Towards clathrates. 2. The frozen states of hydration of tert-butanol

Abstract

Abstract A new crystal structure of tert-butanol and water crystallizing as the decahydrate is reported. The crystallization of the mixture in the desired molar ratio was performed in a capillary placed directly on a goniometer of a single crystal diffractometer at 200 K and ambient pressure using focused IR laser radiation. The crystals were grown while the melting zone formed by the IR laser was moved along the capillary. Usually the crystallization process should be long enough (hours) in order to obtain a good quality single crystal. However, in the case of tert-butanol decahydrate, such a long process led to separation of the ice and alcohol. Only fast crystallization taking tens of seconds allowed crystallization of the desired crystalline phase. In the decahydrate tert-butanol molecules are located in channels formed by water molecules. Hydroxyl groups are anchored to the water framework via hydrogen bonds. All water molecules in the structure have hydrogen atoms disordered equally over two sites; the hydroxyl group is likewise disordered. This effect is observed at both, 200 K and 100 K. Raman spectra recorded for the crystalline phase suggest dynamic disorder at higher temperature, converting to static at lower T. The decahydrate of tert-butanol, together with already known its di- and hepta-hydrates, display similar features to those observed for series of tert-butylamine hydrates. The latter structures behave as frozen steps of amine hydration observed as crystal structures leading, at maximum dilution, to hexagonal ice. Hydrates of tert-butanol nicely follow this tendency completing the relationship found for the tert-butylamine: water system.