Acta Crystallographica Section B

Structural Science

Volume 61, Part 3 (June 2005)

research papers

Acta Cryst. (2005). B61, 329-334 [ doi:10.1107/S010876810500546X ]

Ab initio structure determination of the high-temperature phase of anhydrous caffeine by X-ray powder diffraction

P. Derollez, N. T. Correia, F. Danède, F. Capet, F. Affouard, J. Lefebvre and M. Descamps

Abstract: 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 diffractometer 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.

Keywords: ab initio structure determination; high-temperature phase; X-ray powder diffraction; anhydrous caffeine; rigid-body constraints.