Probing alkenoxy radical electronic structure using anion PEI spectroscopy

Abstract

Photoelectron imaging spectra of three alkenoxide radical anions (3-buten-1-oxide, 3-buten-2-oxide, and 2-propenoxide) are presented and analyzed with supporting results of density functional theory calculations. In all spectra, intense detachment features are observed at approximately 2 eV electron binding energy, which is similar to the electron affinities of saturated neutral alkoxy radicals [Ramond et al., J. Chem. Phys. 112, 1158]. Photoelectron angular distributions suggest the presence of several overlapping transitions which are assigned to theX andà states of multiple energetically competitive conformers. The term energy of theà state of the 2-propenoxy radical, 0.17 eV, is higher than that of 3-buten-2-oxy (0.13 eV) and 3-buten-1-oxy (0.05 eV) radicals. Comparing the butenoxy radicals, we infer that stronger interactions between the non-bonding O 2p orbitals and the π bond increase the splitting between the ground and the first excited state in the 3-buten-2-oxy radical relative to the 3-buten-1-oxy radical.

Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

“…The lower-energy, lower-intensity transition labeled X′ has an origin of 0.8 ± 0.1 eV and a VDE of 1.14 ± 0.05 eV, and the intense transition, X , has an origin of 1.88 ± 0.05 eV and a VDE of 2.00 ± 0.05 eV. The energy and spectral profile of band X are consistent with the PE spectrum of an alkenoxide, − whereas band X′ requires further explanation.…”

“…Again, considering that the isoprene/O 2 /UV conditions have much higher concentrations of O 2 and O 3 relative to the isoprene/O 2 /H 2 gas mixture, and given the typical alkenoxy radical EA of ca . 1.9 eV, ,,− we suggest that this species has a more oxidized composition, such as C 4 H 5 O 2 – , particularly as m / z 85 is 16 units heavier than the abundant m / z 69 anion. For comparison, the EA of acetyloxyl radical is 3.250 ± 0.010 eV …”

“…Alkyl radical species typically have modest EAs below 1 eV, − while alkoxy and alkenoxy radicals have EAs in the 1.8–2 eV range. − Given the H 2 -rich composition of the gas mixture, the lower O 2 concentration relative to the isoprene/O 2 /UV ion source conditions, along with the known typical alkyl and alkoxy radical EAs, we suggest that the m / z 39, m / z 53, and m / z 67 ions, the spectra of which are shown in Figure a–c respectively, correspond to (a) C 3 H 3 – , (b) C 4 H 5 – , and (c) C 5 H 7 – , and the m / z 69 and m / z 83 ions, the PE spectra of which are shown in Figure a,b, respectively, are C 4 H 5 O – and C 5 H 7 O – . The PE spectra of both C 4 H 5 – and C 5 H 7 – exhibit features at higher e – BE values, which will be addressed below.…”

“…The 2.330 eV PE spectra of the m / z 69 and 83 anions, shown in Figure a,b, respectively, exhibit a single and similarly narrow transition ( X ) at approximately 2 eV, which is typical for alkoxide and alkenoxide PE spectra. ,,− These ions correspond to C 4 H 5 O – and C 5 H 7 O – for m / z 69 and 83, respectively.…”

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

“…Finally, we note that band x″ shows a profile and binding energy comparable to the alkenoxides studied previously. 32 The simulation of a representative alkenoxide spectrum, with oxygen addition to C1 [inset in Figure 3c], is shown superimposed on the 2.330 eV spectrum of m/z 85 in Figure 3c. Again, we assume that there would be an ensemble of m/z 85 alkenoxides contributing to this portion of the spectrum, and the single simulation is shown as a representative of the other potential species present.…”