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In situ high-pressure crystallization and diffraction techniques have been applied to obtain two very structurally distinct semi-clathrates of the tert-butylamine-water system with hydration numbers 5.65 and 5.8, respectively, thereby considerably reducing a notable hydration gap between the monohydrate and the 71/4 -hydrate that results when crystallization space is explored by temperature alone. Both structures can be considered as an intriguing solid-state example of hydrophobic hydration, in which the water network creates wide tert-butylamine-filled channels stabilized by cross-linking hydrogen bonds. The existence of interconnected channels might also add low hydration structures to a list of potential targets for hydrogen storage. A detailed analysis of the topology of host water and host-guest interactions is reported and extended to those of other hydrates of the compound. This analysis offers new insight into properties of the tert-butylamine-water system and provides some clues as to the occurrence of the sizable number of hydrates of this compound.

The structure of a 1:1 β-cyclodextrin-dimethylformamide hydrated complex has been determined from single-crystal X-ray diffraction data. A complete study of the structure is presented herein, including invariom refinement and interaction energy calculations. The structure has unit-cell parameters that are different from those of other β-cyclodextrin complexes crystallizing in the same space group, but exhibits the known herringbone packing type. A structural comparison of these complexes has been carried out with XPac in order to understand the origin of the differences in packing and unit-cell parameters. The results show that the differences are most likely ascribed to variations in hydration and in the hydrogen-bonded network.

RMS root mean square Z N P T partition function, or RSS residual sum of squares number of molecules per unit cell s reduced coordinate xviii Database study on hydrate formation at high pressure Water is the most common solvent present in molecular organic materials, 9 and is easily incorporated in their structure due to its small size and H-bond forming capabilities. 10 Despite its ubiquitous presence and a considerable amount of research on the topic (Ch. 2), our understanding of the role of water in organic crystals is still limited, and prediction and control of hydrate formation remains an open question in crystal engineering. 11 A complete database study of hydrates formed at high pressure has been performed to bring to light trends on hydrate formation which can further contribute to systematise our overall knowledge on phase transformations. An equation of state for the compressibility of lattice water has been modelled as part of this study, providing an use-

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