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

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 E_a= 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.