Structure, Stoichiometry, and Morphology of Bromine Hydrate

Abstract

The hydrate of bromine was one of the first clathrate hydrates discovered. It has played a significant role in the development of the solid solution theory of clathrate hydrates, yet its detailed structure remains unknown. This hydrate again has become a test case for two different views of clathrates: the solid solution model, after van der Waals and Platteeuw, that sees clathrates as unstable lattices which derive stability from a minimum degree of cage filling and thus are nonstoichiometric, and a view promoted by Dyadin and Aladko that all large cages in a hydrate structure need to be filled. In light of the latter view, existing data obtained over the last ∼160-year period on the composition and morphology of bromine hydrate would require the existence of four different hydrate structures. Our single crystal diffraction study of 16 different crystals of distinct compositions (Br2·8.62H2O to Br2·10.68H2O) and morphologies showed that there is just a single structure (tetragonal, P42/mnm, a = 23.04 Å, c = 12.07 Å, the structure originally proposed by Allen and Jeffrey) with considerable variation in the degree of occupancy of the large cages. The results favor the solid solution model for clathrates, and settle the question of long standing regarding the structure(s) of bromine hydrate. The bromine atoms occupy the large 14- and 15-hedral cages with up to 15 different crystallographically independent sites per cage and fractional occupancies from 0.19 to <∼0.01. The bromine hydrate structure is unique, so far. 129Xe NMR results suggest that when attempts were made to produce a double hydrate of bromine and xenon, a transient cubic structure II hydrate resulted, which slowly converted to the tetragonal form.

The First Silicon‐Based Cationic Clathrate III with High Thermal Stability: Si172−xPxTey (x=2y, y>20)

“…Description of the crystal structure: The atomic arrangement of Si 172Àx P x Te y corresponds to that of clathrate III and is generally analogous to the bromine hydrate [(Br 2 ) 20 5] 172 four-coordinate Si and P atoms build the host framework with the cages filled by Te atoms. Three types of polyhedral cages can be distinguished (Figures 2 and 3).…”

13C NMR Studies of Hydrocarbon Guests in Synthetic Structure H Gas Hydrates: Experiment and Computation

“…25,26 However, X-ray structural analysis of the sH hydrate with 2,2-dimethylpentane guests The Journal of Physical Chemistry A ARTICLE in the large cages shows that this guest is closer to the anti configuration with some deviation from ideal 180°angles. 27 The structural optimization at the B3LYP/6-311þþG(d,p) level shows that the energy difference between the anti and gauche conformers of 2,2-dimethylpentane is high (10.65 kJ 3 mol -1 ) and this may lead to unfavorable gauche conformations for this guest. The other alkane guests in this work have much lower minimum dihedral rotation barriers (between 3.5 to 4.1 kJ 3 mol -1 , see Table S2 of the Supporting Information) and gauche conformations may be accessible to the enclathrated molecules.…”

Compressibility of Gas Hydrates

“…Some guest substances form low-pressure gas hydrates with other structural types of host frameworks. These are bromine [13] and dimethyl ether. [14] The gas hydrate of xenon with the structure of the bromine hydrate [13] was also obtained as a relaxed version of a high-pressure sH gas hydrate of xenon.…”