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.