On the mechanism of the HCCO + O2 reaction: Probing multiple pathways to a single product channel

Abstract

We explore the mechanism of the HCCO + O₂ reaction using time-resolved Fourier transform spectroscopy. Utilizing an isotopically labeled reactant (¹⁸O₂) and state-selective product detection, we determine the relative flux through different paths on the potential energy surface leading to a single asymptote: H + CO + CO₂. In the labeled reaction, the dominant isotopic products are C¹⁸O and ¹⁶OC¹⁸O. Combined with data from the corresponding reaction in natural isotopic abundance, these results show that at least 85% of the reactive flux passes through a four-membered OCCO ring intermediate. An alternative reaction path through an energetically allowed three-membered COO ring intermediate represents less than 15% of the total reactive flux. The average energy deposited in vibration of CO and CO₂ is in reasonable agreement with a statistical model using the separate statistical ensembles method.