Ring rotation and anomalous metal atom motion in octamethyl ferrocene and spin-lattice relaxation in octamethyl ferrocenium hexafluorophosphate

The dynamics of the metal atom motion in sym octamethyl ferrocene (OMF) has been elucidated over the temperature range 85≤T≤350 K by 57Fe Mössbauer effect spectroscopy, and shows a marked increase in the mean-square-amplitude of vibration at ∼348 K, some 80° below the melting point of the neat solid. Differential scanning calorimetry shows an endothermic peak at about the same temperature, and ΔH is ∼1.50 kJ mol−1 and ΔS is ∼4.31 J mol−1 K−1. Corresponding data for OMF+PF6− can be fitted by a relaxation algorithm and confirm the intra-molecular nature of the transition. The spin-lattice relaxation over the above temperature range is fast compared to the characteristic Mössbauer time scale and can be accounted for by a Raman process in the high temperature limit. The transition at ∼348 K is associated with the onset of ring rotation/libration in the neat solid.