W0077

The Delta (Monoclinic) Polymorph of D-Mannitol. Frank R. Fronczek, Dept. of Chemistry, Louisiana State Univ., Baton Rouge, Louisiana, 70803-1804; Haidy Nasr Kamel and Marc Slattery, Dept. of Pharmacognosy, Univ. of Mississippi, University, MS 38677.

The acyclic sugar alcohol D-mannitol, C₆H₁₄O₆, is one of the classic examples of a compound which crystallizes in several polymorphs, often simultaneously. Previously, at least three polymorphs have been crystallographically well characterized. Cell dimensions for a monoclinic (P2₁) polymorph, designated _ (or modification III by some authors), were reported 35 years ago. Despite the fact that the delta polymorph has physical properties best suited for its pharmaceutical use as an excipient, and several methods exist for crystallizing it exclusively, the crystal structure has not been previously reported. This is probably a result of the difficulty in growing high-quality single crystals.

We have isolated D-mannitol from the brown algae Dictyota dichotoma by accelerated solvent extration, and suitable crystals of the _ form were obtained directly from the extract. We report here its structure at 100K, and compare its conformation and hydrogen bonding to those of the other reported polymorphs. The structure is pseudosymmetric, approximating space group C222₁, and it appears nearly certain that the “_' form”, reported in that space group from 2-dimensional data in 1968, is identical to the polymorph reported here.

Cell dimensions at T=100K are a=4.899(2), b=18.268(6), c=5.043(2) Å, _=118.39(2)˚, R=0.031, 940 data with _<27.5˚ (MoK_).