Hydrogen abstraction reactions of polycyclic aromatic hydrocarbons (PAH) by H atoms play a very important role in both PAH and soot formation processes. However, large discrepancies up to a few orders of magnitude exist among the literature rate constant values. To increase the reliability of the computed rate constants, it is critical to obtain highly accurate potential energy surfaces. For this purpose, we have investigated the energetics of hydrogen abstraction from benzene and naphthalene using both high level-of-theory quantum chemistry methods and a series of density functional theory (DFT) methods, among which M06-2X/6-311g(d,p) has the best performance with a mean unsigned deviation from the CCSD(T)/CBS calculations of 1.0 kcal mol−1 for barrier heights and reaction energies. Thus, M06-2X/6-311g(d,p) has then been applied to compute the potential energy surfaces of the hydrogen abstraction reactions of a series of larger PAH. Based on the quantum chemistry calculations, rate constants are computed using the canonical transition state theory. The effects of the PAH size, structure, and reaction site on the energetics and rate constants are examined systematically. Finally, the hydrogen abstraction rate constants for application in PAH and soot surface chemistry models are recommended.

Unimolecular reactions play an important role in combustion kinetics. An important task of reaction kinetic analysis is to obtain the phenomenological rate coefficients for unimolecular reactions based on the master equation approach. In most cases, the eigenvalues of the transition matrix describing collisional internal energy relaxation are of much larger magnitude than and well separated from the chemically significant eigenvalues, so that phenomenological rate coefficients may be unequivocally derived for incorporation in combustion mechanisms. However, when dealing with unimolecular reactions for a large molecule, especially at high temperatures, the large densities of states of the reactant cause the majority of the population distribution to lie at very high energy levels where the microcanonical reaction rate constants are large and the relaxation and chemical eigenvalues overlap, so that well-defined phenomenological rate coefficients cannot be determined. This work attempts to analyze the effect of overlapping eigenvalues on the high-temperature kinetics of a large oxyradical, based on microcanonical reaction rates and population distributions as well as the eigenvalue spectrum of the transition matrix from the master equation. The aim is to provide a pragmatic method for obtaining the most effective rate coefficients for competing elimination, dissociation, and bimolecular reactions for incorporation in combustion mechanisms. Our approach is demonstrated with a representative example, thermal decomposition and H addition reactions of the corannulene oxyradical.

In order to explore the hydrogen abstraction reaction kinetics of unsaturated methyl esters by hydrogen atoms, we selected two molecules for study, in particular methyl 3-butenoate and methyl 2-butenoate, whose CC double bonds are at different locations. We first determined an accurate and efficient electronic structure method for the investigation by considering eight hydrogen abstraction reactions and comparing their barrier heights and reaction energies computed using several exchange–correlation density functionals to those obtained from CCSD(T)-F12a/jun-cc-pVTZ coupled cluster calculations. In this way, we found the M06-2X/ma-TZVP method to have the best performance with a mean unsigned deviation from the CCSD(T) calculations of 0.51 kcal mol−1. Based on quantum-chemical calculations by using the M06-2X/ma-TZVP method, we then computed rate constants for 298–2500 K by direct dynamics calculations using multi-structural canonical variational transition state theory including tunneling by the multi-dimensional small-curvature tunneling approximation (MS-CVT/SCT). The computed transmission coefficients were compared with those obtained using the zero-curvature tunneling (ZCT) and one-dimensional Eckart tunneling (ET) approximations. We employed the multi-structural torsional method (MS-T) to include the multiple-structure and torsional potential anharmonic effects. The results show that the variational recrossing transmission coefficients range from 0.6 to 1.0, and the multi-structural torsional anharmonicity introduces a factor of 0.5–2.5 into the rate constant, while the tunneling transmission coefficients obtained by SCT can be as large as 17.4 and differ considerably from those determined by the less accurate ZCT and ET approximations. In addition, independent of the location of the CC double bond, the dominant hydrogen abstraction reactions occur at the allylic sites.

In this study, we examined the influence of an embedded five-membered ring on the thermal decomposition of graphene oxyradicals. Their decomposition potential energy surfaces were explored at the B3LYP/6-311g(d,p) level. The temperature and pressure dependence of the rate coefficients was computed by master equation modeling. The results suggest that the embedded five-membered ring leads to a generally slower decomposition rate for CO elimination than that of graphene oxyradicals with only six-membered rings, but the impact of the embedded five-membered ring diminishes when it is two layers away from the edge. Well-behaved first-order kinetics was demonstrated at 1500 K, but collisional relaxation was incomplete on the dissociation timescale at higher temperatures. The ways of determining the effective rate coefficient were discussed and the influence of the uncertainty in rate constants on the predictions of species profiles was also estimated by performing kinetic modeling.

