title: Dielectric study of the slow motional processes in the polymorphic States of anhydrous caffeine.
abstract: Molecular mobility in crystalline anhydrous caffeine was studied by the dielectric technique of thermally stimulated depolarization currents (TSDC). Two relaxational processes were found, one appearing at approximately -10 degrees C that is ascribed to a reorientational glass transition, and a higher temperature one that probably arises from local molecular motions that are precursors of diffusion and sublimation. The experimental results suggest that both crystalline phases II and I of caffeine, that have distinct crystal structures, are solid rotator phases. Furthermore, this dynamic reorientational disorder shows a reorientational glass transition at the same temperature in phase II and in metastable phase I.
authors.affiliation: Centro de Química-Física Molecular, Complexo I, IST, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. mouraramos@ist.utl.pt
authors.forename: Joaquim J
authors.initials: JJ
authors.lastname: Moura Ramos
authors.forename: Natalia T
authors.initials: NT
authors.lastname: Correia
authors.forename: Hermínio P
authors.initials: HP
authors.lastname: Diogo
authors.forename: Marc
authors.initials: M
authors.lastname: Descamps
mesh_terms: D002110:Caffeine / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D003460:Crystallization; D004563:Electrochemistry; D006358:Hot Temperature; D007536:Isomerism; D013816:Thermodynamics
chemical_list: D002110:Caffeine