Polyhedral Clathrate Hydrates. XVI. Structure of Isopropylamine Octahydrate

The crystal structure of isopropylamine octahydrate, 10(CH₃)₂CHNH₂·80H₂O, mp − 4°C, has been determined from three‐dimensional Weissenberg film data obtained at − 160°C with $\mathrm{Cu}\mathrm{K\alpha }$ radiation. The crystals are hexagonal, ${\text{P6}}_3\mathrm{\hspace{0.17em}/\hspace{0.17em}mmc}$ , with $\text{a\hspace{0.17em}=\hspace{0.17em}12.30 Å, c\hspace{0.17em}=\hspace{0.17em}24.85 Å}$ . The structure was determined from the Patterson function and refined anisotropically to an $R$ of 0.09 for 1041 observed reflections. The water structure is a clathrate‐type hydrogen‐bonded framework, which can be related to that of the cubic gas hydrates. It consists of layers of face‐sharing 14‐hedra (4² × 5⁸ × 6⁴), separated by pentagonal dodecahedra (5¹²) and 16‐hedra (5¹² × 6⁴). The amine molecules occupy positions with sixfold orientational disorder in the six 14 hedra and four 16 hedra per unit cell. They are hydrogen bonded to the water structure, singly within the 16‐hedra and doubly within the 14‐hedra. Because of the distortions caused by the inclusion of the amines in the 14‐hedra, there are additional voids in the arrangement of the 12‐, 14‐, and 16‐hedra, which do not occur in the gas hydrates. These are 8‐hedra with six squares and two hexagons (4⁶ × 6²). They are vacant, as are the pentagonal dodecahedra.