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 C₂ 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.