different from that for 1,3- and 1,4-dienes. Hydrogen atom addition and abstraction rate constants are also compared for 1,3-pentadiene, with addition found to be dominant. The subsequent unimolecular reactions on the Ċ5H9 PES are found to be highly complex with reactions taking place on a multiple-well multiple-channel PES. For clarity, the reaction mechanism and kinetics of each Ċ5H9 radical are discussed individually in terms of the computed enthalpy of the reaction and activation, the transition-state structure/reaction class, and also in terms of the combustion species for which the reactions are of potential importance. The reactions on the Ċ5H9 PES are divided into three reaction classes (H-shift isomerization, cycloaddition, and β-scission reactions), and the reactivity-structure-based estimation rules for energy barriers are derived for these three reaction classes and compared to literature results for al...

The temperature-and pressure-dependence of rate constants for several radical and unsaturated hydrocarbon reactions (1,3-C5H8/1,4-C5H8/cyC5H8 + Ḣ, C2H4 + Ċ3H5-a, C3H6 + Ċ2H3) are analysed in this paper. The abstraction reactions of these systems are also calculated and compared with available literature data. Ċ5H9 radicals can be produced via Ḣ atom addition reactions to the pentadiene isomers and cyclopentene, and also by H-atom abstraction reactions from 1-and 2-pentene and cyclopentane. Comprehensive Ċ5H9 potential energy surface (PES) analyses and high-pressure limiting rate constants for related reactions have been explored in Part I of this work (J. Phys. Chem. A 2019, 123(22), 9019-9052). In this work, a chemical kinetic model is constructed based on the computed thermochemistry and high-pressure limiting rate constants from Part I, to further understand the chemistry of different C5H8 molecules. The most important channels for these addition reactions are discussed in the present work based on reaction pathway analyses. The dominant reaction pathways for these five systems are combined together to generate a simplified Ċ5H9 PES including nine reactants, 25 transition states (TSs) and nine products. Spin-restricted single point energies are calculated for radicals and TSs on the simplified PES at the ROCCSD(T)/aug-cc-pVTZ level of theory with basis set corrections from MP2/aug-cc-pVXZ (where X = T and Q). Temperature-and pressure-dependent rate constants are calculated using RRKM theory with a Master Equation analysis, with restricted energies used for minima on the simplified Ċ5H9 PES and unrestricted energies for other species, over a temperature range of 300-2000 K and in the pressure range 0.01-100 atm. The rate constants calculated are in good agreement with the ones in the literature. The chemical kinetic model is updated with pressure-dependent rate constants and is used to simulate the species concentration profiles for Ḣ atom addition to cyclopentane and cyclopentene. Through detailed analyses and comparison, this model can reproduce the experimental measurements of species qualitatively and quantitatively with reasonably good agreement.

Emission of nitrogen oxides (NOx) are one of the major environmental concerns arising from the combustion of syngas. Strategies to reduce emission and improve the efficiency of syngas combustion can be developed using computational fluid dynamic simulations to design cleaner and more efficient combustors. Toward this end, an accurate and efficient chemical kinetic mechanism that can describe the combustion chemistry of syngas with NOx under engine-relevant conditions is critical. In this work, a comprehensive survey of detailed mechanisms available in the literature for syngas/NOx combustion reaction system is first conducted. A systematic and comparative chemical kinetic analysis of five detailed mechanisms is performed based on reaction pathway and sensitivity analyses to identify the key reactions of the nitrogen species for a wide range of mixtures including the formation of NOx during syngas combustion, ignition of NH3, H2/N2O, and H2/NO2 mixtures.Comparisons of the reaction pathways from different detailed mechanisms indicate that the detailed 2 chemistry is controlled by a small set of reactions and species. Recent high-level theoretical studies on HONO and HNO2 chemistry including previously neglected important reactions are updated. The rate constants for HNO + O2 = NO + HȮ2 are calculated using ab initio calculations in this work. An efficient high-fidelity skeletal mechanism consisting of 27 species and 130 reactions is developed based on a combination of the directed relation graph with error propagation (DRGEP) method and the simplified iterative screening and structure analysis (ISSA) method. Compared to the detailed mechanisms, the skeletal mechanism retains the major species and reactions for the syngas/NOx system and is validated against typical experimental data resulting in a very good performance.

Hydrogen atom abstraction from propargyl C-H sites of alkynes plays a critical role in determining the reactivity of alkyne molecules and understanding the formation of soot precursors. This work reports a systematic theoretical study on the reaction mechanisms and rate constants for hydrogen abstraction reactions by hydrogen and hydroxy radicals from a series of alkyne molecules with different structural propargyl C-H atoms. Geometry optimizations and frequency calculations for all species are performed at M06-2X/cc-pVTZ level of theory and the hindered internal rotations are also treated at this level. The high-level W1BD and CCSD(T)/CBS theoretical calculations are used as a benchmark for a series of DFT calculations toward the selection of accurate DFT functionals for large reaction systems in this work. Based on the quantum chemistry calculations, rate constants are computed using the canonical transition state theory with tunneling correction and the treatment of internal rotations. The effects of the structure and reaction site on the energy barriers and rate constants are examined systematically. To the best of our knowledge, this work provides the first systematic study for one of the key initiation abstraction reactions for compounds containing propargyl hydrogen atoms.

No abstract