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

The structures of the plastic crystal (PC), orientational glass (OTG), liquid (LQ) and ordinary glass (OG) phases of 1,6-anhydro-β-D-glucopyranose (levoglucosan) have been investigated using X-ray diffraction and molecular modeling. The experimental diffraction data in the forms of static structure factors and pair distribution functions are analyzed in reciprocal and real spaces and compared with results of model-based simulations. A new approach to modeling the structure of the disordered phases, taking into account the intermolecular scattering contribution in the form of sharp Bragg peaks, the slowly varying intensity associated with intramolecular correlations and the diffusive component resulting from structural disorder, is applied. In the case of the LQ and OG samples, reverse Monte Carlo simulations are also 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. Assuming a rigid molecular skeleton, isotropic free rotations of the molecules about their geometrical center in full and limited angular ranges are generated in theoretical models of the structure. It is demonstrated that the adoption of free rotations of the molecules 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 originating from quasi-Bragg planes of the densely packed face-centered cubic (fcc) type molecular arrangement. Moreover, the slowly varied intensity component of LQ and OG is practically the same as that of PC and OTG, suggesting that the intramolecular structure of these four phases does not change. Interestingly, structural correlations for the disordered LQ and OG states extend surprisingly far, up to about 50 Å. In addition, for all levoglucosan phases investigated, the paracrystalline disorder imposed on the generated models resulted in better compliance with the experimental data.