Formation of Crystalline Hydrates by Nonionic Chaotropes and Kosmotropes: Case of Piperidine

In this Article, a number of single crystal structures of piperidine hydrates are presented. While the lower hydrates, i.e., containing 1/2 or 2 water molecules per amine, present features characteristic for well-defined structures of cocrystals, the higher hydrates (8.1, 9 3/4, 11) have properties more similar to clathrate hydrates with the organic molecules enclosed in cages. However, in these systems some weak hydrogen bonds between piperidine and the water framework exist; thus they cannot be considered as pure host-guest structures. The property of piperidine to crystallize with water giving full enclathration of the amine is compared with the ability of nonionic chaotropes to generate clathrate hydrate-like structures. According to the obtained crystal structures of higher hydrates of piperidine, this amine behaves more like a chaotrope. This finding is supported by Raman spectra in the solid state.