Jahn–Teller distortion and merohedral disorder of C60−· as observed by ESR

We used single crystals of [A+(C6H5)4]2C60−·B− as an ideal model system to determine accurately principal values and principal axes of the g-tensor of the C60 mono radical anion embeded in a crystal field. The experimentally observed non axial g-tensor which corresponds to the molecular Jahn–Teller distortion of C60− shows that the crystal field has stabilized the Jahn–Teller distortion of D2h symmetry along a specific molecular C2 axis. These stabilization occurs below a characteristic temperature Ts for two C60−· orientations (merohedral disorder) and depends on the cations as we could show by the substitution of P+ for As+.