Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(³P)+(CH₃)₂CH→C₃H₆+OH

Abstract

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.

Theoretical Investigation of the Radical–Radical Reaction of O(3P) + C2H3 and Comparison with Gas-Phase Crossed-Beam Experiments

“…The result also shows good agreement with the other theoretical investigations of the reverse reaction of C 2 H 2 + OH, where as the OH radical approaches C 2 H 2 , the association complex exists as the prereaction association complex along the entrance channel . Similar weakly bound adducts were reported in the authors’ studies on the formation of OH in the reactions of O­( 3 P) with ethyl and iso -propyl radicals. , …”

“…This investigation was carried out as part of continuing efforts to deduce the general principles that explain the mechanistic and dynamic traits of atom-radical reactions. In recent years, the authors have reported gas-phase crossed-beam studies of the reaction dynamics of O­( 3 P) with saturated hydrocarbon radicals such as methyl (CH 3 ), ethyl (C 2 H 5 ), and t -butyl ( t -C 4 H 9 ) and unsaturated π-conjugated hydrocarbon radicals such as allyl (C 3 H 5 ) and propargyl (C 3 H 3 ). − A supersonic flash pyrolysis source was adopted to produce clean, jet-cooled radical beams without experiencing recombination and/or secondary dissociation. By means of ab initio simulations and statistical calculations, these experiments revealed that the atom-radical reactions followed microscopic mechanisms with unusual dynamic features.…”

Gas temperature determination from rotational lines in non-equilibrium plasmas: a review

“…The newly formed OH in the ν = 0 level has a rotational temperature parameter of 6000 K while the OH remaining from the parent water molecule has a T r of 2600 K. Both temperature parameters depend on the vibrational level. The chemical reactions which produce OH from hydrocarbons have been shown to be able to yield rotational distributions of the OH(X), which can be described by the superposition of two Boltzmann distributions [90,91]. Similar studies have been performed for other molecules, such as NH(X) [92], HD [93] and NaH [94] which originate from dissociation reactions.…”

Probing the kinetic energy-release dynamics of H-atom products from the gas-phase reaction of O(3P) with vinyl radical C2H3

“…The lab-made crossed-beam machine utilized in this study has been described in detail elsewhere. [3][4][5][6][7][8][9][10][11] Briefly, the machine consisted of two source chambers, each outfitted with a 6-inch baffled diffusion pump, and a scattering chamber equipped with a 10-inch baffled diffusion pump. O( 3 P) was generated by irradiating He samples seeded with 12% NO 2 with a pulsed laser with a wavelength of 355 nm.…”

“…This work develops from this lab's recent progress in crossed-beam investigations of the reaction dynamics of O( 3 P) with a series of prototypical hydrocarbon radicals such as allyl (C 3 H 5 ), propargyl (C 3 H 3 ), t-butyl (t-C 4 H 9 ), ethyl (C 2 H 5 ) and iso-propyl (i-C 3 H 7 ). [4][5][6][7][8][9][10][11] All alkyl radicals were produced using a supersonic flash pyrolysis developed by Chen and coworkers. 12 The labile precursor molecules entrained in a molecular beam experienced a very short residence time inside a high-temperature tube, leading to rapid thermal decomposition into clean, jet-cooled radical beams without recombination and/or secondary dissociation.…”