Emerging Nonvalence Anion States of [Isoprene-H·]·H₂O Accessed via Detachment of OH^–·Isoprene

Abstract

The anion photoelectron imaging spectra of an ion with m/z 85, generated under ion source conditions that optimize •OH production in a coexpansion with isoprene, are presented and analyzed with supporting calculations. A spectroscopic feature observed at a vertical electron detachment energy of 2.45 eV, which dominates the photoelectron spectrum measured at 3.495 eV photon energy, is consistent with the OH–·isoprene ion–molecule complex, while additional signal observed at lower electron binding energy can be attributed to other constitutional isomers. However, spectra measured over a 2.2–2.6 eV photon energy range, i.e., from near threshold of the predominant OH–·isoprene detachment feature through the vertical detachment energy, exhibit sharp features with common electron kinetic energies, suggesting autodetachment from a temporary anion prepared by photoexcitation. The photon energy independence of the electron kinetic energy of these features along with the low dipole moment predicted for the neutral •OH·isoprene van der Waals complex, suggest a complex photon-driven process. We present calculations supporting a hypothesis that near-threshold production of the •OH···isoprene reactive complex results in hydrogen abstraction of the isoprene molecule. The newly formed activated complex anion supports a dipole bound state that temporarily traps the near zero-kinetic energy electron and then autodetaches, encoding the low-frequency modes of the dehydrogenated neutral isoprene radical in the electron kinetic energies.

Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

“…Figure c shows the PE spectrum of an anion with m / z 85, which is mass-coincident with the OH – ·isoprene IMC produced using the isoprene/O 2 /H 2 gas mix, studied previously . The spectrum shown here is additional evidence of disparate speciation between the different ion source conditions and reactants used in this study, as it is different from the spectrum assigned to the IMC (the two distinct spectra are shown superimposed in the Supporting Information).…”

“…Pulsed discharge in combination with pulsed molecular beam valves are useful tools in the study of anions, cations, and unstable radical species, though species formed in discharge sources can be surprising in terms of connecting the structures produced by the precursor molecules. − The results presented above show that the source used in this study produces a combination of fragments, oxidized fragments, and unfragmented species such as the O 2 – ·isoprene IMC, methylglyoxal, and dimethylglyoxal. ,, While in many cases our results do not allow for an unambiguous structural assignment of species that have clearly undergone some fragmentation, we can unambiguously distinguish oxidized and unoxidized species, and there is evidence that the most prevalent fragment ions are formed by breaking the weakest carbon–carbon or carbon–hydrogen bonds and that single O-atom or atomic ion addition takes place. Indeed, the fact that the mass spectra show distinct product distributions under different conditions, rather than a common, statistical distribution of ions ranging from carbon clusters to sequentially reduced and oxidized clusters, is evidence of a hierarchy of processes, with low-energy events such as electron attachment and ion–molecule association being the most important.…”

“…In this report, we present and characterize anions resulting from passing isoprene (2-methyl-1,3-butadiene)/oxidant gas mixtures through a pulsed discharge setup using both mass spectrometry and anion photoelectron (PE) spectroscopy. This study represents an extension of our previous studies on the O 2 – ·isoprene and OH – ·isoprene ion–molecule complexes (IMCs). , What we describe below are ions resulting from fragmentation, some of which undergo further oxidation reactions, rather than simple ion–molecule associations. As described below, two oxidizing gas mixtures were explored for generating product anions.…”

“…Ions generated from expansion of this gas irradiated for different periods of time are included in the Supporting Information. Finally, the isoprene/O 2 /H 2 gas mixture was prepared with 27.5% H 2 , ∼2% O 2 , ∼1% isoprene (Alfa Aesar, 99%), and balance Ar, which generates the oxidizing • OH radical and OH – ions upon passing through the discharge. , To reduce the risk of explosion, we systematically added the reagents (isoprene, then O 2 , buffer Ar, then H 2 , and finally, the remaining Ar) to the electrically grounded tank.…”

Autodetachment over Broad Photon Energy Ranges in the Anion Photoelectron Spectra of [O2–M]− (M = Glyoxal, Methylglyoxal, or Biacetyl) Complex Anions

“…Autodetachment is therefore observed only when the photon energy is resonant with a transition to a quasibound vibronic level of the anion, and the resulting free electron carries away the excess energy between the intermediate excited anion and the final neutral vibronic level. Several examples of this effect are included in refs − . In contrast with the more typical case of autodetachment, the near-threshold autodetachment signal is observed over a range of photon energies for all three [O 2 – M ] − ( M = glyoxal, methylglyoxal, or biacetyl) complexes.…”