Conformational polymorphism of d-sorbitol (d-glucitol): the crystal and molecular structures of d-glucitol 2/3-hydrate and epsilon d-glucitol

The crystal structures of d-glucitol 2/3-hydrate (1) and epsilon d-glucitol (2) were determined by X-ray crystallography and refined to final conventional parameters of R=0.034 and 0.050, respectively. The conformations of the three independent molecules of 1 and one of the two independent molecules of 2 are similar and exhibit, bent-chain, sickle conformations of the carbon chain, thus avoiding the unfavourable 1,3-parallel O//O interactions. However, the orientation of a terminal hydroxyl group differs from the one observed in the bent-chain conformation of the known A form. An even more striking observation is the unexpected, extended, zigzag conformation of the second independent molecule of 2, which results in a 1,3-parallel interaction between O-2 and O-4. Thus in the class of alditols, the crystals of the epsilon and A forms of d-glucitol constitute the rarely occurring type of conformational polymorphism.