The crystal structure of an unstable polymorph of β- d -allose

The crystal structure of a new polymorph, Form II, of β-D-allose has been determined by X-ray powder diffraction. The unit cell is hexagonal, a = b = 16.598 Å, c = 4.856 Å, α = β = 90°, γ = 120°, space group P62 with Z = 6, Z′ = 1. The molecule adopts the 4C1 chair-conformation, with a torsional change of conformation of the O6 side-chain compared to the orthorhombic Form I. The two polymorphs share a common feature of a stacked hydrogen bonded column of molecules in the short axis direction. The structures differ in hydrogen linking of these columns. The more stable Form I has more immediate neighbours linked by hydrogen bonds to a reference molecule, and higher crystal density than Form II.