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

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 View the MathML sourceR3¯m with lattice parameters View the MathML sourcea=b=12.5568(1)Å, View the MathML sourcec=9.6045(1)Å, which in the rhombohedral setting correspond to View the MathML sourcea=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 View the MathML source(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.