Anion Photoelectron Imaging Spectroscopy of C₆HF₅^–, C₆F₆^–, and the Absence of C₆H₂F₄^–

Abstract

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.