Analysis of nascent rotational energy distributions and reaction mechanisms of the gas-phase radical-radical reaction O(3P)+(CH3)2CH→C3H6+OH.

This paper reports on the gas-phase radical-radical dynamics of the reaction of ground-state atomic oxygen [O((3)P), from the photodissociation of NO(2)] with secondary isopropyl radicals [(CH(3))(2)CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O((3)P)+(CH(3))(2)CH→C(3)H(6) (propene)+OH, is examined by high-resolution laser-induced fluorescence spectroscopy in crossed-beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X(2)Π) products with low- and high-N'' components in a ratio of 1.25:1. No significant spin-orbit or Λ-doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS-QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential-energy surface: direct abstraction, giving the dominant low-N'' components, and formation of short-lived addition complexes that result in hot rotational distributions, giving the high-N'' components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast-flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O((3)P) with primary and tertiary hydrocarbon radicals allows molecular-level discussion of the reactivity and mechanism of the title reaction.

A gas-phase crossed-beam study of OH produced in the radical-radical reaction of O((3)P) with iso-propyl radical (CH3)2CH.

The gas-phase radical-radical reaction dynamics of ground-state atomic oxygen [O((3)P)] with iso-propyl radicals, (CH(3))(2)CH, were investigated by applying a combination of high-resolution laser-induced fluorescence spectroscopy in a crossed-beam configuration and ab initio calculations. The nascent distributions of OH (X(2)Π: υ'' = 0) from the major reaction channel O((3)P) + (CH(3))(2)CH → C(3)H(6) (propene) + OH showed substantial internal excitations with a bimodal feature of low- and high-N'' components with neither spin-orbit nor Λ-doublet propensities. Unlike previous kinetic results, proposed to proceed only through the direct H-atom abstraction process, on the basis of the population analysis and comparison with the statistical theory, the title reaction can be described in terms of two competing mechanisms at the molecular level: direct abstraction process and indirect short-lived addition-complex-forming process with a ratio of 1.25 : 1.

A combined crossed-beam and ab initio study of the atom-radical reaction dynamics of O((3)P) + C(2)H(5)--> C(2)H(4) + OH: analysis of nascent internal state distributions of the OH product.

The oxidation reaction dynamics of ethyl radicals (C(2)H(5)) 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((3)P)) and ethyl (C(2)H(5)) reactants are produced by photodissociation of NO(2) and supersonic flash pyrolysis of a synthesized precursor (azoethane), respectively. An exothermic channel leading to the C(2)H(5) + OH (X(2)Pi: upsilon'' = 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 Lambda-doublet propensities in the ground and first excited vibrational states. The averaged vibrational population (P(upsilon'')), partitioning with respect to the low-N'' components of the upsilon'' = 0 level, shows a comparable population ratio of P(0)ratioP(1) = 1 ratio 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.

Crossed-beam investigation of O(3P) + C2H5 --> C2H4 + OH.

The reaction dynamics of ground-state atomic oxygen [O((3)P)] with an ethyl radical (C(2)H(5)) in the gas phase was investigated using high-resolution laser spectroscopy in a crossed-beam configuration. An exothermic channel of O((3)P) + C(2)H(5) --> C(2)H(4) + OH was identified, and the nascent distributions of OH (X (2)Pi: upsilon'' = 0, 1) showed significant internal excitations with an unusual bimodal feature of low and high rotational N''-components with neither spin-orbit nor Lambda-doublet propensities. On the basis of the ab initio and statistical calculations, the reaction mechanism can be rationalized by two competing mechanisms: abstraction vs addition. The low N''-components with significant vibrational excitation can be described in terms of the direct abstraction process as a major channel. The extraordinarily hot rotational distribution of high N''-components implies that a portion of the fraction proceeds through the indirect short-lived addition-complex forming process. From the comparative analysis of the reactions of O((3)P) + several hydrocarbon molecules and radicals, the reactivity and mechanistic characteristics of the title reaction are discussed.