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 (I) 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.

The structure of succinonitrile in its orientationally disordered phase was reexamined through extended X-ray diffraction measurements. It was solved by using both the analytic procedures of symmetry-adapted functions and a Frenkel model assuming discrete orientations. A possible translation-rotation coupling was included in this latter case via an offset vector e. The study confirms that the nitrogen atoms are localised along the fourfold axis of the cubic cell but evidences a strong offset of the centre of mass for gauche conformations. It is shown that this can be explained by steric hindrance between some configurations of neighbouring molecules.

No abstract