DYNAMICS OF HYDROGEN-ATOM ABSTRACTION IN THE O-- PRODUCT ENERGY DISPOSAL AND ANGULAR-DISTRIBUTIONS(CH4 REACTION )

Energy and angular distributions for the hydrogen abstraction reaction O-+CK4-->OH-+CH3, exothermic by 0.26 eV, and a prototype ionic pathwa y for methane oxidation in hydrocarbon flames have been studied in a c rossed molecular beam experiment at collision energies of 0.34, 0.44, and 0.64 eV. At the two lower collision energies, two mechanisms contr ibute to the differential cross section: In the first, low impact para meter rebound collisions form sharply backward-scattered products, whi le in the second, larger impact parameter collisions produce a broad d istribution of forward scattered products. We suggest that the first g roup of products is formed by collisions with hydrogen atoms oriented essentially along the relative velocity vector and proceeding through a near-collinear O ... H ... CH3 geometry, while the second group corr esponds to collisions with One of the three off-axis hydrogens. The pr oducts 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 v(2) umbrella b ending mode of CH3. At the highest collision energy, the product angul ar distribution shifts entirely to the forward direction, suggesting t hat the low impact parameter collisions are no longer important in the reactive process. At this energy, the average product internal excita tion corresponds to 59% of the total available energy. The data sugges t that the majority of product internal excitation resides in the v(2) umbrella bending mode of CH3, with OH in its ground vibrational state . (C) 1997 American Institute of Physics.