Ab Initio Study of Key Branching Reactions in Biodiesel and Fischer–Tropsch Fuels

Diastereomers and Low-Temperature Oxidation

Diastereomers have historically been ignored when building kinetic mechanisms for combustion. Low-temperature oxidation kinetics, which continues to gain interest in both combustion and atmospheric communities, may be affected by the inclusion of diastereomers in radical chain-branching pathways. In this work, key intermediates and transition states lacking stereochemical specification in an existing diethyl ether low-temperature oxidation mechanism were replaced with their diastereomeric counterparts. Rate coefficients for reactions involving diastereomers were computed with ab initio transition state theory master equation calculations. The presence of diastereomers increased rate coefficients by factors of 1.2-1.6 across various temperatures and pressures. Ignition delay simulations incorporating these revised rate coefficients indicate that the diastereomers enhanced the overall reactivity of the mechanism by almost 15% and increased the peak ketohydroperoxide concentration by 30% in the negative temperature coefficient region at combustion-relevant pressures. These results provide an illustrative indication of the important role of stereomeric effects in oxidation kinetics.

Ab initio derived group additivity model for intramolecular hydrogen abstraction reactions

A set of group additivity values for intramolecular hydrogen abstraction reactions of alkanes, alkenes and alkynes is reported. Calculating 448 reaction rate coefficients at the CBS-QB3 level of theory for 1-2 up to 1-7 hydrogen shift reactions allowed the estimation of ΔGAV° values for 270 groups. The influence of substituents on (1) the attacking radical, (2) the attacked carbon atom, and (3) the carbon chain between the attacking and attacked reactive atom has been systematically studied. Substituents have been varied between hydrogen atoms and sp3, sp2 and sp hybridized carbon atoms. It has been assumed that substituents further away from the reactive atoms or their connecting carbon chain have negligible influences on the kinetics. This group additivity model is applicable to a wide variety of reactions in the 300-1800 K temperature range. Correlations for tunneling coefficients have been generated which are complementary to the ΔGAV°'s to obtain accurate rate coefficients without the need for imaginary frequencies or electronic energies of activation. These correlations depend on the temperature and activation energy of the exothermic step. The group additivity model has been successfully applied to a test set of reactions also calculated at the CBS-QB3 level of theory. A mean absolute deviation of 1.18 to 1.71 has been achieved showing a good overall accuracy of the model.

Performance of First-Principles-Based Reaction Class Transition State Theory

Performance of the Reaction Class Transition State Theory (RC-TST) for prediction of rates constants of elementary reactions is examined using data from its previous applications to a number of different reaction classes. The RC-TST theory is taking advantage of the common structure denominator of all reactions in a given family combined with structure activity relationships to provide a rigorous theoretical framework to obtain rate expression of any reaction within a reaction class in a simple and cost-effective manner. This opens the possibility for integrating this methodology with an automated mechanism generator for "on-the-fly" generation of accurate kinetic models of complex reacting systems.

Towards high-level theoretical studies of large biodiesel molecules: an ONIOM [QCISD(T)/CBS:DFT] study of hydrogen abstraction reactions of CnH2n+1COOCmH2m+1 + H

Recent interest in biodiesel combustion urges the need for the theoretical chemical kinetics of large alkyl ester molecules.

Hydroxyalkoxy radicals: importance of intramolecular hydrogen bonding on chain branching reactions in the combustion and atmospheric decomposition of hydrocarbons

During both the atmospheric oxidation and combustion of volatile organic compounds, sequential addition of oxygen can lead to compounds that contain multiple hydrogen-bonding sites. The presence of two or more of these sites on a hydrocarbon introduces the possibility of intramolecular H-bonding, which can have a stabilizing effect on the reactants, products, and transition states of subsequent reactions. The present work compares the absolute energies of two sets of conformations, those that contain intramolecular H-bonds and those that lack intramolecular H-bonds, for each reactant, product, and transition state species in the 1,2 through 1,7 H-migrations and Cα-Cβ, Cα-H, and Cα-OH-bond scission reactions in the n-hydroxyeth-1-oxy through n-hydroxyhex-1-oxy radicals, for n ranging from 1 to 6. The difference in energy between the two conformations represents the balance between the stabilizing effects of H-bonds and the steric cost of bringing the two H-bonding sites together. The effect of intramolecular H-bonding and the OH group is assessed by comparing the net intramolecular H-bond stabilization energies, the reaction enthalpies, and barrier heights of the n-hydroxyalkoxy radical reactions with the corresponding alkoxy radicals values. The results suggest that there is a complex dependence on the location of the two H-bonding groups, the location of the abstraction or bond scission, and the shape of the transition state that dictates the extent to which intramolecular H-bonding effects the relative importance of H-migration and bond scission reactions for each n-hydroxyalkoxy radical. These findings have important implications for future studies on hydrocarbons with multiple H-bonding sites.

First-principles kinetics of n-octyl radicals

Kinetics of the isomerisation and unimolecular degradation of n-octyl radicals have been studied with the reaction class transition state theory (RC-TST) method. To explore the kinetics of the 1,7-H migration reactions family, the accurate high-pressure limits of the rate constants for the reference reaction of this class (1-heptyl → 1-heptyl) have been calculated. Finally, both the achievements reported in this paper and previous developments are employed to obtain theoretical branching ratios of intramolecular H-transfers and unimolecular degradations of all possible n-octyl radicals; the results are in satisfactory agreement when compared to experiment. The application of the rates obtained to the simulation of a simple reactor is also reported.