A combined crossed-beam and ab initio study of the atom-radical reaction dynamics of O(³P) + C₂H₅→ C₂H₄ + OH: Analysis of nascent internal state distributions of the OH product

The oxidation reaction dynamics of ethyl radicals (C₂H₅) in the gas phase are investigated by applying a combination of high-resolution laser induced fluorescence spectroscopy in a crossed-beam configuration and ab initio theoretical calculations. The supersonic atomic oxygen (O(³P)) and ethyl (C₂H₅) reactants are produced by photodissociation of NO₂ and supersonic flash pyrolysis of a synthesized precursor (azoethane), respectively. An exothermic channel leading to the C₂H₅ + OH (X²Π: υ″ = 0, 1) products is identified. The nascent rovibrational state distributions of the OH product show substantial bimodal internal excitations consisting of low- and high-N″ components with neither spin-orbit nor Λ-doublet propensities in the ground and first excited vibrational states. The averaged vibrational population (P_υ″), partitioning with respect to the low-N″ components of the υ″ = 0 level, shows a comparable population ratio of P₀: P₁ = 1: 1.06. On the basis of comparison between the population analyses using ab initio and prior statistical calculations, the title atom-radical reactive scattering processes are governed by dynamic characteristics. The reaction mechanism can be rationalized by two competing mechanisms: abstraction versus addition. The major low N″-components can be described in terms of the direct abstraction process responsible for the comparable vibrational populations, while the minor but hot rotational distribution of the high N″-components implies that some fraction of radical reactants is sampled to proceed through the short-lived addition-complex forming process.