Spectroscopic signatures of halogens in clathrate hydrate cages. 2. Iodine.
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 omega 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(-1), 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.