Polyhedral Clathrate Hydrates. XVI. Structure of Isopropylamine Octahydrate

Abstract

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 ...

Structural transformation and tuning behavior induced by the propylamine concentration in hydrogen clathrate hydrates

“…The lattice parameters agree with values of pure i-PA hydrates reported in the literature (a = 12.30 and c = 24.85 Å at 113 K). 31 On the other hand, the pattern of 5.6 mol% (i-PAÁ17.0H 2 O) in Fig. 1c was identified as a mainly cubic Fd% 3m structure (a = 17.253(1) Å) with little fraction of hexagonal ice (the Le Bail fittings of the patterns are shown in the ESI †).…”

“…The i-PAÁ8.0H 2 O hydrate contains octahedral (4 6 6 2 ), dodecahedral (5 12 ), tetradecahedral (4 2 5 8 6 4 ), and hexadecahedral (5 12 6 4 ) cages, 31 while the n-PAÁ6.5H 2 O hydrate contains the 'concave' polyhedron, which is the fused structure of 5 12 6 2 , 4 1 5 10 6 2 , 4 1 5 10 6 5 , and 4 2 5 8 6 5 convex polyhedra, and additional convex polyhedron (4 2 5 8 6 1 ) cages. 32 The amino groups of both hydrates are hydrogen-bonded to water molecules in each host framework.…”

“…A hypothetical framework, which consists of 68 water molecules, four 5 12 6 4 and eight 5 12 cages per unit cell with the hexagonal P6 3 /mmc structure (a = 12.3 and c = 19.3 Å), is known to be an idealized version of the pure i-PA clathrate hydrate, which consists of 80 water molecules, four 5 12 6 4 , six 4 2 5 8 6 4 , two 5 12 , and two 4 6 6 2 cages in a hexagonal unit cell. 31,33 This ideal structure, sometimes termed as structure V (sV) 7 or hexagonal structure II (HS-II), can be achieved by removal of squares from 4 2 5 8 6 4 cages to transform into dodecahedra. 31 The unindexed reflections shown in Fig.…”

“…31,33 This ideal structure, sometimes termed as structure V (sV) 7 or hexagonal structure II (HS-II), can be achieved by removal of squares from 4 2 5 8 6 4 cages to transform into dodecahedra. 31 The unindexed reflections shown in Fig. 1a 33,34 A possible interpretation of this unexpected crystal phase is that highpressure hydrogen induced the 'imperfect' structural transformation to increase the number of 5 12 cages, suitable for hydrogen enclathration (2 cages per 80 H 2 O molecules versus 8 cages per 68 H 2 O molecules).…”

Structural chemistry of piperidine hydrates

“…Packing diagrams of these two structures are presented in the Figure below. Moreover, piperidine hydrate 8.10 were isostructural with iso-propylamine hydrate 8.00 [6,7]. All synthesised crystals were also characterised with Raman spectroscopy and X-ray powder diffraction.…”