No abstract

Single crystals of (()-modafinil have been grown in gel medium obtained from the hydrolysis and condensation of tetramethoxysilane (TMOS). Three different crystal morphologies have been highlighted, corresponding to two modifications and a twin. The first polymorphic form was found to match with the known monoclinic form I (P2 1 /a, Z′ ) 2, a ) 14.502 Å, b ) 9.678 Å, c ) 20.844 Å, and β ) 110.17°). The determination of the second form allowed us to validate the previously predicted form III (orthorhombic Pna2 1 , Z′ ) 2, a ) 14.510 Å, b ) 9.710 Å, c ) 19.569 Å). The analysis of the twinned bodies and the structural similarities between form I and form III allowed us to identify the twin law: a stacking fault of (001) form I leading to a form III like interface. The analysis of the structures also suggested possible multitwinning.

Polymorphic forms I and III of (() modafinil have been studied. In contrast to the structure of form I which could be solved by using conventional single crystal X-ray diffraction, the crystal structure of form III could not be elucidated due to the shape and the size of the single crystals resulting from the high supersaturation under which this phase is obtained. To bypass the problem, the derived crystal packing (DCP) model has been successfully applied by recombining the slice (002)A extracted from form I. The calculated XRPD pattern of form III fits well with the experimental data. The extensive similarities between the two crystal structures are carefully detailed. † Crystallographic data are deposited at the CCDC under number 229171.

Although (±)-modafinil form I crystallizes in the centrosymmetric space group P21/a, a significant proportion of twins crystallized in polymerized TMOS gel exhibit a polar dissolution. In a slightly undersaturated solution, one body of the twin dissolves smoothly. leading to a progressive disappearance of every face; the second body dissolves heterogeneously, leading to lamellar fragments. This phenomenon could be explained by the insertion of (001) layers of form IIIa metastable noncentrosymmetric modification (SG Pna21)inside a single body of the twinned form I. Because the slice (001) is common to polymorphs I and III, the interlacing of the two forms is compatible with the formation of well-shaped crystals. Regrowth experiments on partially dissolved twins in slightly supersaturated solutions confirms the sustainability of the heterogeneous character for a single body only of the twins.