The Pentafluorophenyl Cation: A Superelectrophile and Diradical

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• The pentafluorophenyl radical is a reactive intermediate.
• Understanding the properties of fluorinated aryl cations is crucial for reaction mechanisms.

Purpose of the Study:

• To directly observe and characterize the pentafluorophenyl cation in the gas phase.
• To determine the electronic structure and stability of the pentafluorophenyl cation.
• To investigate the reactivity of the pentafluorophenyl radical.

Main Methods:

• Vacuum ultraviolet (VUV) photoionization (PI) of thermally generated pentafluorophenyl radical.
• Photoelectron photoion coincidence (PEPICO) spectroscopy using synchrotron radiation.
• High-level ab-initio calculations.

Main Results:

• Direct observation of the pentafluorophenyl cation with an adiabatic ionization energy (AIE) of 9.84 ± 0.02 eV.
• The cation exhibits a π⁵σ¹ diradical character with an open-shell singlet ground state (¹A₂).
• The triplet state (³A₂) is energetically close to the ground state (1.5 ± 0.4 kcal mol⁻¹).
• The closed-shell singlet state (¹A) is highly distorted and lies >4 kcal mol⁻¹ above the ground state.
• The pentafluorophenyl cation shows a ~40 kcal mol⁻¹ higher hydride affinity (HA) compared to the phenyl cation.
• Pentafluorophenyl radical reactivity investigated, forming tetrafluoro ortho-benzyne and other fluorinated species.

Conclusions:

• The pentafluorophenyl cation is a unique aryl carbenium ion with distinct electronic properties.
• Its high hydride affinity explains its reactivity and elusive nature.
• The study provides fundamental insights into the chemistry of fluorinated aromatic species.