Product study of the reaction of CH ₃ with OH radicals at low pressures and temperatures of 300 and 612 K

The product distribution for the title reaction was studied using our time-of-flight mass spectrometer (TOFMS) connected to a tubular flow reactor. The methyl and hydroxyl radicals were produced by an excimer laser pulse (λ = 193 nm) photolyzing acetone and nitrous oxide in the presence of excess water or hydrogen. Helium was used as the bath gas; the total density was held constant at 1.2 × 10 ¹⁷ cm ^-3. At 300 K the observations were consistent with singlet methylene ( ¹CH ₂) and water as the main product channel with a small contribution of methanol. In contrast, at about 610 K three channels - formaldehyde isomers and methanol in addition to ¹CH ₂ + H ₂O - are formed with similar yields. When acetone-d ₆ was used, the production of both CHDO and CD ₂O was observed, indicating that two different formaldehyde-producing channels are operating simultaneously. These experimental results are compared with RRKM and master equation calculations on the basis of the properties of the methanol potential energy surface from a recent ab initio study.