Crystals of Benzamide, the First Polymorphous Molecular Compound, Are Helicoidal

The growth of spontaneously twisted crystals is a common but poorly understood phenomenon. An analysis of the formation of twisted crystals of a metastable benzamide polymorph (form II) crystallizing from highly supersaturated aqueous and ethanol solutions is given here. Benzamide, the first polymorphic molecular crystal reported (1832), would have been the first helicoidal crystal observed had the original authors undertaken an analysis by light microscopy. Polymorphism and twisting frequently concur as they are both associated with high thermodynamic driving forces for crystallization. Optical and electron microscopies as well as electron and powder X‐ray diffraction reveal a complex lamellar structure of benzamide form II needle‐like crystals. The internal stress produced by the overgrowth of lamellae is shown to be able to create a twist moment that is responsible for the observed non‐classical morphologies. The metastable polymorph of benzamide, first described in 1832, is shown to have a twisted morphology. Through the use of several microscopic and diffraction techniques, metastable, twisted benzamide needles are shown to have complex hierarchical intergrowths of lamellae with different orientations/structures. The internal stresses generated due to such intergrowths are a likely reason for benzamide twisting.