Structure of 1,6-anhydro-β-D-glucopyranose in plastic crystal, orientational glass, liquid and ordinary glass forms: molecular modeling and x-ray diffraction studies

Jurkiewicz, Karolina and Glajcar, Wojciech and Kamiński, Kamil and Temleitner, László and Burian, Andrzej (2021) Structure of 1,6-anhydro-β-D-glucopyranose in plastic crystal, orientational glass, liquid and ordinary glass forms: molecular modeling and x-ray diffraction studies. ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE, 77. pp. 138-149. ISSN 0108-7681

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Abstract

The structure of the plastic crystal (PC), orientational glass (OTG), liquid (LQ) and ordinary glass (OG) phases of 1,6-anhydro-β-D-glucopyranose (levoglucosan) has been investigated by means of x-ray diffraction and molecular modelling. The x-ray intensities were measured at 450 K for PC, at 230 K for OTG, at 470 K for LQ and at 230 K for OG. Diffraction data in the forms of static structure factors and pair distribution functions have been analyzed in reciprocal and real spaces and compared with results of molecular modelling. In the case of the LQ and OG samples, Reverse Monte Carlo simulations were used. The PC and OTG phases show long-range ordering of the hexagonal close-packed (hcp) type structure up to 120 Å with random orientation of the molecules. Inter-molecular correlations in the form of sharp Bragg peaks for PC and OTG are isolated from the diffusional intensity associated with intra-molecular correlations and structural disorder. Assuming a rigid molecular skeleton, isotropic free rotations of the molecules about their geometrical center in the full and limited angular ranges have been generated in theoretical models of the structure. It was demonstrated that the adoption of free rotations of the molecules in models leads to the best fits to experimental data for each studied phase of levoglucosan. The diffraction patterns of the LQ and OG samples show a relatively sharp first peak and a diffuse component being practically the same as that of the PC and OTG phases. It was showed that for the LQ and OG forms the models based on face-centered cubic (fcc) type arrangement are in good agreement with the experimental data. Interestingly, structural correlations for the disordered LQ and OG phases extend surprisingly far, up to about 50 Å. In addition, for all substances investigated, the paracrystalline disorder imposed on the generated models resulted in better compliance with the experimental data.

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