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

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

The photoelectron (PE) spectra of C6F5X- (X = Cl, Br, I) and computational results on the anions and neutrals are presented and compared to previously reported results on C6F6- [McGee, C. J. J. Phys. Chem. A 2023, 127, 8556-8565.]. The spectra all exhibit broad, vibrationally unresolved detachment transitions, indicating that the equilibrium structures of the anions are significantly different from the neutrals. The PE spectrum of C6F5Cl- exhibits a parallel photoelectron angular distribution (PAD), similar to that of the previously reported C6F6- spectrum, while the PE spectra of C6F5Br- and C6F5I- have isotropic PADs, and also exhibit a prominent X- PE feature due to photodissociation of C6F5X- resulting in X- formation. Identification of the C6F5X- detachment transition origins, which is equivalent to the neutral electron affinity (EA), in all three cases is difficult, since the broadness of the detachment feature is accompanied by vanishingly small detachment cross section near the origin. Upper limits on the EAs were determined to be 1.70 eV for C6F5Cl, 2.10 eV for C6F5Br, and 2.00 eV for C6F5I, all significantly higher than the 0.76 eV upper limit determined for C6F6 with the same experiment. The broad detachment transitions are consistent with computational results, which predict very large differences between the neutral and anionic C-X (X = Cl, Br, I) bond lengths. Based on differences between the MBIS atom charges in the anions and neutrals, the excess charge in the anion is on the unique C atom and X, in contrast to the nonplanar C2v structured C6F6- anion, for which the charge is delocalized over the molecule. In C6F5Cl-, the C-Cl bond is predicted to be bent out of the plane, while both C6F5Br- and C6F5I- are predicted to be planar on average. The impact of the interruption of the symmetry in the hexafluorobenzene neutral and anion on the molecular and electronic structure of C6F5X/C6F5X- is considered, as well as the possible dissociative state leading to X- (X = Br, I) formation, and the nature of the C-X bond.

The overlooked role of excited anion states in NiO2- photodetachment

Photodetachment spectra of anionic species provide significant insights into the energies and nature of ground and excited states of both the anion and resultant neutral molecules. Direct detachment of the excess electron to the continuum may occur via formally allowed or forbidden transitions (perhaps as the result of intensity borrowing through vibronic coupling). However, alternate indirect pathways are also possible and often overlooked. Here, we report a two-dimensional photoelectron spectral study, combined with correlated electronic structure calculations, to elucidate the nature of photodetachment from NiO2-. The spectra are comprised of allowed and forbidden transitions, in excellent agreement with previously reported slow electron velocity mapped imaging spectra of the same system, which were interpreted in terms of direct detachment. In the current work, the contributions of indirect processes are revealed. Measured oscillations in the branching ratios of the spectral channels clearly indicate non-direct detachment processes, and the electronic structure calculations suggest that excited states of the appropriate symmetry and degeneracy lie slightly above the neutral ground state. Taken together, the results suggest that the origin of the observed forbidden transitions is the result of anion excited states mediating the electron detachment process.

Anion Photoelectron Imaging Spectroscopy of C6HF5-, C6F6-, and the Absence of C6H2F4. J Phys Chem A 2023;127:8556-8565. [PMID: 37816145 DOI: 10.1021/acs.jpca.3c04016] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Substituents have a profound effect on the electronic structure of the benzene molecule. In this paper, we present new photoelectron spectra of the C5HF5- molecular anion, to test predictions [ Int. J. Quant. Chem. 2017, 188, e25504] that pentafluorobenzene has a positive electron affinity, as hexafluorobenzene was already known to have. The PE spectrum of C6HF5- exhibits a broad and vibrationally unresolved band due to significant differences between the structure of the anion and the neutral. The vertical detachment energy (VDE) of C5HF5- is determined to be 1.33 ± 0.05 eV, and the lowest binding energy at which the signal is observed is 0.53 ± 0.05 eV, which, if taken as the electron affinity, is in good agreement with the computed value. In addition, we attempted to generate intact C6H2F4- molecular ions using the 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, and 1,2,4,5-tetrafluorobenzene precursors, as tetrafluorobenzene was predicted to have a near-zero but marginally positive electron affinity. Using a photoemission anion source, we were not able to produce the intact tetrafluorobenzene anion. Density functional theory calculations support a more detailed discussion of the impact of fluorine substitution on the electronic structure of these species.

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

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

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

Anion photoelectron spectroscopy has been used to investigate the structure and dynamics of CH₃OOI^- van der Waals complexes. Peaks within the photoelectron spectrum are attributed to photodetachment to the perturbed ²P_3/2 state of I···CH₃OO (3.46 eV) and the two ²P states of bare iodine. A broad feature at 1.7-2.4 eV is attributed to detachment to the excited singlet states from two O₂^-···CH₃I complexes. This represents the first anion photoelectron spectroscopy of a halide-bound methylperoxy radical species. Complex structures have been optimized using MP2/aug-cc-pVQZ with single-point energies at W1w theory for ground-state complexes and NEVPT2 for photodetachment to excited O₂. Interactions are dominated by electrostatics, with the anion species interacting with the methyl pocket of the solvating molecule, suggesting conversion via an S_N2 mechanism, and excess energy leading to complex dissociation within the timescale of mass spectrometry. The calculated W1w Gibbs energies suggest that while an electron transfer (ET) pathway to conversion is available, it is comparatively unfavored.

Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

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.