Trend in the Electron Affinities of Fluorophenyl Radicals ·C₆H_5-xF_x (1 ≤ x ≤ 4)

Abstract

The electron affinities (EAs) of a series of •C 6 H 5−x F x (1 ≤ x ≤ 4) fluorophenyl radicals are determined from the photoelectron spectra of their associated fluorophenide anions generated from C 6 H 6−x F x (1 ≤ x ≤ 4) fluorobenzene precursors. The spectra show a near-linear incremental increase in EA of 0.4 eV/ x. The spectra exhibit vibrationally unresolved and broad detachment transitions consistent with significant differences in the molecular structures of the anion and neutral radical species. The experimental EAs and broad spectra are consistent with density functional theory calculations on these species. While the anion detachment transitions all involve an electron in a non-bonding orbital, the differences in structure between the neutral and anion are in part due to repulsion between the lone pair on the C-center on which the excess charge is localized and neighboring F atoms. The C 6 H 5−x F x − (2 ≤ x ≤ 4) spectra show features at lower binding energy that appear to be due to constitutional isomers formed in the ion source.

Isomer-Dependent Electron Affinities of Fluorophenyl Radicals, •C6H5–xFx (2 ≤ x ≤ 4)

“…Difluorophenides. Figure 1 shows the PE spectra of the anions formed from (a) 1,2-difluorobenzene, (b) 1,3difluorobenzene, and (c) 1,4-difluorobenzene, the latter of which was published previously 21 but included here for direct comparison. The black and red arrows pointing to the e BE − axes indicate computed adiabatic EAs, which are described below.…”

“…17 In a series of related studies on fluorine-substituted benzenes 18−20 and fluorophenyl radicals, 21 our group demon-strated how fluorination of benzene and the phenyl radical affect how strongly these molecules bind electrons 19,21 or molecular anions 18,20 using anion photoelectron (PE) spectroscopy and supporting density functional theory (DFT) calculations. 21 The fluorine-substituted phenyl radicals, • C 6 H 5−x F x (1 ≤ x ≤ 4), in particular, showed a straightforward, monotonic increase in EA. This effect is readily explained: The excess charge resides in the in-plane sp 2 orbital that coincides with the radical center in the neutral.…”

“…The specific regioisomers of the C 6 H 6−x F x (2 ≤ x ≤ 4) fluorobenzene molecule precursors used to generate the radicals in the previous study 21 were those with zero dipole moments by symmetry (1,4-difluorobenzene, 1,3,5-trifluorobenzene, and 1,2,4,5-tetrafluorobenzene). The rationale behind this selection of regioisomers was the avoidance of dipoles introducing nonvalence-bound anions, which has been observed in other systems.…”

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

“…A third explanation is that the signal is due to the pentafluorophenide anion; the resolution of the mass spectrometer operating with settings necessary to generate sufficient ion signal to acquire the spectrum was not sufficient to fully separate m / z values of 167 and 168 ( vide infra ). The computed EA of the pentafluorophenyl radical (·C 6 F 5 ) was reported elsewhere to be 3.22 eV, which is in good agreement with the observed signal.…”

“…The anion of the tetrafluorophenide anion is observed at 1 unit below m / z 150. Panel (c) adapted with permission from ref . Copyright 2023 American Chemical Society.…”