Crystallization of (±)-Modafinil in Gel: Access to Form I, Form III, and Twins

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 (P21/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 Pna21, 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.