Kosmotropic Behavior of 3‑Pyrroline during Crystalline Hydrates Formation

In this work, we thoroughly analyze the structural behavior of 3-pyrroline (unsaturated analogue of pyrrolidine) in the context of generating possible clathrate hydrates. The crystals of neat amine and its two hydrates tri- and hexahydrate have been obtained with the in situ crystallization technique assisted by IR laser focused radiation. The neat amine crystallizes in the P21/c space group with three ordered nonequivalent amine molecules in the unit cell. The structure of 3-pyrroline trihydrate is characterized by layers formed by amine and water molecules (P21/c), whereas 3-pyrroline hexahydrate contains amine moieties incorporated in a three-dimensional water network, thus resembling hydrates characteristic for kosmotropes. The latter system is isostructural with pyrrolidine hexahydrate and undergoes a phase transition (P21/m ! Cmcm). The analyses were completed by PXRD experiments performed for mixtures with different ratios of amine to water, Raman spectroscopy, differential scanning calorimetry studies, and ab initio calculations. A relatively small change in the ring of 3-pyrroline in comparison to fully saturated pyrrolidine seems to not affect the kosmotropic behavior of the molecule described here. This is not the case of a ring size which is the key feature defining the kosmo- or chaotropic character of the compounds. Indeed, the six-membered piperidine tends to form hydrates characteristic for chaotropes