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

The gas-phase reaction dynamics of ground-state atomic oxygen O( ³P) 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 J. Phys. Chem. Ref. Data 34, 757 (2005)10.1063/1.1748524, a new exothermic channel of O(³P) C ₂H₃ → C₂H₂ OH (X ²Π: 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 C₂H₂-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.