Solvation of O2- and O4- by p -difluorobenzene and p -xylene studied by photoelectron spectroscopy

Anion photoelectron spectra of the O2- arene and O4- arene complexes with p -xylene and p -difluorobenzene are presented and analyzed with the aid of calculations on the anions and corresponding neutrals. Relative to the adiabatic electron affinity of O2, the O2- arene spectra are blueshifted by 0.75-1 eV. Solvation energy alone does not account for this shift, and it is proposed that a repulsive portion of the neutral potential energy surface is accessed in the detachment, resulting in dissociative photodetachment. O2- is found to interact more strongly with the p -difluorobenzene than the p -xylene. The binding motif involves the O2- in plane with the arene, interacting via electron donation along nearby C-H bonds. A peak found at 4.36 (2) eV in the photoelectron spectrum of O2- p -difluorobenzene (p -DFB) is tentatively attributed to the charge transfer state, O2- p- DFB+. Spectra of O4- arene complexes show less blueshift in electron binding energy relative to the spectrum of bare O4-, which itself undergoes dissociative photodetachment. The striking similarity between the profiles of the O4- arene complexes with the O4- spectrum suggests that the O4- molecule remains intact upon complex formation, and delocalization of the charge across the O4- molecule results in similar structures for the anion and neutral complexes.