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

This paper reports on the gas-phase radical-radical dynamics of the reaction of ground-state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH ₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃) ₂CH→C₃H₆ (propene)+OH, is examined by high-resolution laser-induced fluorescence spectroscopy in crossed-beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Î) products with low- and high-N′′ components in a ratio of 1.25:1. No significant spin-orbit or Î-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(³P) with primary and tertiary hydrocarbon radicals allows molecular-level discussion of the reactivity and mechanism of the title reaction. Two competing dynamic pathways, namely, direct abstraction and indirect, short-lived, addition-complex formation (see picture), were revealed by analysis of the distributions of the nascent rotational states of the OH products from the major channel O( ³P)+(CH₃)₂CH→OH+C₃H ₆ of the gas-phase reaction of ground-state atomic oxygen with isopropyl radicals.