Discrete Cuboidal 15- and 16-Membered Water Clusters in Brucine 3.86-Hydrate, Water Release and Its Consequences

Up
to now, three brucine hydrates are known, brucine di-, tetra-,
and 5.25-hydrate. All of them were obtained from solutions containing
the additive diethanolamine, adenosine, and urea, respectively. Studying
the role of the additives on crystallization of the brucine hydrates,
we obtained a new, kinetically favored brucine 3.86-hydrate. In crystals
of brucine 3.86-hydrate, large 15- and 16-membered water clusters
of cuboidal topology are encapsulated in cages formed between honeycomb-like
brucine layers. Dehydration of the brucine hydrate leads to formation
of the known anhydrous brucine, giving insight into a mechanism of
the dehydration process, in which a shift of brucine ribbons in the
honeycomb-like layers leads to an openining of channels and water
release. A collapse of brucine layers after the water release results
in formation of the common anhydrous brucine. The anhydrous brucine
undergoes a phase transition at 249 K in the cooling mode and at 277
K in the heating mode. The phase transition is attributable to a huge
shift of brucine corrugated layers in relation to each other. The
phase transition for anhydrous brucine obtained by dehydration is
accompanied by thermal effects one order larger than anhydrous brucine,
obtained by crystallization from acetone solution.