Dynamics of hydrogen atom abstraction in the O−+CH4 reaction: Product energy disposal and angular distributions

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Energy and angular distributions for the hydrogen abstraction reaction O−+CH4→OH−+CH3, exothermic by 0.26 eV, and a prototype ionic pathway for methane oxidation in hydrocarbon flames have been studied in a crossed molecular beam experiment at collision energies of 0.34, 0.44, and 0.64 eV. At the two lower collision energies, two mechanisms contribute to the differential cross section: In the first, low impact parameter rebound collisions form sharply backward-scattered products, while in the second, larger impact parameter collisions produce a broad distribution of forward scattered products. We suggest that the first group of products is formed by collisions with hydrogen atoms oriented essentially along the relative velocity vector and proceeding through a near-collinear O(centered ellipsis)H(centered ellipsis)CH3 geometry, while the second group corresponds to collisions with one of the three off-axis hydrogens. The products are formed on average with 65% of the total available energy in product internal excitation. The product kinetic energy distribution shows structure that correlates with excitation of the ν2 umbrella bending mode of CH3. At the highest collision energy, the product angular distribution shifts entirely to the forward direction, suggesting that the low impact parameter collisions are no longer important in the reactive process. At this energy, the average product internal excitation corresponds to 59% of the total available energy. The data suggest that the majority of product internal excitation resides in the ν2 umbrella bending mode of CH3, with OH in its ground vibrational state. © 1997 American Institute of Physics.

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