Reactions of HO₂ with n-propylbenzene and its phenylpropyl radicals

Abstraction of an H atom from the propyl side chain by hydroperoxyl radicals (HO₂) constitutes a central reaction in the low-temperature oxidation of n-propylbenzene (nPB). Herein, we calculate reaction rate constants for H abstraction from primary, secondary and benzylic sites in nPB. Rate of abstraction of a benzylic H atom dominates that of a secondary H atom with negligible abstraction of the primary H atom at T≥600K. We present the reaction enthalpies for 1-phenyl-1-propyl (R1), 1-phenyl-2-propyl (R2) and 1-phenyl-3-propyl (R3) radicals, and compare the computed reaction rate constants and bond dissociation enthalpies with analogous scarce literature values. Addition of HO₂ radicals to radical sites in R1, R2 and R3 proceeds in a highly exothermic process and results in the formation of HO₂-phenylpropyl adducts. Mapped potential energy surfaces illustrate all plausible exit channels of the three HO₂-phenylpropyl adducts. Master equation calculations for the three phenylpropyl+HO₂ reactions indicate that direct O-OH bond fission and water elimination control the fate of the adducts leading to the formation of ketonic-type structures. Results from this study should be useful to update kinetic models for the low-temperature oxidation of alkylbenzenes in general.