Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

Abstract

We present a study on the oxidation of isoprene under several different conditions that may model both atmospheric and combustion chemistry. Anions, formed by passing isoprene/oxidant gas mixtures through a pulsed discharge generating a range of species, are separated via mass spectrometry and characterized by anion photoelectron (PE) spectroscopy supported by computations. Specifically, a UV-irradiated isoprene/O2 mixture, which additionally produces O3, and an isoprene/O2/H2 mixture, which generates •OH when passed through the discharge, were sampled. The mass spectra of ions generated under both conditions show the production of intact molecular ions, ion–molecule complexes (e.g., O2 –, O4 –, and O2 –·isoprene), and singly deprotonated species (e.g., deprotonated isoprene, C5H7 –). In addition, both smaller and oxidized fragments are observed using both gas mixtures, though relative abundances differ. From the UV-irradiated isoprene/O2 gas mixture, additional intact molecular products of reactions initiated by ozonolysis of isoprene, methylglyoxal, and dimethylglyoxal were observed. Fragmentation and oxidation of isoprene observed in both gas mixtures included species with m/z 39, 53, 67, 69, and 83 that we attribute to a series of alkyl- and alkenoxide-based anions. The coexistence of intact molecules and complexes with fragments and reaction products demonstrates the versatility of this ion source as a simple and efficient anion formation method for studying species that may be relevant in atmospheric and combustion chemistry.

Photoelectron Spectroscopy and High-Level Ab Initio Calculations of the Iodide–Methylperoxy Radical Complex

“…The 32 m / z peak is assigned to the bare superoxide anion (O 2 – ). Literature examples of O 2 – cluster formation via electric discharge and direct attachment of low-kinetic-energy electrons support the formation of the superoxide complex. − In the high m / z range (Figure b), a strong I – peak suggests that if any CH 3 OO is formed, complex formation by direct addition is also available. While previous work within the Wild Group has generated halide–molecule complexes readily in the gas phase by simple association, − the presence of a relatively intense O 2 – peak in the mass spectrum suggests that more complex dynamics may be present.…”

Probing Anion–Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

“…Insights from these studies on the fate of primary anions in the troposphere as well as transient neutral collision and reaction complexes will be highlighted. Note that there is a rich history of applying this technique to atmospherically relevant radicals and other reactive species like ozone, , several examples of which are included in the references. − This topic would make for an extensive review on its own.…”

Anion Photoelectron Imaging Spectroscopy of C6F5X– (X = F, Cl, Br, I)

“…Small closed-shell organic molecules generally do not form stable negative ions unless they have electron-withdrawing substituents, such as carbonyl groups, − halogens, − cyano groups, or nitro groups. , In contrast with larger, nonsubstituted polycyclic hydrocarbons, − which can support an excess electron occupying what would correspond to the delocalized lowest unoccupied molecular orbital (LUMO) of the neutral, the charge in smaller, substituted molecules can be more localized, with a substantial impact on the structure of the anion relative to the neutral. For example, ethanedial, or glyoxal (HCOCHO) can form a stable anion in which the excess charge is in an out-of-plane π orbital that is C–C bonding and CO antibonding, resulting in an anion with significantly different C–O and C–C bond lengths relative to the corresponding neutral. , …”