期刊
JOURNAL OF PHYSICAL CHEMISTRY B
卷 109, 期 33, 页码 16092-16098出版社
AMER CHEMICAL SOC
DOI: 10.1021/jp040494c
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The present paper focuses on the high temperature form I of caffeine and on its low temperature metastable form. Structural, dynamic, and kinetic information has been obtained by X-ray, dielectric, and calorimetric investigations. This study shows the following features: (1) The high temperature phase (1) of caffeine is in a state of dynamically orientationally disordered crystalline state (so-called plastic, or rotator, phase). (2) This high-symmetry hexagonal phase can be maintained at low temperature in a metastable situation. (3) Under deep undercooling of form I a glass transition occurs in the disordered crystalline state near room temperature. It is associated with the orientational freezing in of the molecular motions. Otherwise stated, the metastable state I enters into a nonergodic unstable state, so-called glassy crystal state. These findings rationalize the difficulties seen with caffeine in pharmaceutical science.
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