S. Albert, J. Ripmeester
Mar 20, 1972
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Journal
Journal of Chemical Physics
Abstract
The unusual polymorphism of beryllium oxyacetate, Be4O(CH3COO)6, was investigated by means of pulsed NMR between −196°C and the melting point (286.7°C). In the α modification of this substance, fast methyl group reorientation takes place, and the spin‐spin and spin‐lattice relaxation times proved to be insensitive to the presence of the reported second or higher order phase transitions. The onset of a relatively slow motion with an activation energy of 15.5± 0.8 kcal/mole was found above the first order phase transition at ∼148°C, and this motion is likely to be over‐all molecular tumbling motion. The transition from a high symmetry cubic phase to a lower symmetry orthorhombic structure on raising the temperature is discussed. No self‐diffusion is present up to the melting point. The high temperature γ phase of beryllium oxyacetate does not seem to be a plastic crystal as was proposed previously. In light of our results, the isotropic optical property by itself is of questionable usefulness as a criterion...