Reactive scattering of a supersonic oxygen-atom beam: O + C2H4, C2H2

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Abstract
Reactive scattering of O atoms with C2H4 and C2H2 molecules has been studied at an initial translational energy E≈ 27 kJ mol–1 using a supersonic beam of O atoms seeded in He. Reactive scattering of C2H3O and C2HO radicals is found to be nearly isotropic but slightly favours forward and sideways scattering, respectively. High product translational energies E′ av≈ 30 kJ mol–1 are observed in both cases. The total reaction cross-sections were measured as a function of initial translational energy, using a He/Ne buffer-gas mixture to vary the O-atom velocity. The threshold energy E0= 5 ± 3 kJ mol–1 for O + C2H4 is in good agreement with the activation energy for the overall reaction of O atoms with C2H4 but the threshold energy E0= 8 ± 3 kJ mol–1 for O + C2H2 is slightly lower than the activation energy Ea= 12.5 kJ mol–1. Hence the H-atom displacement reactions represent important reaction pathways for the O + C2H4, C2H2 reactions which are initiated by bonding of the electrophilic O atom to the unsaturated bonds of the hydrocarbon molecules. The structures of the triplet intermediates for these reactions are compared with that proposed for the O + H2S reaction.

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