UC San Diego

The hydroxyl radical plays a critical role in several contexts including combustion, atmospheric chemistry, and low-temperature extraterrestrial environments. OH has been observed in the interstellar medium and is important in the gas phase chemistry of interstellar clouds. Additionally, the hydroxyl radical is the dominant oxidizer of volatile organic compounds in the atmosphere. Here, the reactions between OH and simple alkanes, alkenes, and alcohols were investigated as these bimolecular reactions can expand fundamental understanding of gas phase reaction dynamics. Photoelectron-photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH4 → H2O + CH3 reaction initiated by photodetachment of the OH−(CH4) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable and dissociative channels. Interpretation of the experimental results was supported by quantum chemistry and quasiclassical trajectory calculations showing that most of the trajectories yield slowly recoiling OH + CH4 reactants while some are trapped in the entrance channel van der Waals well. Extending the study to hydroxy radical reactions with the simplest alkene, ethylene, yielded similar dissociative photodetachment dynamics (DPD) to PPC studies on OH−(CH4). The dominant channel was DPD to OH + C2H4 + e− with a low kinetic energy release (KER) of the dissociating fragments, consistent with weak repulsion between the OH + C2H4 reactants near the transition state. To model the DPD process, an impulsive model was used to account for rotational energy partitioning in C2H4 photofragments and showed good agreement with the experimental results. PPC studies probing the OH + CH3OH → H2O + CH3O reaction had the best Franck-Condon overlap with the exit channel. High-level coupled cluster calculations of the stationary points on the anion surface show that CH3O−(H2O) is the stable minimum. Photodetachment leads to H2O + CH3O + e− products with both direct dissociative photodetachment as well as resonance mediated processes observed on the neutral surface. The partitioning of total kinetic energy in the system indicates that water stretch and bend excitation is induced in DPD, and evidence for long-lived complexes consistent with production of vibrational Feshbach resonances in the exit channel is reported.