Identification of a rotator phase of octamethyl ferrocene and correlations between its structural and dynamical properties

The low- and high-temperature phases of octamethyl ferrocene were studied in detail, using high-resolution X-ray powder diffraction, differential scanning calorimetry and nuclear resonant scattering, in particular the novel technique of synchrotron radiation perturbed angular correlations (SRPAC). Much as in the case of an analogous but more unsymmetrical molecule, octamethyl ethinyl ferrocene, the high-temperature phase possesses the space group R3¯m with lattice parameters a=b=12.5568(1)Å, c=9.6045(1)Å, which in the rhombohedral setting correspond to a=7.9251(1)Å, α=104.79∘α=104.79∘. An increase of the volume per formula unit of about 12% across the phase transition is observed.The rotation of the electric field gradient, which can be identified with the rotation of the entire molecule within the lattice, follows Arrhenius behaviour with a high activation energy of (40.3±3.3)kJmol-1. Whereas precursor effects and a change in activation energy were observed for octamethyl ethinyl ferrocene, no such effects are observed for octamethyl ferrocene. We relate this difference to the absence of the ethinyl substituent in octamethyl ferrocene.