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

Abstract

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.