2,5-O-Methyl­ene-d-mannitol: two polymorphs and the NaI complex

Two polymorphs of the title compound, (4R,5R,6R,7R)-4,7-bis­(hydroxy­methyl)-1,3-dioxepane-5,6-diol, C₇H₁₄O₆, both have Z′ = 2 at 100 K, and differ in their hydrogen-bonding patterns. The sodium iodide complex, NaI·C₇H₁₄O₆, is isomorphous with the NaCl complex, and has the mannitol, cation and anion all lying on twofold axes. The dioxepane rings of all three mol­ecules are in the twist-chair conformation.