Summary: Polymorphism in Br2 Clathrate Hydrates
I. U. Goldschleger, G. Kerenskaya,* K. C. Janda, and V. A. Apkarian
Department of Chemistry, UniVersity of California IrVine, IrVine, California 92697
ReceiVed: September 19, 2007; In Final Form: NoVember 6, 2007
The structure and composition of bromine clathrate hydrate has been controversial for more than 170 years
due to the large variation of its observed stoichiometries. Several different crystal structures were proposed
before 1997 when Udachin et al. (Udachin, K. A.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am.
Chem. Soc. 1997, 119, 11481) concluded that Br2 forms only the tetragonal structure (TS-I). We show
polymorphism in Br2 clathrate hydrates by identifying two distinct crystal structures through optical microscopy
and resonant Raman spectroscopy on single crystals. After growing TS-I crystals from a liquid bromine-
water solution, upon dropping the temperature slightly below -7 °C, new crystals of cubic morphology form.
The new crystals, which have a limited thermal stability range, are assigned to the CS-II structure. The two
structures are clearly distinguished by the resonant Raman spectra of the enclathrated Br2, which show long
overtone progressions and allow the extraction of accurate vibrational parameters: e ) 321.2 ( 0.1 cm-1
and exe ) 0.82 ( 0.05 cm-1 in TS-I and e ) 317.5 ( 0.1 cm-1 and exe ) 0.70 ( 0.1 cm-1 in CS-II. On
the basis of structural analysis, the discovery of the CS-II crystals implies stability of a large class of bromine
hydrate structures and, therefore, polymorphism.
Clathrate hydrates are a ubiquitous class of crystalline
inclusion compounds with nonstoichiometric composition,
consisting of guest molecules trapped in a lattice of polyhedral