Rate coefficients and product branching ratios for the oxidation of phenyl and naphthyl radicals: A theoretical RRKM-ME study

Abstract

Theoretical VRC-TST/RRKM-ME calculations were performed to evaluate total rate coefficients and product branching ratios for the oxidation of phenyl and 1- and 2-naphthyl radicals with O2 at temperatures relevant to combustion (1500, 2000, and 2500K) and pressures of 0.01, 0.1, 1.0, and 10atm. The results give the rate coefficients in the range of 3.0–5.5×10−11cm3 molecule−1s−1 with slightly positive temperature dependence, activation energies varying within 2.3–3.3kcal/mol, and pre-exponential factors of 7–10×10−11cm3 molecule−1s−1. The dominant reaction channel in all three cases is elimination of the oxygen atom from peroxy complexes formed at the initial O2 addition step and leading to the phenoxy and naphthoxy radical products. The contribution of this channel increases with temperature. Chemically-activated phenoxy and naphthoxy radicals either decompose to the cyclopentadienyl+CO and indenyl+CO products, respectively, or undergo thermal equilibration. The relative yields of the decomposition/equilibration products strongly depend on temperature and pressure in the way that a temperature growth favors decomposition, whereas an increase in pressure favors equilibration. At the lowest temperature considered, 1500K, the reactions also yield significant amounts of pyranyl+CO (phenyl+O2) or 1-benzopyranyl+CO (1-naphthyl+O2). A comparison of the phenyl+O2 and naphthyl+O2 reactions reveals that although the general trends in the oxidation kinetics of phenyl and naphthyl radicals are similar, the size and especially the position of the radical site in the aromatic moiety may affect the details of the mechanism and relative product yields.