Rate constants for the reaction OH + OH + M -H202 + M (M = N2, H20) have been determined by using flash photolysis of H20 vapor in combination with quantitative OH resonance spectrometry. For M = N2 experiments were performed at 253, 298, and 353 K and at pressures between 26 and 1100 mbar. Under these conditions the reaction is found to be primarily in the low-pressure limit with £1iN2°( T = 298 K) = (6.9*lf) x 10~31 cm6/s and a temperature dependence of T~o i. Both the absolute value of k^N°a nd its temperature variation are in very satisfactory agreement with theoretical predictions and extrapolations from high-temperature dissociation data. A pressure falloff of £1N2 is also observed. On the basis of a theoretical analysis of the falloff behavior, a high-pressure limiting rate coefficient of kf = 1.5 X 10"" cm3/s, independent of temperature, is predicted. From experiments in N2/H20 mixtures with xHl0 = 0.11 at pressures up to 140 mbar a low-pressure limiting rate coefficient for H20 as a third body of ki,H2o°( T = 298 K) = (4.022o) x 10'30 cm6/s is obtained.

Rate constants and branching ratio for the reaction channels CH3 + 02 -»• CH30 + O (la) and CH3 + 02 -* CH20 + OH (lb) have been derived from RRKM theory by assuming a common CH302 intermediate. The computation is based on a 160 kJ /mol isomerization barrier between CH302 and CH2OOH as derived recently in direct studies of the reverse CH30 + O reaction at low temperatures. In contrast to most previous conclusions, channel lb is predicted to dominate over channel la at all temperatures below 2800 K, with the individual rate coefficients being kH= 1.1 X 1013 exp(-13990/T) and klb = 3.4 X 1011 exp(-4500/T) cm1 23 4567/(rnol-s).

The rate constant for the reaction (1) CH3O + O → products at 298 K has been determined using 248 nm laser co‐photolysis of CH3ONO/O3 mixtures for the generation of CH3O radicals and O atoms combined with LTF for time resolved detection of CH3O and product OH. k1 is found to be (2.5±0.7) 10−11 cm3/s with a relative yield of OH of Φ = ‐Δ[OH]/Δ[CH3O] = 0.12+0.08−0.04. The results are shown to be consistent with a primary capture mechanism on an electronically adiabatic surface forming highly vibrationally excited CH3O *2 and followed by rapid subsequent decomposition or isomerization, viz. . From modelling of the OH yield using RRKM theory the isomerization barrier between CH3O2 and CH2OOH is placed near 160 kJ/mol in good agreement with recent ab initio calculations. As a consequence the CH3 + O2 combustion reaction is predicted to proceed primarily via the low energy CH2O + OH channel, in contrast to most previous suggestions.