In a crossed molecular-beam apparatus, we reacted atomic O in states (3)P and (1)D with ethene (C(2)H(4)) at collision energy 3 kcal mol(-1). Employing two mixtures, 20% O(2) + 80% He and 3% O(2) + 12.5% Ar + 84.5% He, as discharge media allowed us to generate two sources of oxygen atoms that have the same mean velocity but different ratios of (1)D/(3)P populations, 0.0017 and 0.035. We identified six reactions and recorded time-of-flight spectra of products CH(2)CHO, CH(2)CO, and CH(3) as a function of laboratory angle. Reaction O((3)P) + C(2)H(4) --> CH(2)CHO + H has a fraction f(t) = 0.43 of energy release in translation, and product CH(2)CHO has a maximal probability at scattering angle of 140 degrees. For reaction O((1)D) + C(2)H(4) --> CH(2)CO + 2H, f(t) = 0.26, and the angular distribution of product CH(2)CO shows a backward preference. For reaction O((3)P) + C(2)H(4) --> CH(2)CO + H(2), f(t) = 0.35, and the angular distribution of product CH(2)CO has a slight preference for a sideways direction. In contrast, reaction O((1)D) + C(2)H(4) --> CH(2)CO + H(2) has f(t) = 0.26 and an angular distribution with forward and backward peaking and symmetry. Reactions O((3)P and (1)D) + C(2)H(4) --> CH(3) + HCO have f(t) = 0.09 and 0.08, respectively, and angular distributions with forward and backward peaking and nearly symmetric. The reactivity of O (1)D with ethene is ca. 38 and 90 times that of O (3)P for channels to eliminate H(2) and CH(3), respectively. For reactions of O (1)D, the branching ratio for elimination of 2H is ca. 3.3 times that for elimination of H(2).

We investigated the title reaction at collision energy 3.5 kcal mol(-1) in a crossed molecular beam apparatus using undulator radiation as an ionization source. Time-of-flight (TOF) spectra of product C(3)H(3) were measured in laboratory angles from 20° to 100° using two photoionization energies 9.5 and 11.6 eV. These two sets of experimental data exhibit almost the same TOF distributions and laboratory angular distributions. From the best simulation, seven angle-specific kinetic-energy distributions and a nearly isotropic angular distribution are derived for product channel C(3)H(3) + H that has an average kinetic-energy release of 15.5 kcal mol(-1), corresponding to an average internal energy of 33.3 kcal mol(-1) in C(3)H(3). Furthermore, TOF spectra of product C(3)H(3) were measured at laboratory angle 52° with ionizing photon energies from 7 to 12 eV. The appearance of TOF spectra remains almost the same, indicating that a species exclusively contributes to product C(3)H(3); the species is identified as H(2)CCCH (propargyl) based on the ionization energy of 8.6 ± 0.2 eV and the maximal kinetic-energy release of 49 kcal mol(-1). Theoretical calculations indicate that the rapid inversion mechanism and rotation in intermediate H(2)CCCH(2) can result in a forward-backward symmetric angular distribution for product C(3)H(3) + H. The present work avoids the interference of reactions of C((1)D) and C(2) radicals with C(2)H(4) and rules out the probability of production of other isomers like c-C(3)H(3) and H(3)CCC proposed in the previous work at least at the investigated collision energy.

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Some of the polyynes (HC2n+2H, 1 ≤ n ≤ 4) are observable in planetary atmospheres, interstellar space, and flames. Polyynes are proposed to play an important role in synthesis of large carbonaceous molecules. We explore the dynamics of reactions of C2nH (n = 1-4) radicals with C2H2 by interrogating time-of-flight spectra and photoionization efficiency spectra of products C2n+2H2. The reactions of n = 2-4 were investigated for the first time. The translational energy release is biased to low energy but extends to the energetic limit of product HC2n+2H + H, corresponding to a fraction of 0.34-0.36 on translational energy. Product C2n+2H2 has a deconvoluted ionization threshold in good agreement with the ionization energy of polyynes. The quantum chemical calculations support the experimental observations. This work verifies that the title reaction is an important source for formation of polyynes that have been observed in interstellar/circumstellar media and combustion processes.

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