A combined crossed-beam and theoretical study of the reaction dynamics of O(3P) + C2H3 → C2H2 + OH: Analysis of the nascent OH products with the preferential population of the Π(A′) component

Abstract

The gas-phase reaction dynamics of ground-state atomic oxygen [O((3)P) from the photo-dissociation of NO(2)] with vinyl radicals [C(2)H(3) from the supersonic flash pyrolysis of vinyl iodide, C(2)H(3)I] has been investigated using a combination of high-resolution laser-induced fluorescence spectroscopy in a crossed-beam configuration and ab initio calculations. Unlike the previous gas-phase bulk kinetic experiments by Baulch et al. [J. Phys. Chem. Ref. Data 34, 757 (2005)], a new exothermic channel of O((3)P) + C(2)H(3) → C(2)H(2) + OH (X (2)Π: υ" = 0) has been identified for the first time, and the population analysis shows bimodal nascent rotational distributions of OH products with low- and high-N" components with a ratio of 2.4:1. No spin-orbit propensities were observed, and the averaged ratios of Π(A('))∕Π(A") were determined to be 1.66 ± 0.27. On the basis of computations at the CBS-QB3 theory level and comparison with prior theory, the microscopic mechanisms responsible for the nascent populations can be understood in terms of two competing dynamical pathways: a direct abstraction process in the low-N" regime as the major pathway and an addition-complex forming process in the high-N" regime as the minor pathway. Particularly, during the bond cleavage process of the weakly bound van der Waals complex C(2)H(2)-OH, the characteristic pathway from the low dihedral-angle geometry was consistent with the observed preferential population of the Π(A') component in the nascent OH products. A molecular-level discussion of the reactivity, mechanism, and dynamical features of the title reaction are presented together with a comparison to gas-phase oxidation reactions of a series of prototypical hydrocarbon radicals.

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. 9 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. 7,8 A competing but straightforward process responsible for reaction channel 10 is the direct H-atom abstraction from the CH 2 side through the collinear geometry.…”

“…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.…”

A Gas-Phase Investigation of Oxygen-Hydrogen Exchange Reaction of O(3P) + C2H5→ H(2S) + C2H4O

“…After considering the barrier height and reaction exothermicity along the reaction coordinate, the title reaction is, in fact, predicted to be a major channel, which is consistent with the gas-phase kinetic experiments. [5][6][7][8][9][10][11] Statistical Analyses and Reaction Dynamics. The partitioning of the energy into the H products and the microscopic kinetic rate constants were derived using the aforementioned ab initio calculations.…”

“…The crossed-beam apparatus used in this study was made in-house and has been described in detail elsewhere. 3,[5][6][7][8][9][10][11] In brief, a pulsed O( 3 P) radical beam was produced by irradiating 2 atm of He (ultra-high-purity helium: 99.999%) seeded with 12% NO 2 (99.5%, MG Industries) with 30 mJ pulses from a 355 nm Nd:YAG laser beam (Continuum Surelite II-10) near the throat of the supersonic expansion. A second C 2 H 5 radical beam was prepared by supersonic flash pyrolysis of the precursor, azoethane (C 2 H 5 -N=N-C 2 H 5 ), synthesized using the method of Renaud and Leitch.…”

“…4 By adopting the supersonic flash pyrolysis source, a series of combined crossed-beam and theoretical studies of the prototypal oxidation reactions of both π-conjugated hydrocarbon radicals such as allyl (C 3 H 5 ), propargyl (C 3 H 3 ), vinyl (C 2 H 3 ), and saturated hydrocarbon radicals such as t-butyl (t-C 4 H 9 ), ethyl (C 2 H 5 ), and iso-propyl (i-C 3 H 7 ) have recently been executed in our laboratory. [5][6][7][8][9][10][11] The analyses of the energy distributions of the nascent reaction products have demonstrated extraordinary reactivity and mechanistic characteristics, which have not previously been observed in the reactions of O( 3 P) + closed-shell hydrocarbon molecules. 12,13 Based on the authors' theoretical calculations, Leonori et al also studied the oxidation reaction of allyl radicals using a universal crossed-beam apparatus.…”