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.