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Spectroscopic Signatures of Halogens in Clathrate Hydrate Cages. 1 13792 J. Phys. Chem. A 2006, 110, 13792-13798 Spectroscopic Signatures of Halogens in Clathrate Hydrate Cages. 1. Bromine Galina Kerenskaya,* Ilya U. Goldschleger, V. Ara Apkarian, and Kenneth C. Janda Department of Chemistry, UniVersity of California IrVine, IrVine, California 92697 ReceiVed: July 17, 2006; In Final Form: October 19, 2006 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 σ* 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. Introduction synthesis conditions since there is usually incomplete filling of the cages by the bromine molecules. No explanation for the Although the halogen clathrate hydrates have been known wide variety of crystal shapes that can be obtained has been for nearly 200 years, we have not been able to locate any optical offered. spectroscopic studies of these species. This is rather surprising since spectroscopy is often the first tool of choice for studying Dihalogen molecules are often employed as spectroscopic bonding and intermolecular interactions. Chlorine hydrate, the model systems due to their intense optical spectra and the earliest gas clathrate hydrate discovered, was first prepared by interesting variations in their properties depending on which Davy in 1811;1 bromine hydrate was prepared about 20 years two halogen atoms are bound together. For instance, since the later.2 The stoichiometry and structure of these clathrate hydrates visible transitions are nominally spin-forbidden, their intensities were quite controversial until those of chlorine hydrate were vary dramatically from F2 to I2, even though the identities of 8 conclusively determined by X-ray crystallography.3 It was found the states involved are similar. Also, the symmetry breaking that the water lattice is formed from two types of cages, denoted in an interhalogen such as ICl results in selection rules different 512 and 51262. The 512 cages are dodecahedra of 20 water from those of the homonuclear species. As diatomics with molecules with 12 pentagonal faces, as shown in Figure 1. The relatively easily assignable orbital symmetries, the halogens 9 51262 cages are made from 24 water molecules with 12 serve as model systems for nonlinear optical studies, energy 10,11 12,13 pentagonal faces and two hexagonal faces. To achieve three- transfer studies, and cluster studies. Similarly, they are dimensional packing, the crystal has three 51262 cages for every often used to observe how the local environment affects 14 512 cage. Such a clathrate hydrate structure is called “type I”. electronic structure, for example, in noble gas matrices, 15-17 18 In a perfect crystal with complete cage filling by chlorine, the solvents, or microporous SiO2. Given how often halogen molecules are the probe molecule of choice for spectroscopic overall stoichiometry of the unit cell would be (Cl2)8(H2O)46. However, the small cages are usually not fully occupied, and and dynamics studies, and given that the halogen clathrate therefore the chlorine-to-water ratio is typically somewhat hydrates have been known for nearly two centuries, it is quite smaller.4 surprising that there are no published studies of the electronic The structure of bromine clathrate hydrate is considerably spectra of halogen clathrate hydrates. more complicated than that of chlorine clathrate hydrate. The In 1975, Raman spectra for both chlorine and bromine stoichiometry is variable enough that, although the correct unit clathrate hydrate were reported, along with that of a BrCl 19 cell was first determined in 1963,5 it was later postulated that hydrate. The frequencies measured for the Cl2 and Br2 there are several different stable structures.6 In 1997, Ripmeester stretching vibrations were equal to those of the gas phase, within et al.7 determined that a single structure accounts for all stable an experimental error, indicating “little interaction between the cage-filling ratios. The crystal consists of three types of cages, guest and the host molecules”. The frequency of BrCl was red- -1 512,51262, and 51263, in a 10:16:4 ratio. In a perfect clathrate shifted by 17 cm , presumably due to electrostatic interactions crystal, bromine molecules would occupy the 51262 and 51263 since BrCl has a permanent dipole moment. cages, but not the 512 cages, for an