| Abstract
| - X-ray structure determinations of two different single crystals of octamethylferrocenium tetrafluoroborate (OMFc+BF4-) revealed conformational polymorphism with ligand twist angles of 180° and 108°,respectively. Their concomitant occurrence could be explained by the small lattice energy difference of 3.2kJ mol-1. Temperature-dependent Moessbauer spectroscopy of 57Fe-labeled OMFc+BF4- over the range90 < T< 370 K did not show the anomalous sudden increase in the motion of the metal atom as observedin neutral OMFc. Broadened absorption curves characteristic of relaxation spectra were obtained with anisomer shift of 0.466(6) mm s-1 at 90 K. The temperature dependence of the isomer shift corresponded toan effective vibrating mass of 79 ± 10 Da and, in conjunction with the temperature dependence of therecoil-free fraction, to a Moessbauer lattice temperature of 89 K. The spin relaxation rate could be betterdescribed by an Orbach rather than a Raman process. At 400 K, a reversible solid−solid transition to aplastic crystalline mesophase was noted.
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