The anion radical of [18]annulene

The anion radicals of [18]annulene and of [6]annulene undergo Jahn-Teller distortions from the Dol conformations of their respective neutral molecules. The conformation of lowest energy for the anion radical of benzene has a C-2nu symmetry, and that for the anion radical of [18]annulene is D-2h. The very rapid pseudorotation of the benzene C-2nu system causes the six protons of the benzene anion radical to appear as if they were equivalent in the EPR experiment. Fast pseudo-rotation rendering the uniform distribution of spin density in the [18]annulene anion radical system is prevented by the nonplanarity of the radical. Hence, the EPR spectrum of the [18]annulene anion radical must be interpreted in terms of a static (on the EPR time scale) D-2h conformation, which pseudo-rotates over a C-2h transition state. B3LYP calculations support these conclusions.