Using density functional theory and master equation modeling, we have studied the kinetics of small unsaturated aliphatic molecules reacting with polycyclic aromatic hydrocarbon (PAH) molecules having a diradical character. We have found that these reactions follow the mechanism of carbon addition and hydrogen migration (CAHM) on both spin-triplet and open-shell singlet potential energy surfaces at a rate that is about ten times those of the hydrogen-abstraction-carbon-addition (HACA) reactions at 1500 K in the fuel-rich postflame region. The results also show that the most active reaction sites are in the center of the zigzag edges of the PAHs. Furthermore, the reaction products are more likely to form straight rather than branched aliphatic side chains in the case of reacting with diacetylene. The computed rate constants are also found to be independent of pressure at conditions of interest in soot formation, and the activation barriers of the CAHM reactions are linearly correlated with the diradical characters.

Biodiesel contains a large proportion of unsaturated fatty acid methyl esters. Its combustion characteristics, especially its ignition behavior at low temperatures, have been greatly affected by these C═C double bonds. In this work, we performed a theoretical analysis of the effect of C═C double bonds on the low-temperature reactivity of alkenylperoxy radicals, the key intermediates from the low-temperature combustion of biodiesel. To understand how double bonds affect the fate of peroxy radicals, we selected three representative peroxy radicals from heptane, heptene, and heptadiene having zero, one, and two double C═C bonds, respectively, for study. The potential energy surfaces were explored at the CBS-QB3 level, and the reaction rate constants were computed using canonical/variational transition state theories. We have found that the double bond is responsible for the very different bond dissociation energies of the various types of C–H bonds, which in turn affect significantly the reaction kinetics of alkenylperoxy radicals.

Using density functional theory, a possible pathway of soot surface growth is studied in the low-temperature, postflame region in which spin-triplet polycyclic aromatic hydrocarbon (PAH) molecules with a small singlet–triplet energy gap react with unsaturated aliphatics such as acetylene via the carbon-addition-hydrogen-migration (CAHM) reaction. Results show that a PAH-core-aliphatic-shell structure is formed and the mass growth rate of this triplet soot surface growth reaction is one order of magnitude larger than that of the surface hydrogen-abstraction-carbon-addition (HACA) reaction at temperatures below 1500 K.

A possible pathway of soot nucleation, in which localized π electrons play an important role in binding the polycyclic aromatic hydrocarbon (PAH) molecules having multiradical characteristics to form stable polymer molecules through covalent bonds, is studied using density functional and semiempirical methods. Results show that the number of covalent bonds formed in the dimerization of two identical PAHs is determined by the radical character, and the sites to form bonds are related to the aromaticity of individual six-membered ring structure. It is further shown that the binding energy of dimerization increases linearly with the diradical character in the range relevant to soot nucleation.

In this study, spatially resolved measurement of soot particle size distribution functions (PSDFs) down to ∼1 nm from a laminar premixed burner-stabilized stagnation ethylene flame was made by paralleling a commercial 3936 Scanning Mobility Particle Spectrometer (3936 SMPS) and a Diethylene Glycol (DEG) SMPS. While the 3936 SMPS may detect particles with a mobility diameter of 3–150 nm, DEG SMPS can be used to measure incipient soot particles of 1–10 nm. We found that the minimum diameter of the incipient soot particles appeared at ∼1.5 nm (though with some uncertainty caused by the classification device). A complete bimodality of the PSDFs was observed quantitatively when the burner-to-stagnation surface separation distance (Hp) was greater than 0.6 cm. Characterized by a lognormal distribution, the first peak appears to be relatively stable at different Hp, with the geometric standard deviation varying from 1.1 to 1.3 and the peak diameter ranging from 1.9 to 2.9 nm. The absolute number density of particles no bigger than the first peak diameter was found to be positively related to the first peak diameter and the geometric mean diameter of these particles.

Recent studies showed that current H2/CO kinetic models failed to match the measured laminar flame speeds of H2/O2/CO2 mixtures at high pressures. To explore the source of discrepancy, we performed uncertainty analysis using the Data Collaboration method, and obtained dataset inconsistency if these measured values were included. We then conducted experiments at similar conditions and found that our new measured values are consistent with other data in the dataset. Our uncertainty analyses suggest two approaches to improve the model prediction performance: by reducing the uncertainties of model parameters directly, and by designing and conducting experiments that can effectively constrain the model predictions.

