The crystal structure of isopropylamine octahydrate, 1O(CH s )2CHNH2"SOH20, mp -4°C, has been determined from three-dimensional Weissenberg film data obtained at -160°C with CuKa radiation.The crystals are hexagonal, P~/mmc, with a=12.30 A, c=24.85 k 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 (4 2 X5 8 X6 4 ), separated by pentagonal dodecahedra (5 12 ) and 16-hedra (5 12 X6'). 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 (4 6 X 6 2 ). They are vacant, as are the pentagonal dodecahedra. INTRODUCTIONIsopropylamine octahydrate belongs to a group of hexagonal amine hydrates which includes those of npropylamine and trimethylamine.! A detailed structure study2 has shown the latter, (CHa)aN "lOiH 2 0, to have a hydrogen-bonded water framework related to the host structures found in the isoamyl ammonium salt hydrates,a.4 the 17 -A cubic (type II) gas hydrates,5.6 and the monoclinic n-butyl sulfonium fluoride hydrate. 7 The motif common to these water structures 8 is the extended hexagonal layer of face-sharing pentagonal dodecahedra, shown in projection in Fig. 1. These 12hedra are generally unoccupied, and the guest species are situated in polyhedral voids between the layers. This hydrate, (CHa)2CHNH 2 "8H 2 0, has similar layers of face-sharing 14-hedra, which present the same view as the 12-hedra when projected down the hexagonal axis. These 14-hedra can be derived from the 12-hedraby an expansion, which permits the isopropylamine molecules to be accommodated as hydrogen-bonded "guests." CRYSTAL DATAHexagonal crystals of (CHa)2CHNH2"8H20 were obtained from 12-mol % aqueous amine solutions at -SoC, Their diffraction symmetry is 6/mmm. The space group, from the extinctions (hh2fil) with I odd, is P62c, P6 a mc, or P6 a /mmc. The latter (No. 194) was observed to undergo a structure change within the range from -S to 160°. EXPERIMENTAL MEASUREMENTSThe crystal for diffraction studies was grown in situ on a Stoe Weissenberg camera from a 12-mol % solution sealed in a O.S-mm capillary. Selection of the single crystal was aided by use of polarizing filters. Cell dimensions were determined from a (Okl) photograph calibrated with NaCI powder diffraction lines. The diffraction intensities were recorded on multiple film equiinclination Weissenberg photographs at -160° with CuKa radiation. Eleven levels were obtained about the a axis, giving a ...

9) show systematic differences in the structure factors from those predicted by a model which assumes harmonic thermal vibration of the atoms. These differences indicate an anharmonic component in the thermal vibration of the magnesium atoms, consistent with their tetrahedral site symmetry. An analysis of these data has therefore been carried out in terms of a model which includes anharmonic thermal vibration and a least-squares refinement gave a value of the anharmonicity parameter (/~g=-2"39 x 10-1a erg./~-3) which is similar in magnitude to those found for other systems with the fluorite structure.The significance of anharmonic thermal vibrations in materials with the fluorite structure has been established by a series of recent accurate neutron diffraction studies on UO2 (Rouse, Willis & Pryor, 1968), BaF2 (Cooper, Rouse & Willis, 1968 and CaFz and SrFz (Cooper & Rouse, 1970). In this structure the anion has a tetrahedral environment which allows an anharmonic contribution to the thermal vibration of the anion, consistent with the introduction of a cubic teim pxyz in the potential, where x, y and z are the coordinates defining the instantaneous displacement of the anion (see Dawson, Hurley & Maslen, 1967). The magnitude of the anharmonicity parameter/? in these materials is of the order of -3 x 10-12 erA A,-3 and the anharmonic component of the thermal vibration results in significant deviations in the Bragg intensities from those predicted by a harmonic model. For X-ray diffraction the angular dependence of the scattering factors and the predominance of the cation scattering result in the X-ray Bragg intensities for these matelials being less dependent on the anharmonic effects, although a detailed analysis of recent accurate X-ray diffraction measurements on CaFa (Cooper, 1970a) has shown that these also are consistent with an anharmonicity of this magnitude. Panke & W61fel (1969) have recently carried out a careful X-ray diffraction study of Mg2Si, which also has the fluorite structure and which has a more favourable scattering factor ratio. It is interesting to note, therefore, that their results show systematic differences from the predictions of a model which assumes harmonic thermal vibration of the atoms, for example the different values obtained for the structure factors of the 933, 771 and 755 reflexions are indicative of anharmonic effects as described above, whereas a harmonic model would require these three reflexions to have equal structure factors. Because of these apparent systematic effects we have therefore reconsidered the experimental data in terms of an anharmonic model. The X-ray measurements on Mg2Si were made on both single crystals and powder samples and a comprehensive correction for extinction was carried out. Care was also taken to minimize errors due to multiple diffraction (Panke & W~51fel, 1968). No correction was made for the contribution of thermal diffuse scattering, but it is most probable that such a correction would result only in a change in the derived temperature f...

The crystal structure determination of 4(CHa)sN·41H20 shows that the trimethylamine guest molecules are hydrogen bonded within voids formed by a polyhedral host lattice of hydrogen-bonded water. The water framework consists mainly of sheets formed by face-sharing dodecahedra which are linked by additional water molecules. The amine molecules occupy large irregular cages between the layers of dodecahedra. The amine and the linking water molecules are statistically disordered so that a hexagonal structure is observed, whereas an ordered arrangement would have monoclinic symmetry. 14 P. T. Beurskens and G.

The crystal structure of the monoclhtic modification of n-propylamine clathrate hydrate has been determined at -lOOT. The space group is P 21 / n with cell dimensions. a = 12.43, b = 20,73. c = 17.28 A, and ,8=89.3°. The water framework is described as a packing of small unoccupied II hedra and large concave polyhedra. The disordered amine molecules occupy convex regions of the concave polyhedra with the amine nitrogens hydrogen bonding to the framework.

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