Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse β‐Scission Reactions: Modeling of Activation Energies and Pre‐Exponential Factors

Abstract

The group additivity method for Arrhenius parameters is applied to hydrogen addition to alkenes and alkynes and the reverse β-scission reactions, an important reaction family in thermal processes based on radical chemistry. A consistent set of group additive values for 33 groups is derived to calculate the activation energy and preexponential factor for a broad range of H-addition reactions. The group additive values are determined from CBS-QB3 ab initio calculated rate coefficients. A mean factor of deviation between the CBS-QB3 and the experimental rate coefficients for 7 reactions of only 2 in the range 300-1000 K is found. Tunneling coefficients for these reactions were found to be significant below 400 K and a correlation accounting for tunneling is presented. Application of the obtained group additive values to predict the kinetics for a set of 11 additions and β scissions yields rate coefficients within a factor 3.5 from the CBS-QB3 results except for 2 β scissions with severe steric effects. The mean factor of deviation with experimental rate coefficients is 2.0, showing that the group additive method with tunneling corrections can accurately predict the kinetics, at least as accurate as the most commonly used density functional methods. The constructed group additive model can hence be applied to predict the kinetics of hydrogen radical additions to a broad range of unsaturated compounds. IntroductionThe addition of hydrogen radicals to alkenes and its reverse β scission, are important elementary steps in radical processes such as polymerization, pyrolysis, steam cracking, partial oxidation and combustion.[1] Therefore, the reaction family of hydrogen addition/β scission forms an indispensable part of any radical reaction network.A reliable reactor optimization requires an accurate kinetic model based on elementary reactions. For radical chemistry, on which many of the world largest scale chemical processes are based, the reactive nature of the radical intermediates results in huge reaction networks typically involving hundreds of species and thousands of elementary reactions. [2][3][4] Currently, most elementary reaction networks are automatically generated using advanced algorithms for the selection of the relevant reactions. [5][6][7][8][9][10][11] Sensitivity studies on these reaction networks point out that the main part of the uncertainty on the product yields stems from inaccurate knowledge of kinetic data. [12,13] Therefore, accurate kinetic data are essential to obtain reliable process simulations. Moreover, if rate-based network construction algorithms are applied, [14] accurate rate data are even more important as inaccuracies can result in the construction of an incomplete network that is not capable of grasping the underlying chemistry of the process.A quantitative description of radical processes thus requires that rate parameters be known for all of the different reactions comprising the reaction network. However, it is very difficult to measure kinetic parameters for individual radical r...

Kinetics of α hydrogen abstractions from thiols, sulfides and thiocarbonyl compounds

“…Besides hydrogen abstraction reactions, this additivity scheme has also proven to yield accurate kinetic data for addition reactions between hydrocarbons. 24,34,49 As the details of the method have been presented elsewhere, 25,26 only a brief overview is given here. Within Benson's method a group is defined as a polyvalent atom together with all of its ligands.…”

“…The group additivity values (GAVs) can be obtained from high-level quantum chemical calculations. The method has proven to be successful in predicting rate coefficients of addition reactions and hydrogen abstractions for hydrocarbons 26,34 and H 2 additions, 1,2-hydrogen shifts and cyclization reactions for silicon-containing compounds. [35][36][37] The aim of this work is to extend the previously developed additivity schemes for Arrhenius parameters of hydrogen abstraction reactions between hydrocarbons to sulfur containing compounds.…”

Understanding the reactivity of unsaturated alcohols: Experimental and kinetic modeling study of the pyrolysis and oxidation of 3-methyl-2-butenol and 3-methyl-3-butenol

“…Reaction rate coefficients of reactions belonging to the reaction families (i) hydrogen abstraction by carbon-centered radicals, (ii) hydrogen abstraction by hydrogen atoms, (iii) carbon-centered β-scission/carbon-centered radical addition and (iv) hydrogen-centered β-scission/hydrogen atom addition were calculated using the group additive databases developed by Marin and coworkers [56][57][58][59]. These authors applied Benson's group additivity concept to transition state theory and derived group additive values from CBS-QB3 calculations at the high-pressure limit.…”

Ab initio based kinetic Monte Carlo analysis to unravel the propagation kinetics in vinyl acetate pulsed laser polymerization

“…27 Standard Gibbs free energies of solvation in vinyl acetate, Δ solv G°, were calculated using COSMO-RS 28 the differences in standard Gibbs free energy between the transition state and the reactants was used to calculate rate coefficients at a temperature of 298 and 333 K, leading to pre-exponential factors and activation energies using the Arrhenius relation. It is known that quantum tunneling has an important contribution to hydrogen transfer reactions 30,31 such as backbiting. This has been taken into account according to the Eckart tunneling scheme.…”