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| - Spectroscopic Signatures of Halogens in Clathrate Hydrate Cages. 1. Bromine
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| - 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 intothe 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 spectrawhile shifting to the blue. In contrast, spectra of bromine in liquid water or amorphous ice are broadened andsignificantly more blue-shifted. The absorption bands shift by about 360 cm-1 for bromine in large 51264cages of type II clathrate, by about 900 cm-1 for bromine in a combination of 51262 and 51263 cages of purebromine hydrate, and by more than 1700 cm-1 for bromine in liquid water or amorphous ice. The dramaticshift and broadening in water and ice is due to the strong interaction of the water lone-pair orbitals with thehalogen 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 shiftsobserved in the clathrate hydrate cages are related to the spatial constraints on the halogen excited states bythe cage walls.
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