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 Br 2 forms only the tetragonal structure (TS-I). We show polymorphism in Br 2 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 brominewater 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 Br 2 , which show long overtone progressions and allow the extraction of accurate vibrational parameters: ω e ) 321.2 ( 0.1 cm -1 and ω e x e ) 0.82 ( 0.05 cm -1 in TS-I and ω e ) 317.5 ( 0.1 cm -1 and ω e x e ) 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 water cages. 1 The resurgence of interest in this fascinating class of solids is, in part, motivated by the recognition of the vastness of natural deposits of methane hydrates 2 and their potential global implications with respect to energy and the environment. 3,4 Understanding the stability and structures of clathrate hydrates remains a challenge since they are controlled by a delicate balance of hydrogen bonding within the lattice and weak interactions between the water lattice and the guest molecule. Bromine hydrate was one of the first clathrate hydrates discovered and played an important role in the development of models describing thermodynamic stability and physical properties of clathrate hydrates. 5 Nevertheless, since its discovery in 1828 by Lowig, 6 the structure and composition of Br 2 hydrates has been controversial. The reported hydration numbers, which varied between 7 and 12, 7-9 were previously assumed to arise from polymorphism, that is, more than one crystal structure. 10

We report the first UV-vis spectroscopic study of bromine molecules confined in clathrate hydrate cages. Bromine in its natural hydrate occupies 51262 and 51263 lattice cavities. Bromine also can be encapsulated into the larger 51264 cages of a type II hydrate formed mainly from tetrahydrofuran or dichloromethane and water. The visible spectra of the enclathrated halogen molecule retain the spectral envelope of the gas-phase spectra while shifting to the blue. In contrast, spectra of bromine in liquid water or amorphous ice are broadened and significantly more blue-shifted. The absorption bands shift by about 360 cm-1 for bromine in large 51264 cages of type II clathrate, by about 900 cm-1 for bromine in a combination of 51262 and 51263 cages of pure bromine hydrate, and by more than 1700 cm-1 for bromine in liquid water or amorphous ice. The dramatic shift and broadening in water and ice is due to the strong interaction of the water lone-pair orbitals with the halogen sigma* orbital. In the clathrate hydrates, the oxygen lone-pair orbitals are all involved in the hydrogen-bonded water lattice and are thus unavailable to interact with the halogen guest molecule. The blue shifts observed in the clathrate hydrate cages are related to the spatial constraints on the halogen excited states by the cage walls.

We report transient grating measurements carried out on single crystals of bromine clathrate hydrates and on bromine dissolved in water. In all cases, excitation into the B-state of Br2 leads to prompt predissociation, followed by cage-induced recombination on the A/A' electronic surfaces. In liquid water, the vibrationally incoherent recombinant population peaks at t=1 ps and decays with a time constant of 1.8 ps. In the hydrate crystals, the recombination is sufficiently impulsive to manifest coherent oscillations of the reformed bond. In tetragonal TS-I crystals, with the smaller cages, the recombination is fast, t=360 fs, and the bond oscillation period is 240 fs. In cubic CS-II crystals, the recombination is slower, t=490 fs, and the visibility of the vibrational coherence, which shows a period of 290 fs, is significantly reduced due to the larger cages and the looser fit around bromine. The mechanical cage effect is quantified in terms of the recombination time-distribution, the first three moments of which are associated with size, structural rigidity, and anelasticity of the cage. In the crystalline cages, the distribution is symmetric about the mean: mean time tm=300 fs, 400 fs and standard deviation sigma=70 fs, 100 fs, in TS-I and CS-II, respectively. The finding is consistent with the assignment of occupied cages: principally 5(12)6(2) polyhedra in TS-I and 5(12)6(4) polyhedra in CS-II. In liquid water, with diffuse cages, the distribution characterized by tm=555 fs and sigma=400 fs, is strongly skewed (gamma1=1.88) toward delayed recombination-the effective liquid phase hydration shell is larger than that in a hydrate phase, structurally disordered, and anelastic. Information about dipolar disorder, comparable in all three media, is extracted from electronic predissociation rates of the B-state, which is sensitive to the symmetry in the guest-host interaction.

Time-and frequency-resolved coherent anti-Stokes Raman scattering is used to carry out systematic measurements of vibrational dephasing on I 2 (v ¼ 1-19) isolated in solid Kr, as a function of temperature, T ¼ 7-45 K. The observed quantum beats, o v 0 ,v 00 allow an accurate reconstruction of the solvated molecular potential, which is well represented by the Morse form: o e ¼ 211.56 AE 0.14, o e x e ¼ 0.658 AE 0.006. Near T ¼ 7 K, the coherence decay rates g v,0 become independent of temperature and show a linear v-dependence, indicative of dissipation, which must be accompanied by the simultaneous creation of at least four phonons. At higher temperatures, the T-dependence is exponential and the v-dependence is quadratic, characteristic of pure dephasing via pseudo-local phonons. A normal mode analysis suggests librations as the principle modes responsible for pure dephasing.

UV-vis and Raman spectroscopy were used to study iodine molecules trapped in sII clathrate hydrate structures stabilized by THF, CH 2 Cl 2 , or CHCl 3 . The spectra show that the environment for iodine inside the water cage is significantly less perturbed than either in aqueous solution or in amorphous water-ice. The resonance Raman progression of I 2 in THF clathrate hydrate can be observed up to V ) 6 when excited at 532 nm. The extracted vibrational frequency ω e ) 214 ( 1 cm -1 is the same as that of the free molecule to within experimental error. At the same time, the UV-vis absorption spectrum of I 2 in the sII hydrate exhibits a relatively large, 1440 cm -1 , blue-shift. This is mainly ascribed to the differential solvation of the I 2 electronic states. We conclude that iodine in sII hydrate resides in a 5 12 6 4 cavity, in which the ground-state I 2 potential is not significantly perturbed by the hydrate lattice. In contrast, in water and in ice, the valence absorption band of I 2 is dramatically broadened and blue-shifted by 3000 cm, and the resonance Raman scattering is effectively quenched. These observations are shown to be consistent with a strong interaction between water molecule and iodine through the lone pair of electrons on water as in the case of bromine in the same media. The results presented here, and the stability of other halogen hydrates, were used to test the predictions of simple models and force-field calculations of the host cage-guest association energy.