Thermochemical Kinetic Analysis on Chain Branching in Alkyl Radical Reactions with O2: n-Propyl andHydroperoxide-Propyl Radicals

摘要

Ab initio and density functional calculations are performed to determine thermochemical and kinetic parameters inanalysis association reactions of the n-propyl (CCC?) and the hydroperoxide-propyl radicals, C?CCOOH andCC?COOH, with O2. The CBS-Q//B3LYP/6-31G(d,p) composite method is utilized to calculate energies from theB3LYP/6-31G(d,p) optimized geometries. The n-propyl + O2 reaction forms an energized peroxy adduct with acalculated well depth of 36 kcal mol-1. The transition states of direct molecular elimination to C=CC + HO2, 1,5H-shift isomerization to form a hydroperoxide-propan-1-yl adduct (C?CCOOH), 1,4 H-shift isomerization tohydroperoxide-propan-2-yl adduct (CC?COOH), and products of C=CC + HO2 and CYCOC + OH all haveenergy barriers below the reactants. The hydroperoxide-propan-1-yl radical (C?CCOOH) + O2 system has threereaction paths, which have lower energies than the entrance channel; two of which lead to chain branching. Thehydroperoxide-propan-2-yl adduct (CC?COOH) + O2 system is more complex; but intramolecular reactions of theadduct have higher barriers then that of (C?CCOOH) + O2. An elementary reaction mechanism is constructed tomodel experimental data on negative temperature behavior data in propane oxidation. The chain branching neededfor the negative temperature coefficient (NTC) comes from the isomerization of the propyl peroxy radicalCCCOO? to C?CCOOH, with subsequent addition of another O2 to produce ?OCCC=O + 2 OH and fromchemical activation reaction to ?OOCCCO? + OH products. A small amount of chain branching comes frompropyl-hydroperoxide, dissociation.