Pressure dependence of the rate coefficients and product yields for the reaction of CH₃CO radicals with O₂

Relative rate coefficients for the reaction of acetyl (CH₃CO) radicals with O₂ (k₄) and Cl₂ (k₇,) have been obtained at 298 K and 228 K as a function of total pressure, using FTIR/environmental chamber techniques. Measured values of k₄/k₇ were placed on an absolute basis using k₇ = 2.8 × 10^-11 exp(-47/T) cm³ molec^-1 s^-1. At 298 K, the value of k₄ is constant ((7 ± 2) × 10^-13 cm³ molec^-1 s^-1) at pressures from 0.1 to 2 torr, then increases to a high pressure limiting value of (3.2 ± 0.6) × 10^-12 cm³ molec^-1 s^-1. which is approached at pressures above 300 torr. At 228 K, the low-pressure value of k₄ increases by about 20-30% while the high pressure value remains unchanged. Experiments designed to elucidate the products of reaction (4) as a function of pressure at 298 K indicate that the reaction occurs via a concerted mechanism in which CH₃CO radicals combine with O₂ to give an excited acetylperoxy radical (CH₃COO₂*) which is increasingly stabilized at high pressure at the expense of a low pressure decomposition channel. The yield of acetylperoxy radicals from reaction (4) decreases from >95% at pressures above 100 torr, to about 90% at 60 torr, and 50% at 6 torr. Indirect evidence for formation of OH radicals from the low pressure decomposition is presented, although the carbon-containing coproduct(s) of this channel could not be identified.