Characterization and crystal structure of D-mannitol hemihydrate

The objectives of this study were (i) to isolate and characterize mannitol hydrate, and (ii) to solve its crystal structure from high-resolution synchrotron X-ray powder diffraction data. Mannitol hydrate was prepared by freeze-drying aqueous mannitol solutions (5% w/v) under controlled conditions. X-ray powder diffractometry, differential scanning calorimetry, and thermogravimetric analyses indicated that mannitol exists as a hemihydrate (C₆H₁₄O₆ · 0.5H₂O). Synchrotron data were collected on the X3B1 beamline at the National Synchrotron Light Source. The simulated annealing program PSSP was used to solve the structure, which was subsequently refined by Rietveld analysis using the program package GSAS. The compound crystallizes in space group P1, with a = 9.8963 Å, b = 10.5424 Å, c = 4.7860 Å, α = 102.589°, β = 86.092°, and γ = 116.079°. The unit cell contains two dissimilar D-mannitol molecules and one water molecule, forming a hydrogen bonding pattern significantly different from that seen in the anhydrous polymorphs.