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.