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

Abstract

The gas-phase reaction dynamics of ground-state atomic oxygen [O(P-3) from the photo-dissociation of NO2] with vinyl radicals [C2H3 from the supersonic flash pyrolysis of vinyl iodide, C2H3I] 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(P-3) + C2H3 -> C2H2 + OH (X (2)Pi: upsilon '' = 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 Pi(A')/Pi(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 C2H2-OH, the characteristic pathway from the low dihedral-angle geometry was consistent with the observed preferential population of the Pi(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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767772]