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

Abstract

The photoelectron (PE) spectra of C 6 F 5 X − (X = Cl, Br, I) and computational results on the anions and neutrals are presented and compared to previously reported results on C 6 F 6 − [McGee, C. J. et al. 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 C 6 F 5 Cl − exhibits a parallel photoelectron angular distribution (PAD), similar to that of the previously reported C 6 F 6 − spectrum, while the PE spectra of C 6 F 5 Br − and C 6 F 5 I − have isotropic PADs, and also exhibit a prominent X − PE feature due to photodissociation of C 6 F 5 X − resulting in X − formation. Identification of the C 6 F 5 X − 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 C 6 F 5 Cl, 2.10 eV for C 6 F 5 Br, and 2.00 eV for C 6 F 5 I, all significantly higher than the 0.76 eV upper limit determined for C 6 F 6 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 C 2v structured C 6 F 6 − anion, for which the charge is delocalized over the molecule. In C 6 F 5 Cl − , the C−Cl bond is predicted to be bent out of the plane, while both C 6 F 5 Br − and C 6 F 5 I − 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 C 6 F 5 X/ C 6 F 5 X − is considered, as well as the possible dissociative state leading to X − (X = Br, I) formation, and the nature of the C−X bond.