Computational Kinetics of Hydroperoxybutylperoxy Isomerizations and Decompositions: A Study of the Effect of Hydrogen Bonding.

Hydroperoxyalkylperoxy (OOQOOH) radicals are important intermediates in combustion chemistry. The conventional isomerization of OOQOOH radicals to form ketohydroperoxides has been long believed to be the most important chain branching reaction under the low-temperature combustion conditions. In this work, the kinetics of competing pathways (alternative isomerization, concerted elimination, and H-exchange pathways) to the conventional isomerization of different ß-, γ- and Δ-OOQOOH butane isomers are investigated. Six- and seven-membered ring conventional isomerizations are found to be the dominant pathways, whereas alternative isomerizations are more important than conventional isomerization, when the latter proceeded via a more strained transition state ring. The oxygen atoms in OOQOOH radicals introduce intramolecular hydrogen bonding (HB) that significantly affects the energies of reacting species and transition states, ultimately influencing chemical kinetics. Conceptually, HB has a dual effect on the stability of chemical species, the first being the stabilizing effect of the actual intramolecular HB force, and the second being the destabilizing effect of ring strain imposed by the HB conformer. The overall effect can be quantified by determining the difference between the minimum energy conformers of a chemical species or transition state that have HB and that do not have HB (non-hydrogen bonding (NHB)). The stabilization effect of HB on the species and transition sates is assessed, and its effect on the calculated rate constants is also considered. Our results show that, for most species and transition states, HB stabilizes their energies by as much as 2.5 kcal/mol. However, NHB conformers are found to be more stable by up to 2.7 kcal/mol for a few of the considered species. To study the effect of HB on rate constants, reactions are categorized into two groups ( groups one and two) based on the structural similarity of the minimum energy conformers of the reactant and transition state, for a particular reaction. For cases where the reactant and transition state conformers are similar (i.e., both HB or NHB structures), group one, the effect of HB on reaction kinetics is major only if the magnitudes of the stabilization energy of the reactant and transition state are quite different. Meanwhile, for group two, where the reactant and transition state prefer different conformers (one HB and the other NHB), HB affects the kinetics when the stabilization energy of the reactant or transition state is significant or the entropy effect is important. This information is useful in determining corrections accounting for HB effects when assigning rate parameters for chemical reactions using estimation and/or analogy, where analogies usually result in inaccuracies when modeling atmospheric and combustion processes.

High-Pressure Limit Rate Rules for α-H Isomerization of Hydroperoxyalkylperoxy Radicals.

Hydroperoxyalkylperoxy (OOQOOH) radical isomerization is an important low-temperature chain branching reaction within the mechanism of hydrocarbon oxidation. This isomerization may proceed via the migration of the α-hydrogen to the hydroperoxide group. In this work, a combination of high level composite methods-CBS-QB3, G3, and G4-is used to determine the high-pressure-limit rate parameters for the title reaction. Rate rules for H-migration reactions proceeding through 5-, 6-, 7-, and 8-membered ring transitions states are determined. Migrations from primary, secondary and tertiary carbon sites to the peroxy group are considered. Chirality is also investigated by considering two diastereomers for reactants and transition states with two chiral centers. This is important since chirality may influence the energy barrier of the reaction as well as the rotational energy barriers of hindered rotors in chemical species and transition states. The effect of chirality and hydrogen bonding interactions in the investigated energies and rate constants is studied. The results show that while the energy difference between two diastereomers ranges from 0.1-3.2 kcal/mol, chirality hardly affects the kinetics, except at low temperatures (atmospheric conditions) or when two chiral centers are present in the reactant. Regarding the effect of the H-migration ring size, it is found that in most cases, the 1,5 and 1,6 H-migration reactions have similar rates at low temperatures (below ∼830 K) since the 1,6 H-migration proceeds via a cyclohexane-like transition state similar to that of the 1,5 H-migration.

Modeling Ignition of a Heptane Isomer: Improved Thermodynamics, Reaction Pathways, Kinetics, and Rate Rule Optimizations for 2-Methylhexane.

Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2-methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2-methylhexane, based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxy-alkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new high-pressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate the ability to optimize the kinetic model using both manual and automated techniques for rate parameter tunings to improve agreement with the measured ignition delay time data. Finally, additional low temperature chain branching reaction pathways are shown to improve the model's performance. The present approach to model development provides better performance across extended operating conditions while also strengthening the fundamental basis of the model.

PAH growth initiated by propargyl addition: mechanism development and computational kinetics.

Polycyclic aromatic hydrocarbon (PAH) growth is known to be the principal pathway to soot formation during fuel combustion, as such, a physical understanding of the PAH growth mechanism is needed to effectively assess, predict, and control soot formation in flames. Although the hydrogen abstraction C2H2 addition (HACA) mechanism is believed to be the main contributor to PAH growth, it has been shown to under-predict some of the experimental data on PAHs and soot concentrations in flames. This article presents a submechanism of PAH growth that is initiated by propargyl (C3H3) addition onto naphthalene (A2) and the naphthyl radical. C3H3 has been chosen since it is known to be a precursor of benzene in combustion and has appreciable concentrations in flames. This mechanism has been developed up to the formation of pyrene (A4), and the temperature-dependent kinetics of each elementary reaction has been determined using density functional theory (DFT) computations at the B3LYP/6-311++G(d,p) level of theory and transition state theory (TST). H-abstraction, H-addition, H-migration, ß-scission, and intramolecular addition reactions have been taken into account. The energy barriers of the two main pathways (H-abstraction and H-addition) were found to be relatively small if not negative, whereas the energy barriers of the other pathways were in the range of (6-89 kcal·mol(-1)). The rates reported in this study may be extrapolated to larger PAH molecules that have a zigzag site similar to that in naphthalene, and the mechanism presented herein may be used as a complement to the HACA mechanism to improve prediction of PAH and soot formation.

Heterogeneous ozonolysis of folpet and dimethomorph: a kinetic and mechanistic study.

The research study discussed in this paper concerns determination of the kinetic rate constants for heterogeneous degradation of the pesticides folpet and dimethomorph by ozone at room temperature. The study also involves identification of the degradation products of the analyzed compound in the condensed phase by solid-phase microextraction/gas chromatography/mass spectrometry (SPME/GC/MS). The heterogeneous O3 reactivity of the identified degradation product of dimethomorph, 4-chlorophenyl 3,4-dimethoxyphenyl methanone (CPMPM), is also evaluated experimentally. The obtained results show that the rate constant values of the analytes are (1.7 ± 0.5) × 10(-19), (2.1 ± 0.8) × 10(-19), (2.6 ± 0.2) × 10(-20), and (2.7 ± 0.2) × 10(-20) cm(3)·molecule(-1)·s(-1) for (Z)-dimethomorph, (E)-dimethomorph, folpet, and CPMPM, respectively. Such values implicate heterogeneous ozone lifetimes that vary from a few days to several months, meaning that, depending on their reactivity with respect to other atmospheric oxidants, these compounds might be relatively persistent and may be transported to regions far from their point of application.