overall unit cell stoichiometry Perhaps one reason for the lack of optical spectroscopic data of (Br2)20(H2O)172. The actual stoichiometry depends on the on the halogen clathrate hydrates is that their optical density in a pure crystal is extremely high. To circumvent this issue, we * Corresponding author. E-mail: [email protected]. make thin films of polycrystalline bromine clathrate hydrate and 10.1021/jp064523q CCC: $33.50 © 2006 American Chemical Society Published on Web 12/10/2006 Signatures of Halogens in Clathrate Hydrate Cages J. Phys. Chem. A, Vol. 110, No. 51, 2006 13793 Figure 1. Clathrate hydrate cages of type I and type II and the bromine clathrate hydrate structures. Larger cages are shown with a halogen guest molecule inside. study them at temperatures where the halogen vapor pressure in the bromine clathrate hydrate, and the 51264 cage that can is negligible. Also, we studied low concentrations of trapped hold a bromine molecule in either THF or CH2Cl2 clathrate bromine molecules in type II hydrate cages formed with either hydrate. Although the figures are only schematic, they give an tetrahydrofuran (THF, C4H8O) or methylene chloride (CH2Cl2) idea of the size ranges of the cages being studied. as the principle hydrate-former guest molecule. Type II hy- drates consist of a combination of 512 and 51264 cages, in a 2:1 Experimental Section ratio. The unit cell is face-centered cubic (space group Fd3m, Nanopure distilled water (resistivity greater than 16 MΩ) was a ) 17.3 Å) with 136 waters and eight guests.20 The THF or used in all experiments. The stated manufactured purity of CH2Cl2 molecules occupy most of the larger cages, but in this bromine, tetrahydrofuran (THF, C4H8O), cyclohexane (C6H12), study a small fraction of the large cages is occupied by bromine and methylene chloride (CH2Cl2) was greater than 99%. No molecules. further purification was performed. Absorption measurements Although no UV-vis spectroscopy has been reported on in the 190-700 nm spectral range were conducted in a Cary clathrate hydrate systems, other spectroscopic techniques have UV-visible spectrometer. For low-temperature scans, the been employed. Infrared spectra for ethylene oxide hydrate spectrometer was equipped with a glass Dewar with quartz clathrate were first reported in 1973.21 Subsequently, Devlin et windows and evaporating liquid nitrogen served as the refrig- al. used FTIR spectroscopy to measure the formation kinetics erating fluid. The temperature was monitored by a thermocouple of a variety of hydrate clathrate systems.22-24 Also, Devlin and probe. Fleyfel reported data in 1988 that show that stretching vibra- Absorption spectra of gas-phase Br2 and Br2 in solutions were tional frequencies of trapped molecules decrease with increasing recorded using 10-mm capped quartz cuvettes at T ) 293K. cage size and that the size of the small cage of a structure I The samples of bromine in ice were prepared by vapor hydrate increases as the size of the molecules occupying the deposition of the premixed gases, in the ratio 1:500, Br2-H2O, large cages increases.25 Sloan et al.26 monitored the methane on a 0.5-mm sapphire window held at 120 K, as described Raman transition at 2900 cm-1 and its shift and splitting from previously.9 This flash freezing technique is expected to yield the value in liquid water, 2911 cm-1, to a pair of values, 2905 an amorphous arrangement of the water and bromine molecules. and 2915 cm-1, that were assigned to methane in each of the Samples of clathrate hydrates were prepared in three different two cages of a type I hydrate. They were also able to use the ways: time dependence of such data to study the rate of hydrate (1) Polycrystalline samples of bromine hydrate were grown formation as a function of temperature and the presence of an on a surface of a conventional 10 × 10 mm quartz cell from “inhibitor” molecule. Spectra for other molecules have been aqueous bromine solutions with Br2-H2O ratios ranging from analyzed with the qualitative “loose cage-tight cage” model of 1:5 to 1:20. The cell was capped to contain the volatile bromine. Pimentel and Charles27, where increases in vibrational frequen- The crystallization process was initiated on a cell wall by cies for guest molecules are an indication that they are being applying dry ice above the mixture level.
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