Geometrical and electronic structure of fluoro-substituted benzene radical anions based on quantum chemical analysis of hyperfine interactions

The INDO method has been employed to calculate the isotropic and anisotropic hyperfine interactions for some geometrical structures of the C₆F₆^-, C₆HF₅^-, 1,2,4,5-F₄C₆H₂^- radical anions. The π and σ states of planar radical anions have been considered together with three types of non-planar structures: (a) with out-of-plane distortions of the benzene ring following the "chair" pattern: (b) with neighbouring fluorine atoms displaced in opposite directions from the ring plane due to their electrostatic repulsion: (c) with symmetric (of type) displacement of CF bonds due to the interaction between the ground and lowest excited σ-electron states within the pseudo-Jahn-Teller effect. It is the last model that describes all experimental data on magnetic resonance parameters. This model is also preferable for energy reasons. Most likely, this type of non-planar structures reflects the basic features of the real geometry of the above radical anions whose ground state may be termed as pseudo-π-electronic since the unpaired electron occupies an orbital that is a linear combination of the π and σ orbitals of the corresponding planar structure with a prevailing π component.