Rate constants for CH3+O2 → CH3O+O at high temperature and evidence for H2CO+O2 → HCO+HO2

The reaction of CH3 with Oz has been studied in a reflected shock tube apparatus between 1600 and 2100 K. CH3 was prepared from the fast thermal decomposition of CH3I, and O atom atomic resonance absorption spectrometry (ARAS) was used to observe absolute [O],. [CH3I](0) was sufficiently low so that most secondary reactions were negligible, allowing for unambiguous determination of the rate constant for CH3 + Oz CH3O + O. The rate constant expression for this reaction derived to match the experimental data is k = (3.90 +/- 0.40) x 10(-11) exp(-16858 +/- 1127 WT) cm(3) molecule(-1) s(-1). To explain O atom concentration buildup at longer times, a fast reaction between H2CO and Oz was postulated. Rate constants for this process were derived by fitting the long-time O atom profiles. Last, the four-center reaction CH3 + Oz --> CH2O + OH was found to be of negligible importance over the temperature range of the study.