Spectroscopic signatures of halogens in clathrate hydrate cages. 1. Bromine

We report the first UV-vis spectroscopic study of bromine molecules confined in clathrate hydrate cages. Bromine in its natural hydrate occupies 5(12)6(2) and 5(12)6(3) lattice cavities. Bromine also can be encapsulated into the larger 5(12)6(4) 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 5(12)6(4) cages of type II clathrate, by about 900 cm(-1) for bromine in a combination of 5(12)6(2) and 5(12)6(3) 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.