COMPARATIVE SO2 INFRARED-SPECTRA - TYPE-I AND TYPE-II CLATHRATE HYDRATE FILMS, LARGE GAS-PHASE CLUSTERS, AND ANHYDROUS CRYSTALLINE FILMS

The mechanism by which SO2 is incorporated into microparticles of ice in the vapor phase is receiving special interest because of the unexpectedly high efficiency with which SO2 is scavenged by ice crystals. A possible explanation of this efficiency might be found in the tendency for small polar molecules, such as the small ring ethers, to form clathrate hydrates at low temperatures and low partial pressures. This possibility has been examined by spectroscopic studies at ∼120 K of large gas-phase clusters formed from anhydrous SO2 and H2O-SO2 mixtures with a ratio appropriate for clathrate hydrate formation. On the basis of a comparison with new thin-film infrared data for the simple type I SO2 hydrate, the mixed SO2-ethylene oxide type I hydrate and the type II double hydrate with tetrahydrofuran, it is apparent that SO2 is not enclathrated under the conditions that are known to cause formation of clathrate hydrate crystalline clusters of ethylene oxide, trimethylene oxide, and tetrahydrofuran. This indication, that the SO2 clathrate hydrate, although stable once formed, grows relatively slowly, reduces the likelihood that SO2 enclathration is the basis for its large uptake in ice crystallites. The nature of the anhydrous-SO2 cluster spectra, together with existing data for anhydrous-CO2 clusters, prompted an examination of large cluster spectra from a macroscopic dielectric approach. A remarkable similarity of cluster spectra with spectra of thin films, at off-normal incidence, has revealed a close relationship between the cluster and thin film spectra. © 1990 American Chemical Society.