Group Additive Kinetics for Hydrogen Transfer Between Oxygenates

Improved Approach for ab Initio Calculations of Rate Coefficients for Secondary Reactions in Acrylate Free-Radical Polymerization

Secondary reactions in radical polymerization pose a challenge when creating kinetic models for predicting polymer structures. Despite the high impact of these reactions in the polymer structure, their effects are difficult to isolate and measure to produce kinetic data. To this end, we used solvation-corrected M06-2X/6-311+G(d,p) ab initio calculations to predict a complete and consistent data set of intrinsic rate coefficients of the secondary reactions in acrylate radical polymerization, including backbiting, β-scission, radical migration, macromonomer propagation, mid-chain radical propagation, chain transfer to monomer and chain transfer to polymer. Two new approaches towards computationally predicting rate coefficients for secondary reactions are proposed: (i) explicit accounting for all possible enantiomers for reactions involving optically active centers; (ii) imposing reduced flexibility if the reaction center is in the middle of the polymer chain. The accuracy and reliability of the ab initio predictions were benchmarked against experimental data via kinetic Monte Carlo simulations under three sufficiently different experimental conditions: a high-frequency modulated polymerization process in the transient regime, a low-frequency modulated process in the sliding regime at both low and high temperatures and a degradation process in the absence of free monomers. The complete and consistent ab initio data set compiled in this work predicts a good agreement when benchmarked via kMC simulations against experimental data, which is a technique never used before for computational chemistry. The simulation results show that these two newly proposed approaches are promising for bridging the gap between experimental and computational chemistry methods in polymer reaction engineering.

On-the-fly kinetics of hydrogen abstraction from polycyclic aromatic hydrocarbons by methyl/ethyl radicals

This work provides a rigorous procedure, within the framework of the Reaction Class Transition State Theory (RC-TST) and the Structure-Activity Relationship (SAR), for predicting reliable thermal rate constants on-the-fly for hydrogen abstraction reactions by methyl/ethyl radicals from Polycyclic Aromatic Hydrocarbons (PAHs) in a temperature range of 300-3000 K. All necessary RC-TST parameters were derived from ab initio calculations for a representative set of 36 reactions on which different error analyses and comparisons with available literature data were carried out. In addition to the good agreement between the RC-TST rate constants and the literature data, the detailed error analyses show that RC-TST/SAR, utilizing either the Linear Energy Relationship (LER) where only the reaction energy is needed or Barrier Height Grouping (BHG) where no additional data is needed, can predict the thermal rate constants for any reaction in the title reaction class with an average systematic error of less than 50% when compared to the explicit rate calculations. Therefore, the constructed RC-TST procedure can be confidently used to obtain reliable rate constants on the fly in an attempt to effectively construct detailed kinetic mechanisms for PAH-related fuels.

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.