In order to explore the hydrogen abstraction reaction kinetics of unsaturated methyl esters by hydrogen atoms, we selected two molecules for study, in particular methyl 3-butenoate and methyl 2-butenoate, whose CC double bonds are at different locations. We first determined an accurate and efficient electronic structure method for the investigation by considering eight hydrogen abstraction reactions and comparing their barrier heights and reaction energies computed using several exchange–correlation density functionals to those obtained from CCSD(T)-F12a/jun-cc-pVTZ coupled cluster calculations. In this way, we found the M06-2X/ma-TZVP method to have the best performance with a mean unsigned deviation from the CCSD(T) calculations of 0.51 kcal mol−1. Based on quantum-chemical calculations by using the M06-2X/ma-TZVP method, we then computed rate constants for 298–2500 K by direct dynamics calculations using multi-structural canonical variational transition state theory including tunneling by the multi-dimensional small-curvature tunneling approximation (MS-CVT/SCT). The computed transmission coefficients were compared with those obtained using the zero-curvature tunneling (ZCT) and one-dimensional Eckart tunneling (ET) approximations. We employed the multi-structural torsional method (MS-T) to include the multiple-structure and torsional potential anharmonic effects. The results show that the variational recrossing transmission coefficients range from 0.6 to 1.0, and the multi-structural torsional anharmonicity introduces a factor of 0.5–2.5 into the rate constant, while the tunneling transmission coefficients obtained by SCT can be as large as 17.4 and differ considerably from those determined by the less accurate ZCT and ET approximations. In addition, independent of the location of the CC double bond, the dominant hydrogen abstraction reactions occur at the allylic sites.

In the search for an accurate yet inexpensive method to predict thermodynamic properties of large hydrocarbon molecules, we have developed an automatic and adaptive distance-based group contribution (DBGC) method. The method characterizes the group interaction within a molecule with an exponential decay function of the group-to-group distance, defined as the number of bonds between the groups. A database containing the molecular bonding information and the standard enthalpy of formation (Hf,298K) for alkanes, alkenes, and their radicals at the M06-2X/def2-TZVP//B3LYP/6-31G(d) level of theory was constructed. Multiple linear regression (MLR) and artificial neural network (ANN) fitting were used to obtain the contributions from individual groups and group interactions for further predictions. Compared with the conventional group additivity (GA) method, the DBGC method predicts Hf,298K for alkanes more accurately using the same training sets. Particularly for some highly branched large hydrocarbons, the discrepancy with the literature data is smaller for the DBGC method than the conventional GA method. When extended to other molecular classes, including alkenes and radicals, the overall accuracy level of this new method is still satisfactory.

In this study, we examined the influence of an embedded five-membered ring on the thermal decomposition of graphene oxyradicals. Their decomposition potential energy surfaces were explored at the B3LYP/6-311g(d,p) level. The temperature and pressure dependence of the rate coefficients was computed by master equation modeling. The results suggest that the embedded five-membered ring leads to a generally slower decomposition rate for CO elimination than that of graphene oxyradicals with only six-membered rings, but the impact of the embedded five-membered ring diminishes when it is two layers away from the edge. Well-behaved first-order kinetics was demonstrated at 1500 K, but collisional relaxation was incomplete on the dissociation timescale at higher temperatures. The ways of determining the effective rate coefficient were discussed and the influence of the uncertainty in rate constants on the predictions of species profiles was also estimated by performing kinetic modeling.

Unimolecular reactions play an important role in combustion kinetics. An important task of reaction kinetic analysis is to obtain the phenomenological rate coefficients for unimolecular reactions based on the master equation approach. In most cases, the eigenvalues of the transition matrix describing collisional internal energy relaxation are of much larger magnitude than and well separated from the chemically significant eigenvalues, so that phenomenological rate coefficients may be unequivocally derived for incorporation in combustion mechanisms. However, when dealing with unimolecular reactions for a large molecule, especially at high temperatures, the large densities of states of the reactant cause the majority of the population distribution to lie at very high energy levels where the microcanonical reaction rate constants are large and the relaxation and chemical eigenvalues overlap, so that well-defined phenomenological rate coefficients cannot be determined. This work attempts to analyze the effect of overlapping eigenvalues on the high-temperature kinetics of a large oxyradical, based on microcanonical reaction rates and population distributions as well as the eigenvalue spectrum of the transition matrix from the master equation. The aim is to provide a pragmatic method for obtaining the most effective rate coefficients for competing elimination, dissociation, and bimolecular reactions for incorporation in combustion mechanisms. Our approach is demonstrated with a representative example, thermal decomposition and H addition reactions of the corannulene oxyradical.