Calculation of the rate constants for concerted elimination reaction class of hydroperoxyl-alkyl-peroxyl radicals

Abstract

Hydroperoxyl-alkyl-peroxyl radicals (O2QOOH) are an important radical intermediate in low-temperature combustion, and its concerted elimination reaction to produce Q″OOH and HO2 is an important reaction class in low-temperature combustion modeling of hydrocarbons. In this paper, isodesmic reaction method is used for the accurate calculation of rate constants for a representative set of 45 reactions in the class. All geometries of the reactants, the transition states and the products are optimized under the B3LYP/6-311 + G(d, p) level. It is shown that the geometries at the reaction center of the transition states in this reaction class are conserved, and hence, the minus of any two reactions for the formation of transition states can be considered as an isodesmic reaction. One reaction is called reference reaction and as is usually done in reaction class-transition state theory, the reaction in the class with the minimum size is chosen as the reference reaction, and other reactions are called target reactions. According to isodesmic reaction theory, it can be derived that the energy difference between the high-level ab initio result and the low-level result for the reference reaction can be used to correct the low-level energy barriers for the target reactions. In this study, B3LYP/6-311 + G(d, p) method is used as the low-level ab initio method and the composite Gaussian-4 (G4) method is used as the high-level ab initio method. CCSD(T)/cc-PVTZ method is chosen as the benchmarking method to validate the accuracy of G4 method through a comparison of the energy barriers for three selected small molecular reactions calculated by G4 method with the results by CCSD(T)/cc-PVTZ method, and it is found that the difference of the energy barriers by these two methods are between 0.35 and 2.11 kJ/mol, indicating that the accuracy of G4 method is close to that of the benchmarking method. Meanwhile, ten representative target reactions in this class from the O2QOOH to Q″OOH + HO2 are selected to validate the correction scheme through a comparison of the energy barriers before and after correction with the energy barriers by G4 method. It is found that the deviations of the energy barriers calculated by direct DFT method are between 16.58 and 24.95 kJ/mol, and after correction, the deviations of the energy barriers are reduced to −3.28 to 5.09 kJ/mol, indicating that the results of the energy barriers corrected by the isodesmic reaction method are close to that of the G4 method. Finally, using this correction scheme, the energy barriers, reaction enthalpies and rate constants of all reactions are calculated and kinetic parameters in the form of (A, n, E) are provided for this important elimination reaction class of hydroperoxyl-alkyl-peroxyl radicals (O2QOOH), which is useful for the simulation of low-temperature combustion for hydrocarbon.