Theoretical Prediction of Rate Constants for Hydrogen Abstraction by OH, H, O, CH₃, and HO₂ Radicals from Toluene

Abstract

Hydrogen abstraction from toluene by OH, H, O, CH3, and HO2 radicals are important reactions in oxidation process of toluene. Geometries and corresponding harmonic frequencies of the reactants, transition states as well as products involved in these reactions are determined at the B3LYP/6-31G(2df,p) level. To achieve highly accurate thermochemical data for these stationary points on the potential energy surfaces, the Gaussian-4(G4) composite method was employed. Torsional motions are treated either as free rotors or hindered rotors in calculating partion functions to determine thermodynamic properties. The obtained standard enthalpies of formation for reactants and some prodcuts are shown to be in excellent agreement with experimental data with the largest error of 0.5 kcal mol(-1). The conventional transition state theory (TST) with tunneling effects was adopted to determine rate constants of these hydrogen abstraction reactions based on results from quantum chemistry calculations. To faciliate its application in kinetic modeling, the obtained rate constants are given in Arrhenius expression: k(T) = AT(n) exp(-EaR/T). The obtained reaction rate constants also agree reasonably well with available expermiental data and previous theoretical values. Branching ratios of these reactions have been determined. The present reaction rates for these reactions have been used in a toluene combustion mechanism, and their effects on some combustion properties are demonstrated.

Radical‐Pair Formation in Hydrocarbon (Aut)Oxidation

“…Turning first to the reaction of the hydroxyl radical ( 2 ) with toluene ( 5 ), we find that hydrogen‐atom transfer proceeds in this case through the initial formation of a weakly bound reactant complex ( RC2 ) and the low‐lying transition state TS2 (Figure ). This is very similar to several recent studies of this process in the context of modeling the combustion chemistry of aromatic hydrocarbons . As can be expected for a low‐barrier process with a large thermochemical driving force, the transition state TS2 for hydrogen abstraction by the hydroxyl radical from toluene is geometrically early in that the breaking C−H bond (118.3 pm) is not much longer than that in the reactant complex RC2 (109.7 pm).…”

“…This is very similar to severalr ecent studies of this process in the context of modeling the combustion chemistry of aromatic hydrocarbons. [34][35][36] As can be expected for al owbarrierp rocess with al arge thermochemical driving force, the transition state TS2 for hydrogen abstraction by the hydroxyl radicalf rom toluene is geometrically early in that the breaking CÀHb ond (118.3 pm) is not much longer than that in the reactant complex RC2 (109.7 pm). Wave-function analysis of the transition-state region shows that this reaction is not am ultireference case, but rather welld escribed by established singledeterminantalm ethods.…”

Modeling of aromatics formation in fuel-rich methane oxy-combustion with an automatically generated pressure-dependent mechanism

“…64 In addition to the pressure-dependent kinetics of C 7 H 8 and C 7 H 9 surfaces, the rates of H-abstraction from toluene determined by Li et al were appended together to the single aromatic ring mechanism. 92 3.2.2 Formation of naphthalene and indene. For most of the ten pathways, the PESs reported by Mebel et al 51 were used in this work.…”

Barrier heights of hydrogen-transfer reactions with diffusion quantum monte carlo method

“…Hydrogen‐transfer reactions are an important class of reactions in a wide range of chemical processes such as combustion chemistry as well as biological processes . For example, H‐transfer reaction is the main chain propagation step to produce alkyl radicals in the combustion of hydrocarbon fuels and it is also the most important consumption path of these fuels in their combustion processes . Reasonable rate constants of H‐transfer reactions are essential for understanding these chemical and biological processes.…”