Pyrrolidine and Its Hydrates in the Solid State

The crystals of new ambient pressure, high temperature (HT) polymorphs of neat pyrrolidine hemi- and hexahydrates were obtained using the IR laser assisted in situ crystallization method. They were subsequently investigated by single crystal X-ray diffraction and Raman spectroscopy. The transition from HT form to the known low temperature (LT) polymorph of the amine was monitored by X-ray powder diffraction. Although the investigated amine is an analogue to tetrahydrofuran (THF) which forms a stable clathrate hydrate, both binary pyrrolidine hydrates are dissimilar to the THF hydrate. The hexahydrate of pyrrolidine represents a class of semiclathrates where “guest” molecules are incorporated in the water framework. While cooling it undergoes a phase transition to the fully ordered phase. At higher temperatures the structure contains disordered pyrrolidine and water molecules. The latter adopts the same type of disorder as typical for H-disordered ice polymorphs (e.g., Ih, Ic, ...) and most clathrate hydrates.