Rate Constants for CH 3 + O 2 → CH 3 O + O at High Temperature and Evidence for H 2 CO + O 2 → HCO + HO 2

The reaction of CH 3 with O 2 has been studied in a reflected shock tube apparatus between 1600 and 2100 K. CH 3 was prepared from the fast thermal decomposition of CH 3 I, and O atom atomic resonance absorption spectrometry (ARAS) was used to observe absolute [O] t . [CH 3 I] 0 was sufficiently low so that most secondary reactions were negligible, allowing for unambiguous determination of the rate constant for CH 3 + O 2 → CH 3 O + O. The rate constant expression for this reaction derived to match the experimental data is k = (3.90 ± 0.40) × 10 -11 exp(−16858 ± 1127 K/ T ) cm 3 molecule -1 s -1 . To explain O atom concentration buildup at longer times, a fast reaction between H 2 CO and O 2 was postulated. Rate constants for this process were derived by fitting the long-time O atom profiles. Last, the four-center reaction CH 3 + O 2 → CH 2 O + OH was found to be of negligible importance over the temperature range of the study.