Spectral fingerprint of stabilized тАвQOOH
Carbon-centered radicals containing the hydroperoxy group, commonly denoted as тАвQOOH, are elusive but are among the most critical intermediate species for kinetic modeling of hydrocarbon oxidation in various atmospheric and combustion processes. Their direct experimental observation is a long-standing challenge, with only one successful previous attempt. Using a combination of infrared activation spectroscopy and ultraviolet laserтАУinduced fluorescence detection, Hansen et al. directly characterized the vibrational structure of a тАвQOOH intermediate in isobutane oxidation, collisionally stabilized and isolated, and followed its dissociative evolution under infrared activation with time and energy resolution. High-level electronic structure calculations revealed an important role of heavy-atom tunneling in this process.
Science, abj0412, this issue p. 679
Abstract
A prototypical hydroperoxyalkyl radical (тАвQOOH) intermediate, transiently formed in the oxidation of volatile organic compounds, was directly observed through its infrared fingerprint and energy-dependent unimolecular decay to hydroxyl radical and cyclic ether products. Direct time-domain measurements of тАвQOOH unimolecular dissociation rates over a wide range of energies were found to be in accord with those predicted theoretically using state-of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by substantial heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling yielded a fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the тАвQOOH intermediateтАФagain increased by heavy-atom tunnelingтАФwhich are required for global models of atmospheric and combustion chemistry.
