research papers Acta Cryst. (2005). B61, 329–334 doi:10.1107/S010876810500546X 329 Acta Crystallographica Section B Structural Science ISSN 0108-7681 Ab initio structure determination of the high- temperature phase of anhydrous caffeine by X-ray powder diffraction Patrick Derollez,* Nata ´lia T. Correia, Florence Dane `de, Fre ´de ´ric Capet, Fre ´de ´ric Affouard, Jacques Lefebvre and Marc Descamps Laboratoire de Dynamique et Structure des Mate ´riaux Mole ´culaires (UMR CNRS 8024), UFR de Physique, Ba ˆtiment P5, Universite ´ des Sciences et Technologies de Lille, 59655 Ville- neuve d’Ascq CEDEX, France Correspondence e-mail: patrick.derollez@univ-lille1.fr # 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The high-temperature phase I of anhydrous caffeine was obtained by heating and annealing the purified commercial form II at 450 K. This phase I can be maintained at low temperature in a metastable state. A powder X-ray diffraction pattern was recorded at 278 K with a laboratory diffract- ometer equipped with an INEL curved position-sensitive detector CPS120. Phase I is dynamically orientationally disordered (the so-called plastic phase). The Rietveld refinements were achieved with rigid-body constraints. It was assumed that on each site, a molecule can adopt three preferential orientations with equal occupation probability. Under a deep undercooling of phase I, below 250 K, the metastable state enters in a glassy crystal state. Received 26 November 2004 Accepted 18 February 2005 1. Introduction Caffeine (1,3,7-trimethylpurine-2,6-dione, C 8 H 10 N 4 O 2 ) is a common agrochemical and therapeutic agent. It is known to occur in a hydrated form and two anhydrous polymorphic varieties. The crystal structure of the hydrated form was determined a long time ago (Sutor, 1958) and confirmed recently (Edwards et al. , 1997). The existence of a fully ordered crystal phase for anhydrous caffeine was discussed (Griesser et al., 1999; Carlucci & Gavezzotti, 2004). The physical characterization of the anhydrous states is really challenging because of the apparent impossibility of producing monophasic caffeine. Depending on the preparation and the history of the sample, a mixture of different forms was found (Mu ¨ ller & Griesser, 2003). The commercial form [designed by (II) in the following] transforms upon heating to form (I). Cesa ` ro & Starec (1980) have shown a phase transformation at T t = 426 K before the melting at T m = 512 K and have suggested, on the basis of calorimetric studies, that the high- temperature phase I is structurally disordered. The crystal structure of phase I has not yet been solved. From X-ray powder diffraction measurements, different rhombohedral space groups have been proposed in which the molecules should be linked by very weak hydrogen bonds to form an ordered network (Edwards et al. , 1997). Stowasser & Lehmann (2002) suggest a monoclinic cell containing 20 molecules with a volume of 4450 A ˚ 3 . In this paper we report the ab initio structure determination of the high-temperature phase I of caffeine from X-ray powder diffraction experiments. We show that this phase can be maintained at low temperature in the metastable state. A deep undercooling of phase I, below 250 K, transforms the metastable state into a glassy crystal state.