The Pentafluorophenyl Cation: A Superelectrophile and Diradical

We report the first direct observation of the pentafluorophenyl cation in the gas phase via vacuum ultraviolet (VUV) photoionization (PI) of the thermally generated pentafluorophenyl radical. The reactive intermediates and stable reaction products were characterized utilizing photoelectron photoion coincidence (PEPICO) spectroscopy with synchrotron radiation. Electron removal from the pentafluorophenyl radical yields the cation with an adiabatic ionization energy (AIE) of 9.84 ± 0.02 eV. Threshold photoelectron spectra combined with high-level ab-initio calculations show that the cation is described as a π⁵σ¹ diradical with an open-shell singlet (¹A₂) ground state, while the triplet (³A₂) state lies only 1.5 ± 0.4 kcal mol^-1 higher in energy. The closed-shell singlet (¹A) state is highly distorted and lies >4 kcal mol^-1 above the ground state. This unique aryl carbenium ion exhibits a ∼40 kcal mol^-1 higher hydride affinity (HA) as compared to the parent phenyl cation, explaining its high reactivity and elusive character. In addition, the radical's reactivity was investigated upon hydrogen abstraction and unimolecular decomposition, forming tetrafluoro ortho-benzyne as well as smaller fluorinated species.